Documents published in 2020 : Scipedia

Blood-flow models of the circle of Willis from magnetic resonance data
J. Cebral, M. Castro, O. Soto, R. Lohner, N. Alperin

Journal of Engineering Mathematics (2003). Vol. 47, pp. 369-386

Abstract
Detailed knowledge of the cerebral hemodynamics is important for a variety of clinical applications. Cerebral perfusion depends not only on the status of the diseased vessels but also on the patency of collateral pathways provided by the circle of Willis. Due to the large anatomical and physiologic variability among individuals, realistic patient-specific models can provide new insights into the cerebral hemodynamics. This paper presents an image-based methodology for constructing patient-specific models of the cerebral circulation. This methodology combines anatomical and physiologic imaging techniques with computer simulation technology. The methodology is illustrated with a finite element model constructed from magnetic resonance image data of a normal volunteer. Several of the remaining challenging problems are identified. This work represents a starting point in the development of realistic models that can be applied to the study of cerebrovascular diseases and their treatment.
Abstract
Detailed knowledge of the cerebral hemodynamics is important for a variety of clinical applications. Cerebral perfusion depends not only on the status of the diseased vessels [...]
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Convergence study for the discrete particle method
D. Pelessone, J. Baum, R. Lohner, C. Charman, J. Baylot

(2003). Proceedings Second MIT Conference on Compurational Fluid and Solid Mechanics June 17–20, pp. 2093-2096

Abstract
The Discrete Particle Method (DPM) is a numerical technique in the class of the discrete element methods for the modeling of cementitious material in the pre- and post-failure regimes. Because the DPM is not based on continuum mechanics, the conventional convergence properties of Galerkin based methods, such as the finite element method, are not expected to apply. This chapter presents the results of a study to assess the convergence properties of the DPM for elastic problems. The DPM belongs to the general class of methods denoted as discrete element methods. The benchmark problem is based on the vibrations of a concrete beam free in space. Axial oscillations are used to adjust DPM model parameters, bending oscillations are used to test convergence characteristics. Results show that the DPM solution converges to the equivalent converged finite element solution.
Abstract
The Discrete Particle Method (DPM) is a numerical technique in the class of the discrete element methods for the modeling of cementitious material in the pre- and post-failure [...]
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Coupling of discrete particle model with embedded mesh flow solver
C. Charman, D. Pelessone, R. Lohner, J. Baum

(2003). Proceedings Second MIT Conference on Compurational Fluid and Solid Mechanics June 17–20, pp. 1282-1284

Abstract
This chapter describes a unique coupling between a discrete particle model and an embedded mesh flow solver, with the focus on solving blast loadings on reinforced concrete structures. The development of a discrete particle method (DPM) model is more complex and time-consuming than the mesh generation of a typical finite element method (FEM) analysis. The DPM process is a bootstrapping process, where the discrete particle solver is used in the development of the model. Two methods are commonly used for creating particle models: “fill and expand” (F+E), and the "depositional" methods. There have been numerous successes in coupling the finite element-based structural models to the finite-element fluid-flow solver (FEFLO). Two methods for solving the coupled fluid/structure interaction problem are: the body-conforming and the embedded grid methods. In the body-conforming method, the external faces of the structure are used to define the fluid domain. In the embedded method, the structure is placed inside a larger flow region with special treatment of the fluid elements close to the structural surfaces.
Abstract
This chapter describes a unique coupling between a discrete particle model and an embedded mesh flow solver, with the focus on solving blast loadings on reinforced concrete [...]
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Simulation of the must field experiment using the FEFLO-URBAN CFD model
F. Camelli, S. Hanna, R. Lohner

Computer Science (2004). Vol. 13 (12), pp. 1-14

Abstract
The Setting Test (MUST) experiment, that was carried out at Dugway Proving Ground, was analyzed using Very Large Eddy Simulation (VLES). The CFD approaches used to simulate transport and dispersion in the atmosphere at the urban scale included Reynold Average Navier-Stokes (RANS) and LES. Improvements of the vertical turbulence production were introduced using the geometrical roughness applied to the ground surface. The results show that small changes in wind direction can produce large localized changes in concentration levels.
Abstract
The Setting Test (MUST) experiment, that was carried out at Dugway Proving Ground, was analyzed using Very Large Eddy Simulation (VLES). The CFD approaches used to simulate [...]
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On the computation of multi-material flows using ALE formulation
H. Luo, J. Baum, R. Lohner

Journal of Computational Physics (2004). Vol. 194 (1), pp. 304-328

Abstract
Computation of compressible multi-fluid flows with a general equation of state using interface tracking and moving grid approach is discussed in this paper. The AUSM+, HLLC, and Godunov methods are presented and implemented in the context of arbitrary Lagrangian–Eulerian formulation for solving the unsteady compressible Euler equations. The developed methods are fully conservative, and used to compute a variety of multi-component flow problems, where the equations of state can be drastically different and stiff. Numerical results indicate that both ALE HLLC and Godunov schemes demonstrate their simplicity and robustness for solving such multi-phase flow problems, and yet ALE AUSM+ scheme exhibits strong oscillations around material interfaces even using a first order monotone scheme and therefore is not suitable for this class of problems.
Abstract
Computation of compressible multi-fluid flows with a general equation of state using interface tracking and moving grid approach is discussed in this paper. The AUSM+, [...]
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Multistage explicit advective prediction for projection-type incompressible flow solvers
R. Lohner

Journal of Computational Physics (2004). Vol. 195 (1), pp. 143-152

Abstract
A multistep advective predictor has been developed within the context of projection schemes for incompressible flows. The key idea is to integrate with schemes of different order the different regions of the domain. In regions where advection dominates, multistepping yields a considerable benefit. In those regions where viscosity dominates, the scheme reverts naturally to the original one-step scheme. Several examples show savings of the order of 1:3–1:10 as compared with standard projection schemes, even for transient problems. Given that these benefits can be achieved with a very modest change in existing codes, the proposed multistage advective predictor should be widely applicable.
Abstract
A multistep advective predictor has been developed within the context of projection schemes for incompressible flows. The key idea is to integrate with schemes of different [...]
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Projective prediction of pressure increments
R. Lohner

Commun. Numer. Meth. Engng (2004). Vol. 21 (4), pp. 201-207

Abstract
A simple projective predictor of pressure increments has been developed. The procedure requires the storage of previous pressure increments and right-hand sides, i.e. a modest amount of storage. Based on this information, the known right-hand sides are projected onto the right-hand side at the new timestep. The projection coefficients are then used to predict the pressure increment at the new timestep. Numerical tests indicate that the number of iterations required is reduced considerably. Furthermore, the main gains are achieved with a very modest number of basis vectors. Typically, no more than 2 previous results have to be stored. The procedure is easy to implement and should be applicable to a large number of codes.
Abstract
A simple projective predictor of pressure increments has been developed. The procedure requires the storage of previous pressure increments and right-hand sides, i.e. a modest [...]
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Large-scale fluid-structure interaction simulations
R. Lohner, J. Cebral, C. Yang, J. Baum, E. Mestreau, C. Charman, D. Pelessone

Computing in Science & Engineering (2004). Vol. 6 (3), pp. 27-37

Abstract
Combining computational-science disciplines, such as in fluid-structure interaction simulations, introduces a number of problems. The authors offer a convenient and cost-effective approach for coupling computational fluid dynamics (CFD) and computational structural dynamics (CSD) codes without rewriting them. With the advancement of numerical techniques and the advent, first, of affordable 3D graphics workstations and scalable compute servers, and, more recently, PCs with sufficiently large memory and 3D graphics cards, public-domain and commercial software for each of the computational core disciplines has matured rapidly and received wide acceptance in the design and analysis process. Most of these packages are now at the threshold mesh generation pre-processor. This has prompted the development of the next logical step: multidisciplinary links of codes, a trend that is clearly documented by the growing number of publications and software releases in this area. In this paper, we concentrate on fluid-structure and fluid-structure-thermal interaction, in which changes of geometry due to fluid pressure, shear, and heat loads considerably affect the flowfield, changing die loads in turn. Problems in this category include: steady-state aerodynamics of wings under cruise conditions; aeroelasticity of vibrating - that is, elastic - structures such as flutter and buzz (aeroplanes and turbines), galloping (cables and bridges), and maneuvering and control (missiles and drones); weak and nonlinear structures, such as wetted membranes (parachutes and tents) and biological tissues (hearts and blood vessels); and strong and nonlinear structures, such as shock-structure interaction (command and control centers, military vehicles) and hypersonic flight vehicles.
Abstract
Combining computational-science disciplines, such as in fluid-structure interaction simulations, introduces a number of problems. The authors offer a convenient and cost-effective [...]
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A stabilized edge-based implicit incompressible flow formulation
O. Soto, R. Lohner, J. Cebral, F. Camelli

Comput. Methods Appl. Mech. Engrg., (2004). Vol. 193 (23-26), pp. 2139-2154

Abstract
An edge-based implementation of an implicit, monolithic, finite element (FE) scheme for the solution of the incompressible Navier–Stokes (NS) equations is presented. The original element formulation is based on the pressure stability properties of an implicit second-order in time fractional step (FS) method, which is conditionally stable. The final monolithic scheme preserves the second-order accuracy of the FS method, and is unconditionally stable. Furthermore, it can be demonstrated that the final pressure stabilizing term is practically the same fourth-order pressure term added by some authors (but following different arguments) to obtain high order accurate results, and that the final discretized convective terms are formally a second-order discretization of the respective continuous one. The development of the edge implementation is supported by two criteria: the properties of the element based one, which has already been extensively tested and for which convergence and stability analysis has already been presented, and on the enforcement of global conservation and symmetry at the discrete level. A monotonicity preserving term which decreases the discretization order in sharp gradient regions to avoid localized oscillations (overshoots and undershoots), is formulated and tested. Some numerical examples and experimental comparisons are presented
Abstract
An edge-based implementation of an implicit, monolithic, finite element (FE) scheme for the solution of the incompressible Navier–Stokes (NS) equations is presented. [...]
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Dynamic deactivation for advection‐dominated contaminant transport
R. Lohner, F. Camelli

Commun. Numer. Meth. Engng (2004). Vol. 20 (8), pp. 639-646

Abstract
A simple dynamic deactivation procedure for advection‐dominated contaminant transport is presented. The key idea is to avoid any work in regions where the solution cannot change, i.e. where sources vanish and unknowns do not exhibit any spatial change. Saving factors in CPU of 1: 3–1: 10 are commonly achieved. The procedure is general and simple to implement, and should be applicable to a large number of codes and transport problems.
Abstract
A simple dynamic deactivation procedure for advection‐dominated contaminant transport is presented. The key idea is to avoid any work in regions where the solution cannot [...]
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An Adjoint-Based Design Methodology for CFD Optimization Problems
O. Soto, R. Lohner, C. Yang

Int. J. Numer. Meth. Heat and Fluid Flows (2004). Vol. 14 (6), pp. 734-759

Abstract
A complete CFD design methodology is presented. The main components of this methodology are a general edge-based compressible/incompressible flow solver; a continuous adjoint formulation for the gradient computations; a steepest descent technique for the change of design variables; evaluation of the gradient of the discretized flow equations with respect to mesh by finite differences; a CAD-free pseudo-shell surface parametrization, allowing every point on the surface to be optimized to be used as a design parameter; and a level type scheme for the movement of the interior points. Several examples are included to demonstrate the methodology developed.
Abstract
A complete CFD design methodology is presented. The main components of this methodology are a general edge-based compressible/incompressible flow solver; a continuous adjoint [...]
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Assessing maximum possible damage for contaminant release events
F. Camelli, R. Lohner

Engineering Computations (2004). Vol. 21 (7), pp. 748-760

Abstract
The combined use of damage criteria, genetic algorithms and advanced CFD solvers provides an effective strategy to identify locations of releases that produce maximum damage. The implementation is simple and does not require any change to flow solvers. A rather general criterion has been formulated to determine the damage inflicted by the intentional or unintentional release of contaminants. Results of two typical cases show that damage can vary considerably as a function of release location, implying that genetic algorithms are perhaps the only techniques suited for this type of optimization problem
Abstract
The combined use of damage criteria, genetic algorithms and advanced CFD solvers provides an effective strategy to identify locations of releases that produce maximum damage. [...]
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Comparison of Classical And Simple Free-surface Green Functions
C. Yang, R. Lohner, F. Noblesse

International Journal of Offshore and Polar Engineering (2004). Vol. 14 (4), pp. 1-8

Abstract
The classical Green function and a related simpler Green function associated with the linearized free-surface boundary condition for 3D diffraction-radiation by a ship advancing in regular waves are considered for the special case of steady flows. Both the classical Green function and the simpler Green function satisfy the radiation condition and the linearized free-surface condition in the farfield (where the linear free-surface condition is valid). The classical Green function also satisfies the linear free-surface condition in the nearfield (where this linear condition is only an approximation, due to nearfield effects), whereas the simple Green function satisfies the linear free-surface condition only approximately in the nearfield. Numerical differences between these alternative free-surface Green functions are shown to vanish in the farfield, as expected, and to be relatively moderate in the nearfield. Copyright © 2004 by The International Society of Offshore and Polar Engineers.
Abstract
The classical Green function and a related simpler Green function associated with the linearized free-surface boundary condition for 3D diffraction-radiation by a ship advancing [...]
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An ALE method for compressible multi-material flows on unstructured grids
H. Luo, J. Baum, R. Lohner

(2005). Shock Waves. Springer, Berlin, Heidelberg

Abstract
Multi-material flows, where a moving interface exists between two immiscible fluids, can be found in a variety of engineering problems. Development of numerically accurate and computationally efficient algorithms for multi-material flow simulations remains one of the unresolved issues in computational fluid dynamics. These flow problems are characterized by the existence of material interfaces. The modeling of these complicated free boundaries poses a difficult numerical challenge, as they are either time dependent or unknown a priori and determined as part of the solution. A number of numerical methods exist for solving the interface problems: interface capturing methods (mixed cell methods), level set methods, volume of fluid and interface reconstruction methods, interface tracking methods, free-Lagrange methods. Unfortunately, there are still limitations and shortcomings attached to each of them. In general, for all these methods that allow mixed cells, the computation of thermodynamical variables such as pressure, speed of sound, and temperature on mixed cells is difficult to achieve correctly. In particular, when the equations of state for different materials are drastically different, a small error on the thermodynamical variables can lead to collapse or meaningless of the computation. For all these methods where the interface is represented and tracked explicitly either by marking it with special marker points, or by treating it like a boundary, it is difficulty to handle very complex free surface problems, especially those involving interface topological changes such as merging or breakup of the interface.
Abstract
Multi-material flows, where a moving interface exists between two immiscible fluids, can be found in a variety of engineering problems. Development of numerically accurate [...]
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Recent development of a coupled CFD/CSD methodology using an embedded approach
J. Baum, E. Mestreau, H. Luo, R. Lohner, D. Pelessone, C. Charman

(2005). Shock Waves. Springer, Berlin, Heidelberg

Abstract
A new algorithm for modeling the response of structures to severe airblast and fragment loading, including the modeling of large plastic deformations, structural failure and break-up, is described in this paper. The coupled Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodologies required to describe these phenomena include the FEFL098 flow solver and DYNA3D structural solver. The original coupling between the two domains was based on the so-called “glued-mesh” approach, where the CFD and CSD interfaces match. Recent failure of this approach to model severe structural deformation, as well as crack propagation in steel and concrete, led us to the development and use of the “embedded-mesh” approach. Here, the CSD objects float through the CFD domain. While each approach has its own advantages, limitations and deficiencies, the embedded approach was proven to be superior for the problems modeled here. Critical applications of both approaches are described, including weapon detonation and fragmentation, airblast interaction with a reinforced concrete wall, and fragment/airblast interaction with a steel wall. The final applications model the interaction of an external airblast with a generic steel ship hull and a generic multi-chamber steel tower.
Abstract
A new algorithm for modeling the response of structures to severe airblast and fragment loading, including the modeling of large plastic deformations, structural failure and [...]
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On the coupling of CFD and CSD methodologies for modeling blast-structure interactions
J. Baum, E. Mestreau, R. Lohner, D. Pelessone, M. Giltrud, J. Gran

WIT Transactions on The Built Environment (2005). Vol. 84, pp. 361-371

Abstract
This paper describes applications of a coupled Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodology to the simulation of blast waves generated by bare explosive charges in a test facility with rigid and deformable walls. The coupled algorithm combines FEFLO98 (CFD) and MARS3D (CSD) via an embedded approach, where the CSD objects float through the CFD domain. This combination enables an easier and more accurate prediction of structural deformation, cracking and failure under blast loading. Several experiments were conducted to characterize blast load and structural response as a function of charge size, weapon ignition point (nose or tail) and orientation (horizontal or vertical). The numerical simulations helped in understanding the experimental results, some of which were not intuitively understood. Good agreement between the experimental results and the numerical predictions were demonstrated for pressure data, blast loading and the corresponding structural response. Keywords: blast-structure interaction, coupled CFD and CSD, blast wave evolution, structural response to blast loading.
Abstract
This paper describes applications of a coupled Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodology to the simulation of blast waves [...]
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High-Reynolds Number Viscous Flow Computations Using Unstructured-Grid Method
H. Luo, J. Baum, R. Lohner

Journal of Aircraft (2005). Vol. 42 (2), pp. 483-462

Abstract
An unstructured grid method is presented to compute three-dimensional compressible turbulent flows for complex geometries. The Navier-Stokes equations along with the one equation turbulence model of Spalart-Allmaras are solved using a parallel, matrix-free implicit method on unstructured tetrahe- dral grids. The developed method has been used to predict drags in the transonic regime for both DLR- F4 and DLR-F6 configurations to assess the accuracy and efficiency of the method. The results obtained are in good agreement with experimental data, indicating that the present method provides an accurate, efficient, and robust algorithm for computing turbulent flows for complex geometries on unstructured grids.
Abstract
An unstructured grid method is presented to compute three-dimensional compressible turbulent flows for complex geometries. The Navier-Stokes equations along with the one equation [...]
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Optimal placement of sensors for contaminant detection based on detailed 3D CFD simulations
R. Lohner, F. Camelli

Engineering Computations (2005). Vol. 22 (3), pp. 260-273

Abstract
Purpose - Develop a method for the optimal placement of sensors in order to detect the largest number of contaminant release scenarios with the minimum amount of sensors. Design/methodology/approach - The method considers the general sensor placement problem. Assuming a given number of sensors, every release scenario leads to a sensor input. The data recorded from all the possible release scenarios at all possible sensor locations allow the identification of the best or optimal sensor locations. Clearly, if only one sensor is to be placed, it should be at the location that recorded the highest number of releases. This argument can be used recursively by removing from further consideration all releases already recorded by sensors previously placed. Findings - The method developed works well. Examples showing the effect of different wind conditions and release locations demonstrate the effectiveness of the procedure. Practical implications - The method can be used to design sensor systems for cities, subway stations, stadiums, concert halls, high value residential areas, etc. Originality/value - The method is general, and can be used with other physics-based models (puff, mass-conservation, RANS, etc.). The investigation also shows that first-principles CFD models have matured sufficiently to be run in a timely manner on PCs, opening the way to optimization based on detailed physics.
Abstract
Purpose - Develop a method for the optimal placement of sensors in order to detect the largest number of contaminant release scenarios with the minimum amount of sensors. [...]
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Efficient simulation of blood flow past complex endovascular devices using an adaptive embedding technique
J. Cebral, R. Lohner

IEEE Transactions on Intelligent Transportation Systems (2005). Vol. 24 (4), pp. 468-476

Abstract
The simulation of blood flow past endovascular devices such as coils and stents is a challenging problem due to the complex geometry of the devices. Traditional unstructured grid computational fluid dynamics relies on the generation of finite element grids that conform to the boundary of the computational domain. However, the generation of such grids for patient-specific modeling of cerebral aneurysm treatment with coils or stents is extremely difficult and time consuming. This paper describes the application of an adaptive grid embedding technique previously developed for complex fluid structure interaction problems to the simulation of endovascular devices. A hybrid approach is used: the vessel walls are treated with body conforming grids and the endovascular devices with an adaptive mesh embedding technique. This methodology fits naturally in the framework of image-based computational fluid dynamics and opens the door for exploration of different therapeutic options and personalization of endovascular procedures.
Abstract
The simulation of blood flow past endovascular devices such as coils and stents is a challenging problem due to the complex geometry of the devices. Traditional unstructured [...]
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Extension of Harten-Lax-van Leer Scheme for Flows at All Speeds
H. Luo, J. Baum, R. Lohner

AIAA Journal (2005). Vol. 43 (6), pp. 1160-1166

Abstract
The Harten, Lax, and van Leer with contact restoration (HLLC) scheme has been modified and extended in conjunction with time-derivative preconditioning to compute How problems at all speeds. It is found that a simple modification of signal velocities in the HLLC scheme based on the eigenvalues of the preconditioned system is only needed to reduce excessive numerical diffusion at the low Mach number. The modified scheme has been implemented and used to compute a variety of flow problems in both two and three dimensions on unstructured grids. Numerical results obtained indicate that the modified HLLC scheme is accurate, robust, and efficient for flow calculations across the Mach-number range.
Abstract
The Harten, Lax, and van Leer with contact restoration (HLLC) scheme has been modified and extended in conjunction with time-derivative preconditioning to compute How problems [...]
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Comparison of Classical And Weakly-singular Representations of Freesurface Potential Flows
C. Yang, R. Lohner, F. Noblesse

International Journal of Offshore and Polar Engineering (2005). Vol. 15 (4), pp. 241-248

Abstract
Within the potential-flow framework, a flow in a domain is determined in terms of the flow at the boundary surface of the flow domain by means of a classical boundary-integral representation, which defines the flow potential фin terms of a Green function G and its gradient ▼G. An alternative flow representation is the weakly-singular representation (which defines фin terms of G and a vector Green function G associated to G via the relation ▼×G = ▼G) given by the authors for diffraction radiation by a ship advancing in regular waves. The alternative mathematical representations of far-field waves associated with the classical and weakly-singular potential representations are compared here in the special case of steady flows.
Abstract
Within the potential-flow framework, a flow in a domain is determined in terms of the flow at the boundary surface of the flow domain by means of a classical boundary-integral [...]
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Adaptividad En El Método Sin Malla De Puntos Finitos
F. Perazzo, R. Lohner, N. Ipinza

Mecánica Computacional (2006). Vol. XXV (11), pp. 1039-1050

Abstract
En este trabajo se presentan un estimador del error a posteriori y un proceso de refinamiento adaptivo para el método sin malla de puntos finitos (MPF). El indicador del error se formula a partir de la evaluación del funcional de mínimos cuadrados, utilizado en el cálculo de la función de forma. Nuevos grados de libertad o nodos adicionales pueden ser incorporados sin dificultad en las regiones donde el estimador del error presenta un valor elevado, mediante las técnicas de refinamiento h y p. La validez del estimador del error propuesto se demuestra, mediante el desarrollo de problemas de la mecánica de sólidos y fluidos tanto 2D como 3D, utilizando un proceso de refinamiento adaptivo de la solución
Abstract
En este trabajo se presentan un estimador del error a posteriori y un proceso de refinamiento adaptivo para el método sin malla de puntos finitos (MPF). El indicador [...]
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30 Years of FCT: Status and Directions
R. Lohner, J. Baum

(2005). Flux-Corrected Transport. Scientific Computation. Springer, Berlin, Heidelberg

Abstract
A somewhat historical perspective of the use of FCT for fluid dynamics is given. The particular emphasis is on large-scale blast problems. A comparison with other high-resolution CFD solvers is included to highlight the differences between them, as well as the relative cost. Results from test runs, as well as several relevant production runs are shown. Outstanding issues that deserve further investigation are identified.
Abstract
A somewhat historical perspective of the use of FCT for fluid dynamics is given. The particular emphasis is on large-scale blast problems. A comparison with other high-resolution [...]
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A p-multigrid discontinuous Galerkin method for the Euler equations on unstructured grids
H. Luo, J. Baum, R. Lohner

Journal of Computational Physics (2006). Vol. 211 (2), pp. 767-783

Abstract
A p-multigrid (p = polynomial degree) discontinuous Galerkin method is presented for the solution of the compressible Euler equations on unstructured grids. The method operates on a sequence of solution approximations of different polynomial orders. A distinct feature of this p-multigrid method is to use different time integration schemes on different approximation levels, resulting in an accurate, fast, and low memory method that can be used to accelerate the convergence of the Euler equations to a steady state for discontinuous Galerkin methods. The developed method is used to compute the compressible flows for a variety of test problems on unstructured grids. The numerical results obtained strongly indicate the order independent property of this p-multigrid method. An overall speed-up factor more than one order of magnitude for both second- and third-order solutions of all test cases in comparison with the explicit method is demonstrated.
Abstract
A p-multigrid (p = polynomial degree) discontinuous Galerkin method is presented for the solution of the compressible Euler [...]
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Electromagnetic scattering calculations using a finite—element solver for the Maxwell equations
R. DeVore, R. Lohner, J. Ambrosiano

(1991). Advances in the Free-Lagrange Method Including Contributions on Adaptive Gridding and the Smooth Particle Hydrodynamics Method. Lecture Notes in Physics, vol 395. Springer, Berlin, Heidelberg

Abstract
We describe a pair of finite-element codes which use unstructured meshes to solve the time-dependent Maxwell equations, with particular emphasis on their application to electromagnetic scattering problems. A two-step, flux-corrected transport scheme is used to effect the time integration, while the spatial structure of the field is determined by a Galerkin procedure. The basis functions are piecewise-linear on three-noded triangles in two dimensions and four-noded tetrahedra in three. For the periodic scattering problems with which we are presently concerned, adaptive remeshing is a convenient and powerful method for improving the quality of the solutions. Results for the analytically tractable case of scattering by a perfectly conducting circular cylinder are used to illustrate the performance of the codes.
Abstract
We describe a pair of finite-element codes which use unstructured meshes to solve the time-dependent Maxwell equations, with particular emphasis on their application to electromagnetic [...]
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Electromagnetic particle codes on unstructured grids
J. Ambrosiano, S. Brandon, R. Lohner

(1991). Advances in the Free-Lagrange Method Including Contributions on Adaptive Gridding and the Smooth Particle Hydrodynamics Method. Lecture Notes in Physics, vol 395. Springer, Berlin, Heidelberg

Abstract
The most widely used computational model of collisionless plasmas is the Lagrangian-Eulerian hybrid technique known as particle-in-cell or PIC. In the electromagnetic version, Maxwell's equations are solved on an Eulerian grid and electromagnetic forces are interpolated form the grid to particle locations. Particles are then moved in Lagrangian fashion while their currents are interpolated back onto the grid to provide sources for the fields on the next cycle. There are many applications where one needs to model plasmas and electromagnetic waves inside regions of complicated shape. Traditional methods for solving Maxwell's equations employ finite differences on regular grids to replace differential operators. These methods are awkward for complicated boundary shapes, often replacing smoothly curved or slanted boundaries with stairsteps. The desire to incorporate realistic boundaries into plasma simulations is motivated by a host of situations in which proper representation of the boundary shape is expected to be critical. Our approach to solving this problem is to design electromagnetic particle codes based on the use of unstructured grids. The arbitrary connectivity of unstructured grids provides the flexibility to place nodes wherever needed to fit the most complex boundary shapes. The most significant problems that must be addressed as a result of this strategy are: grid generation, field solution, and particle tracking. Our solutions to these problems, along with a few preliminary results, are presented in this paper.
Abstract
The most widely used computational model of collisionless plasmas is the Lagrangian-Eulerian hybrid technique known as particle-in-cell or PIC. In the electromagnetic version, [...]
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Adaptive remeshing for transient problems with moving bodies
R. Lohner

(1989). 11th International Conference on Numerical Methods in Fluid Dynamics. Lecture Notes in Physics, vol 323. Springer, Berlin, Heidelberg

Abstract
The accurate simulation of time-dependent compressible flows with moving bodies has been an outstanding goal for a long time. Several major capabilities are required to simulate efficiently this class of problems: a) Flow solvers that can handle moving frames of reference, b) high-order, monotonicity preserving schemes, c) a consistent integration scheme for the rigid body motion, and d) fast regridding capabilities. It is also convenient to incorporate an adaptive refinement capability for the flow domain in order to reduce the overall CPU-cost of the algorithm.
Abstract
The accurate simulation of time-dependent compressible flows with moving bodies has been an outstanding goal for a long time. Several major capabilities are required to simulate [...]
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An unstructured-grid based volume-of-fluid method for extreme wave and freely-floating structure interactions
C. Yang, R. Lohner, H. Lu

Journal of Hydrodynamics (2006). Vol. 18 (1), pp. 405-412

Abstract
A Volume of Fluid (VOF) technique has been further developed and coupled with an incompressible Euler/Navier Stokes solver operating on adaptive, unstructured grids to simulate the interactions of extreme waves and a LNG carrier with full or partially filled tanks. The present implementation follows the classic VOF implementation for the liquid-gas system, considering only the liquid phase. Extrapolation algorithms are used to obtain velocities and pressures in the gas region near the free surface. An arbitrary Lagrangian-Eulerian (ALE) frame of reference is used. The mesh is moved in such a way as to minimize the distortion of the mesh due to body movement. The incompressible Euler/Navier-Stokes equations are solved using projection schemes and a finite element method on unstructured grids, and the free surface is captured by the VOF method. The computer code developed based on the method described above is used in this study to simulate a numerical seakeeping tank, where the waves are generated by the sinusoidal excitation of a piston paddle, and a freely-floating LNG carrier with full or partially filled tanks moves in response to the waves. Both head sea and oblique sea are considered in the simulation. Highly nonlinear wave-body interactions, such as green water on deck and sloshing, have been modeled successfully.
Abstract
A Volume of Fluid (VOF) technique has been further developed and coupled with an incompressible Euler/Navier Stokes solver operating on adaptive, unstructured grids to simulate [...]
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Improving the speed and accuracy of projection-type incompressible flow solvers
R. Lohner, C. Yang, J. Cebral, F. Camelli, O. Soto, J. Waltz

Comput. Methods Appl. Mech. Engrg., (2006). 195 (23-24), pp. 3087-3109

Abstract
Superseding so-called first-generation incompressible flow solvers of the projection type (based on Taylor–Galerkin advection, second-order pressure damping and element-based data structures), the current, second-generation solvers (based on high-order upwind advection, fourth-order pressure damping and edge-based data structures) have now been in use for half a decade and have proven remarkably robust and efficient for many large-scale problems. In order to achieve higher accuracy and speed, these solvers have recently been enhanced in a variety of ways: (a) substepping for advection, (b) implicit treatment of advective terms via SGS and GMRES-LU-SGS iterative solvers, (c) fully implicit, time-accurate advancement of pressure and velocities, and (d) linelet preconditioning for the pressure-Poisson equation. The combined effect of these third-generation improvements leads to speedups of the order of O(1:5−1:10), with similar or even better temporal accuracy, as demonstrated on a variety of academic and industrial problems.
Abstract
Superseding so-called first-generation incompressible flow solvers of the projection type (based on Taylor–Galerkin advection, second-order pressure damping and element-based [...]
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Applications of the method of flux-corrected transport to generalized meshes
R. Lohner, G. Patnaik

(1986). Tenth International Conference on Numerical Methods in Fluid Dynamics. Lecture Notes in Physics, vol 264. Springer, Berlin, Heidelberg

Abstract
A new technique for numerical solution of fluid equations has been developed by combining the Finite-Element Methods with the method of Flux-Corrected Transport. The resulting hybrid method, called FEM-FCT, is useful for problems involving steady and unsteady transonic and supersonic flow irregular geometries. The main computational advance grows out of the need to find a prescription for limiting fluxes through the sides of a triangular or other nonquadrilateral zone when arbitrarily many zones may meet at a given vertex.
Abstract
A new technique for numerical solution of fluid equations has been developed by combining the Finite-Element Methods with the method of Flux-Corrected Transport. The resulting [...]
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A hybrid Cartesian grid and gridless method for compressible flows
H. Luo, J. Baum, R. Lohner

Journal of Computational Physics (2006). Vol. 214 (2), pp. 618-632

Abstract
A hybrid Cartesian grid and gridless method is presented to compute unsteady compressible flows for complex geometries. In this method, a Cartesian grid is used as baseline mesh to cover the computational domain, while the boundary surfaces are addressed using a gridless method. This hybrid method combines the efficiency of a Cartesian grid method and the flexibility of a gridless method for the complex geometries. The developed method is used to compute a number of test cases to validate the accuracy and efficiency of the method. The numerical results obtained indicate that the use of this hybrid method leads to a significant improvement in performance over its unstructured grid counterpart for the time-accurate solution of the compressible Euler equations. An overall speed-up factor of about eight and a saving in storage requirements about one order of magnitude for a typical three-dimensional problem in comparison with the unstructured grid method are demonstrated.
Abstract
A hybrid Cartesian grid and gridless method is presented to compute unsteady compressible flows for complex geometries. In this method, a Cartesian grid is used [...]
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A hybrid building‐block and gridless method for compressible flows
H. Luo, J. Baum, R. Lohner

Int. J. Numer. Meth. Fluids (2006). Vol. 59 (4), pp. 459-474

Abstract
A hybrid building‐block Cartesian grid and gridless method is presented to compute unsteady compressible flows for complex geometries. In this method, a Cartesian mesh based on a building‐block grid is used as a baseline mesh to cover the computational domain, while the boundary surfaces are represented using a set of gridless points. This hybrid method combines the efficiency of a Cartesian grid method and the flexibility of a gridless method for the complex geometries. The developed method is used to compute a number of test cases to validate the accuracy and efficiency of the method. The numerical results obtained indicate that the use of this hybrid method leads to a significant improvement in performance over its unstructured grid counterpart for the time‐accurate solution of the compressible Euler equations. An overall speed‐up factor from six to more than one order of magnitude and a saving in storage requirements up to one order of magnitude for all test cases in comparison with the unstructured grid method are demonstrated.
Abstract
A hybrid building‐block Cartesian grid and gridless method is presented to compute unsteady compressible flows for complex geometries. In this method, a Cartesian mesh based [...]
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A coupled thermo‑mechanical model for the simulation of discrete particle systems
O. Quintana-Ruiz, E. Campello

Osvaldo D. Quintana-Ruiz's personal collection (2020). 1

Abstract
This work presents a computational model for the simulation of problems involving thermo-mechanically active particles forming discrete particle systems. Our approach is based on the discrete element method for description of the particles’ dynamics, combined with simple heat transfer equations to describe the various thermal effects that may take place when the system is excited by temperature gradients and external heat sources. We are able to track the motion of the particles and their thermal states over time under the influence of body (e.g., gravitational) forces, contact and friction forces (and the related moments w.r.t. the particles’ centers), as well as applied heat from external devices, heat transfer through conduction (at the particles’ interfaces upon contact with other particles and objects), convective cooling and radiative effects. Numerical examples are provided to validate our scheme and illustrate its applicability to the simulation of a wide range of engineering applications. We believe that simple, consistent particle models of the type as shown here may be a useful tool to the modeling of discrete particle systems that are consisted of thermo-mechanically active particles and, in a broader sense, many other multiphysical discrete systems.
Abstract
This work presents a computational model for the simulation of problems involving thermo-mechanically active particles forming discrete particle systems. Our approach is based [...]
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An unstructured-grid based volume-of-fluid method for extreme wave and freely-floating structure interactions
C. Yang, R. Lohner, H. Lu

Journal of Hydrodynamics (2006). Vol. 18 (3), pp. 415-422

Abstract
A Volume of Fluid (VOF) technique has been further developed and coupled with an incompressible Euler/Navier Stokes solver operating on adaptive, unstructured grids to simulate the interactions of extreme waves and a LNG carrier with full or partially filled tanks. The present implementation follows the classic VOF implementation for the liquid-gas system, considering only the liquid phase. Extrapolation algorithms are used to obtain velocities and pressures in the gas region near the free surface. An arbitrary Lagrangian-Eulerian (ALE) frame of reference is used. The mesh is moved in such a way as to minimize the distortion of the mesh due to body movement. The incompressible Euler/Navier-Stokes equations are solved using projection schemes and a finite element method on unstructured grids, and the free surface is captured by the VOF method. The computer code developed based on the method described above is used in this study to simulate a numerical seakeeping tank, where the waves are generated by the sinusoidal excitation of a piston paddle, and a freely-floating LNG carrier with full or partially filled tanks moves in response to the waves. Both head sea and oblique sea are considered in the simulation. Highly nonlinear wave-body interactions, such as green water on deck and sloshing, have been modeled successfully.
Abstract
A Volume of Fluid (VOF) technique has been further developed and coupled with an incompressible Euler/Navier Stokes solver operating on adaptive, unstructured grids to simulate [...]
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Image-based computational hemodynamics methods and their application for the analysis of blood flow past endovascular devices
J. Cebral, R. Lohner, S. Appanaboyina, C. Putman

(2007). Biomechanical systems technology, pp. 29-85

Abstract
Knowledge of the hemodynamic conditions in intracranial aneurysms before and after endovascular treatment is important to better understand the mechanisms responsible for aneurysm growth and rupture, and to optimize and personalize the therapies. Unfortunately, there are no reliable imaging techniques for in vivo quantification of blood flow patterns in cerebral aneurysms. Patient-specific, image-based computational models provide an attractive alternative since they can handle any vascular geometry and physiologic flow condition. However, computational modeling of the hemodynamics in cerebral aneurysms after their endovascular treatment is a challenging problem because of the high degree of geometric complexity required to represent and mesh the vascular anatomy and the endovascular devices simultaneously. This paper describes an image-based methodology for constructing patient-specific vascular computational fluid dynamics models and an adaptive grid embedding technique to simulate blood flows around endovascular devices. The methodology is illustrated with several examples ranging from idealized vascular models to patient-specific models of cerebral aneurysms after deployment of stents and coils. These techniques have the potential to be used to select the best therapeutic option for a particular individual and to optimize the design of endovascular devices on a patient-specific basis.
Abstract
Knowledge of the hemodynamic conditions in intracranial aneurysms before and after endovascular treatment is important to better understand the mechanisms responsible for [...]
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A Hermite WENO-based limiter for discontinuous Galerkin method on unstructured grids
H. Luo, J. Baum, R. Lohner

Journal of Computational Physics (2007). Vol. 225 (1), pp. 686-713

Abstract
A weighted essentially non-oscillatory reconstruction scheme based on Hermite polynomials is developed and applied as a limiter for the discontinuous Galerkin finite element method on unstructured grids. The solution polynomials are reconstructed using a WENO scheme by taking advantage of handily available and yet valuable information, namely the derivatives, in the context of the discontinuous Galerkin method. The stencils used in the reconstruction involve only the van Neumann neighborhood and are compact and consistent with the DG method. The developed HWENO limiter is implemented and used in a discontinuous Galerkin method to compute a variety of both steady-state and time-accurate compressible flow problems on unstructured grids. Numerical experiments for a wide range of flow conditions in both 2D and 3D configurations are presented to demonstrate the accuracy, effectiveness, and robustness of the designed HWENO limiter for the DG methods.
Abstract
A weighted essentially non-oscillatory reconstruction scheme based on Hermite polynomials is developed and applied as a limiter for the discontinuous Galerkin finite [...]
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Patient-specific modeling of intracranial aneurysmal stenting
S. Appanaboyina, F. Mut, R. Lohner, C. Putman, J. Cebral

(2007). Proceedings Volume 6511, Medical Imaging 2007: Physiology, Function, and Structure from Medical

Abstract
Simulating blood flow around stents in intracranial aneurysms is important for designing better stents and to personalize and optimize endovascular stenting procedures in the treatment of these aneurysms. However, the main difficulty lies in the generation of acceptable computational grids inside the blood vessels and around the stents. In this paper, a hybrid method that combines body-fitted grid for the vessel walls and adaptive embedded grids for the stent is presented. Also an algorithm to map a particular stent to the parent vessel is described. These approaches tremendously simplify the simulation of blood flow past these devices. The methodology is evaluated with an idealized stented aneurysm under steady flow conditions and demonstrated in various patient-specific cases under physiologic pulsatile flow conditions. These examples show that the methodology can be used with ease in modeling any patient-specific anatomy and using different stent designs. This paves the way for using these techniques during the planning phase of endovascular stenting interventions, particularly for aneurysms that are difficult to treat with coils or by surgical clipping.
Abstract
Simulating blood flow around stents in intracranial aneurysms is important for designing better stents and to personalize and optimize endovascular stenting procedures in [...]
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A fast, matrix-free implicit method for compressible flows on unstructured grids
H. Luo, J. Baum, R. Lohner

(2007). Sixteenth International Conference on Numerical Methods in Fluid Dynamics. Lecture Notes in Physics, vol 515, pp. 73-78, Springer, Berlin, Heidelberg

Abstract
A fast, matrix-free implicit method has been developed to solve the three dimensional compressible flow problems on unstructured meshes. An approximate system of linear equations arising from the Newton linearization is solved by the GMRES (Generalized Minimum RESidual) algorithm with a LU-SGS (Lower-Upper Symmetric Gauss-Seidel) preconditioner. A remarkable feature of the present GMRES+LU-SGS method is that the storage of the Jacobian matrix can be completely eliminated by approximating the Jacobian with numerical fluxes, resulting in a matrix-free implicit method. The developed method has been used to compute compressible flows around 3D complex aerodynamic configurations for a wide range of flow conditions, from subsonic to supersonic. The numerical results indicate that the use of the GMRES+LU-SGS method leads to a significant increase in performance over the best current implicit methods, GMRES+ILU and LU-SGS, while maintaining memory requirements competitive to its explicit counterpart.
Abstract
A fast, matrix-free implicit method has been developed to solve the three dimensional compressible flow problems on unstructured meshes. An approximate system of linear equations [...]
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A Godunov-Type Scheme for Atmospheric Flows on Unstructured Grids: Euler and Navier-Stokes Equations
N. Ahmad, Z. Boybeyi, R. Lohner, A. Sarma

Pure and Applied Geophysics (2007). Vol. 164 (1), pp. 217-244

Abstract
In recent years there has been a growing interest in using Godunov-type methods for atmospheric flow problems. Godunov's unique approach to numerical modeling of fluid flow is characterized by introducing physical reasoning in the development of the numerical scheme (VAN LEER, 1999). The construction of the scheme itself is based upon the physical phenomenon described by the equation sets. These finite volume discretizations are conservative and have the ability to resolve regions of steep gradients accurately, thus avoiding dispersion errors in the solution. Positivity of scalars (an important factor when considering the transport of microphysical quantities) is also guaranteed by applying the total variation diminishing condition appropriately. This paper describes the implementation of a Godunov-type finite volume scheme based on unstructured adaptive grids for simulating flows on the meso-, micro- and urban-scales. The Harten-Lax-van Leer-Contact (HLLC) approximate Riemann solver used to calculate the Godunov fluxes is described in detail. The higher-order spatial accuracy is achieved via gradient reconstruction techniques after van Leer and the total variation diminishing condition is enforced with the aid of slope-limiters. A multi-stage explicit Runge-Kutta time marching scheme is used for maintaining higher-order accuracy in time. The scheme is conservative and exhibits minimal numerical dispersion and diffusion. The subgrid scale diffusion in the model is parameterized via the Smagorinsky-Lilly turbulence closure. The scheme uses a non-staggered mesh arrangement of variables (all quantities are cell-centered) and requires no explicit filtering for stability. A comparison with exact solutions shows that the scheme can resolve the different types of wave structures admitted by the atmospheric flow equation set. A qualitative evaluation for an idealized test case of convection in a neutral atmosphere is also presented. The scheme was able to simulate the onset of Kelvin-Helmholtz type instability and shows promise in simulating atmospheric flows characterized by sharp gradients without using explicit filtering for numerical stability.
Abstract
In recent years there has been a growing interest in using Godunov-type methods for atmospheric flow problems. Godunov's unique approach to numerical modeling of fluid [...]
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Extending the Range and Applicability of the Loose Coupling Approach for FSI Simulations
R. Lohner, J. Cebral, C. Yang, J. Baum, E. Mestreau, O. Soto

(2006). Bungartz HJ., Schäfer M. (eds) Fluid-Structure Interaction. Lecture Notes in Computational Science and Engineering, vol 53. Springer, Berlin, Heidelberg

Abstract
Several algorithms for fluid-structure interaction are described. All of them are useful for the loose coupling of fluid and structural dynamics codes. The first class of algorithms considers the loose coupling of implicit time-marching codes. Of these, a predictor-corrector algorithm that may be interpreted as a Jacobi iteration with block-diagonal terms was found to be a good compromise of simplicity, generality and speed. The second class of algorithms treats the displacement of the surface of the structure that is in contact with the fluid. It is shown that a straightforward treatment of the displacements for arbitrary choice of timesteps can lead to instabilities. For optimal stability, at each timestep the ending time of the fluid should be just beyond the ending time of the structure. The third class of algorithms treats the movement of the flow mesh in an ALE setting. The use of a projective prediction of mesh velocities, as well as linelet preconditioning for the resulting PCG system can reduce significantly the effort required. Examples are included that show the effectiveness of the proposed procedures.
Abstract
Several algorithms for fluid-structure interaction are described. All of them are useful for the loose coupling of fluid and structural dynamics codes. The first class of [...]
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Generation of viscous grids at ridges and corners
R. Aubry, R. Lohner

International Journal for Numerical Methods in Engineering (2007). Vol. 77 (9), pp. 1247-1289

Abstract
An extension of Löhner (AIAA‐93‐3348‐CP , 1993) for the generation of high aspect ratio volume grids on surfaces with ridges and corners is presented for Reynolds‐averaged Navier–Stokes computations. Multiple point normals are introduced along ridges and corners. The original technique generates a semi‐structured boundary layer of prismatic elements growing along point normals. Therefore, extra degenerated faces must be introduced to take into account the multiple growth curves at ridges and corners and produce a valid topological surface triangulation. The major task of the algorithm consists in recovering conformity in the surface mesh triangulation, which has been lost due to the introduction of the virtual faces. The procedure relies on a topological taxonomy of an arbitrary combination of concave and convex ridges. Each case is highlighted in detail. Special boundary conditions such as symmetry planes and periodic boundary conditions are also handled. Several complex geometries have been chosen to illustrate the proposed procedure, and timings are given, showing that the new module does not place any extra burden on the previous semi‐structured approach.
Abstract
An extension of Löhner (AIAA‐93‐3348‐CP , 1993) for the generation of high aspect ratio volume grids on surfaces with ridges and corners is presented for Reynolds‐averaged [...]
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Adaptive Embedded/Immersed Unstructured Grid Techniques
R. Lohner, J. Cebral, F. Camelli, S. Appanaboyina, J. Baum, E. Mestreau, O. Soto

Computer Methods in Applied Mechanics and Engineering (2008). Vol. 197 (25-28), pp. 2173-2197

Abstract
Embedded mesh, immersed body or fictitious domain techniques have been used for many years as a way to discretize geometrically complex domains with structured grids. The use of such techniques within adaptive, unstructured grid solvers is relatively recent. The combination of body-fitted functionality for some portion of the domain, together with embedded mesh or immersed body functionality for another portion of the domain offers great advantages, which are increasingly being exploited. The present paper reviews the methodologies pursued so far, addresses implementational issues and shows the possibilities such techniques offer.
Abstract
Embedded mesh, immersed body or fictitious domain techniques have been used for many years as a way to discretize geometrically complex domains with structured grids. The [...]
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Flowfield and Acoustic radiation from imperfectly expanded supersonic jets: Computational Studies
J. Liu, R. Ramamurti, K. Kailsanath, R. Lohner

(2007). American Physical Society, 60th Annual Meeting of the Divison of Fluid Dynamics, November 18-20.

Abstract
This project involves the study of sound generated by supersonic jets like those emanating from the exhausts of high-performance military aircraft. This is a joint experimental/computational project with the University of Cincinnati. The flowfield and near-field noise from both convergent and convergent-divergent nozzles have been simulated. The emphasis is on imperfectly expanded or off-design conditions. The impact of grid resolution, initial and boundary conditions on the computed solutions have been assessed. Comparisons with analytical predictions on shock-cell spacing show very good agreement. Comparison to the experimental observations are underway and will be presented at the meeting.
Abstract
This project involves the study of sound generated by supersonic jets like those emanating from the exhausts of high-performance military aircraft. This is a joint experimental/computational [...]
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On the ‘most normal’ normal
R. Aubry, R. Lohner

Communications in numerical methods in engineering (2008). Vol. 24 (12), pp. 1641-1652

Abstract
Given a set of normals in ℛ3, two algorithms are presented to compute the ‘most normal’ normal. The ‘most normal’ normal is the normal that minimizes the maximal angle with the given set of normals. A direct application is provided supposing a surface triangulation is available. The set of normals may represent either the face normals of the faces surrounding a point or the point normals of the points surrounding a point. The first algorithm is iterative and straightforward, and is inspired by the one proposed by Pirzadeh (AIAA Paper 94‐0417 , 1994). The second gives more insight into the complete problem as it provides the unique solution explicitly. It would correspond to the general extension of the algorithm presented by Kallinderis (AIAA‐92‐2721 , 1992).
Abstract
Given a set of normals in ℛ3, two algorithms are presented to compute the ‘most normal’ normal. The ‘most normal’ normal is the normal that minimizes [...]
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Parabolic recovery of boundary gradients
R. Lohner, J. Cebral, S. Appanaboyina

Communications in numerical methods in engineering (2008). Vol. 24 (12), pp. 1611-1615

Abstract
A parabolic recovery procedure suited for shear stress and heat flux recovery on surfaces from linear element data is proposed. The information required consists of the usual unknowns at points, as well as gradients recovered at the points that are one layer away from the wall. The procedure has been in use for some time and has consistently delivered superior results as compared with the usual wall shear stress and heat flux obtained from linear finite element method shape functions.
Abstract
A parabolic recovery procedure suited for shear stress and heat flux recovery on surfaces from linear element data is proposed. The information required consists of the usual [...]
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Cache‐efficient renumbering for vectorization
R. Lohner

International Journal for Numerical Methods in Biomedical Engineering (2008). Vol. 26 (5), pp. 628-636

Abstract
A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring techniques, the new renumbering strategy reduces the spread in point‐data access for edge‐based solvers by more than an order of magnitude. The technique is particularly suited for multicore, cache‐based machines that allow for vectorization or pipelining.
Abstract
A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring [...]
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Combination of Body-Fitted and Embedded Grids for External Vehicle Aerodynamics
R. Tilch, A. Tabbal, M. Zhu, F. Decker, R. Lohner

Engineering Computations (2008). Vol. 25 (1), pp. 28-41

Abstract
Body-fitted and embedded mesh techniques are combined to obtain accurate external aerodynamic solutions for realistic car geometries with minimal user intervention. The key idea is to mesh with typical body-fitted RANS grids the external shape of the vehicle, which is smooth and requires detailed physical modeling. The underhood and undercar- riage are treated as embedded surfaces. The flow in this region is massively separated, requiring LES runs and isotropic grids. This makes it a suitable candidate for embedded grids. Comparisons with body-fitted and experimental data for a typical car show that this approach can yield drag predictions with an error less than 5%. Thus, the present approach reduces turnaround times for complete car geometries to 1-2 days, without compromising accuracy.
Abstract
Body-fitted and embedded mesh techniques are combined to obtain accurate external aerodynamic solutions for realistic car geometries with minimal user intervention. The key [...]
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On the computation of steady‐state compressible flows using a discontinuous Galerkin method
H. Luo, J. Baum, R. Lohner

International Journal for Numerical Methods in Engineering (2008). Vol. 73 (5), pp. 597-623

Abstract
Computation of compressible steady‐state flows using a high‐order discontinuous Galerkin finite element method is presented in this paper. An accurate representation of the boundary normals based on the definition of the geometries is used for imposing solid wall boundary conditions for curved geometries. Particular attention is given to the impact and importance of slope limiters on the solution accuracy for flows with strong discontinuities. A physics‐based shock detector is introduced to effectively make a distinction between a smooth extremum and a shock wave. A recently developed, fast, low‐storage p‐multigrid method is used for solving the governing compressible Euler equations to obtain steady‐state solutions. The method is applied to compute a variety of compressible flow problems on unstructured grids. Numerical experiments for a wide range of flow conditions in both 2D and 3D configurations are presented to demonstrate the accuracy of the developed discontinuous Galerkin method for computing compressible steady‐state flows.
Abstract
Computation of compressible steady‐state flows using a high‐order discontinuous Galerkin finite element method is presented in this paper. An accurate representation of [...]
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Adaptive methodology for meshless finite point method
F. Perazzo, R. Lohner, L. Perez-Pozo

Advances in Engineering Software (2008). Vol. 39 (3), pp. 156-166

Abstract
In this work, a posteriori error estimator and an adaptive refinement process for the meshless finite point method (FPM), which is based on point collocation, are presented. The error indicator is formulated by the least-squares functional evaluation, used in the shape function development. New degrees of freedom or additional points can be incorporated without difficulty, in zones where the error estimator presents a high value, by means of h–p refinement processes. The validity of the proposed error estimator can be demonstrated by developments of numerical problems in mechanics of solids, using an adaptive refinement process of the solution.
Abstract
In this work, a posteriori error estimator and an adaptive refinement process for the meshless finite point method (FPM), which is based on point collocation, are presented. [...]
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A Fast p-Multigrid Discontinuous Galerkin Method for Compressible Flows at All Speeds
H. Luo, J. Baum, R. Lohner

AIAA Journal (2008). Vol. 46 (3), pp. 635-652

Abstract
A p-multigrid (p=polynomial degree) discontinuous Galerkin method is presented for the solution of the compressible Euler equations on unstructured grids. The method operates on a sequence of solution approximations of different polynomial orders. A distinct feature of this p-multigrid method is the application of an explicit smoother on the higher level approximations (p > 0) and an implicit smoother on the lowest level approximation (p = 0), resulting in a fast as well as low storage method that can be efficiently used to accelerate the convergence to a steady state solution. Furthermore, this p-multigrid method can be naturally applied to compute the flows with discontinuities, where a monotonic limiting procedure is usually required for discontinuous Galerkin methods. An accurate representation of the boundary normals based on the definition of the geometries is used for imposing slip boundary conditions for curved geometries. A variety of compressible flow problems for a wide range of flow conditions, from low Mach number to supersonic, in both 2D and 3D configurations are computed to demonstrate the performance of this p-multigrid method. The numerical results obtained strongly indicate the order independent property of this p-multigrid method and demonstrate that this method is orders of magnitude faster than its explicit counterpart. The performance comparison with a finite volume method shows that using this p-multigrid method, the discontinuous Galerkin method provides a viable, attractive, competitive and probably even superior alternative to the finite volume method for computing compressible flows at all speeds.
Abstract
A p-multigrid (p=polynomial degree) discontinuous Galerkin method is presented for the solution of the compressible Euler equations on unstructured grids. The method operates [...]
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The backwater effect as a tool to assess formative long-term flood regimes
F. Castelltort, E. Bladé, J. Balasch, M. Ribe

Quaternary international (2020). Vol. 583, pp. 29-43

Abstract
The Ter River drains the south-eastern Pyrenees and flows into the Mediterranean Sea. A lithological constriction affects the normal water flow in the La Plana de Vic area. As a consequence of this disturbance in the flow, the shrinkage in the bedrock activates a backwater effect. Systematic water retention during extreme events has formative consequences. The process involves the creation of a helical flow for the structural misalignment of the channel at the point of narrowness. The backwater effect transmits the secondary currents backwards, resulting in the creation of a sinuous pattern upstream from the shrinkage. A tributary, the Gurri River, flows into the main river just before the constriction, and this too has been affected by the process of water storage and channel pattern change. A two-dimensional numerical flow model (Iber) has modelled several hypothetical cases of flooding. This modelling aims to test the reach of the hydraulic influence upstream from the constriction, both in the main river and its tributary, due to the backwater effect. Moreover, it sought to find the best balance of discharge between the two streams. The upstream reach of the backwater effect was considered as its endpoint. During the flooding, the system reached hydraulic equilibrium between the constriction and the two endpoints when both were at the same water level, and the flow regime was subcritical everywhere. The hydraulic conditions that drove the water flow to the equilibrium are thought to be the ones that promoted formative processes in a sinuous pattern on a long-term basis. The water discharge values obtained from this procedure are, in general terms, 50% above those considered to be a peak flood with a recurrence time of 500 years (Q500), and they accomplish the conditions of extreme events. Thus classified, the calculated discharges can be helpful for comparison with those measured in historical and systematic records, because a water discharge like the one calculated has never been measured at the Ter River.
Abstract
The Ter River drains the south-eastern Pyrenees and flows into the Mediterranean Sea. A lithological constriction affects the normal water flow in the La Plana de Vic area. [...]
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Evaluation of two hydrometeorological ensemble strategies for flash-flood forecasting over a catchment of the eastern Pyrenees
H. Roux, A. Amengual, R. Romero, E. Bladé, M. Sanz-Ramos

Natural hazards and earth system sciences (2020). Vol. 20 (2), pp. 425-450

Abstract
This study aims at evaluating the performances of flash flood forecasts issued from deterministic and ensemble meteorological prognostic systems. The hydro-meteorological modeling chain includes the Weather Research and Forecasting model (WRF) forcing the rainfall-runoff model MARINE dedicated to flash flood. Two distinct ensemble prediction systems accounting for (i) perturbed initial and lateral boundary conditions of the meteorological state and (ii) mesoscale model physical parameterizations, have been implemented on the Agly catchment of the Eastern Pyrenees with three sub-catchments exhibiting different rainfall regimes. Different evaluations of the performance of the hydrometeorological strategies have been performed: (i) verification of short-range ensemble prediction systems and corresponding stream flow forecasts, for a better understanding of how forecasts behave, (ii) usual measures derived from a contingency table approach, to test an alert threshold exceedance, and (iii) overall evaluation of the hydro-meteorological chain using the Continuous Rank Probability Score, for a general quantification of the ensemble performances. Results show that the overall discharge forecast is improved by both ensemble strategies with respect to the deterministic forecast. Threshold exceedance detections for flood warning also benefit from large hydro-meteorological ensemble spread. There are no substantial differences between both ensemble strategies on these test cases in terms both of the issuance of flood warnings and the overall performances, suggesting that both sources of external-scale uncertainty are important to take into account
Abstract
This study aims at evaluating the performances of flash flood forecasts issued from deterministic and ensemble meteorological prognostic systems. The hydro-meteorological [...]
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Las ecuaciones de Saint Venant para la modelización de avalanchas de nieve densa
M. Sanz-Ramos, E. Bladé, A. Torralba, P. Oller

Ingeniería del agua (2020). Vol. 24 (1), pp. 65-79

Abstract
La creciente preocupación por los riesgos naturales, como las avalanchas de nieve, ha propiciado el desarrollo de modelos numéricos ad hoc como una herramienta de soporte para su análisis y evaluación. Los modelos existentes para simulación de aludes se basan en la conservación de la masa y de la cantidad de movimiento, que son unas ecuaciones similares a las ecuaciones de Saint Venant para agua con diferencias sólo en los términos de fricción (modelo reológico). Este documento muestra las posibilidades de estas ecuaciones para simular avalanchas de placa-densa y el tratamiento numérico realizado en Iber. Se ha empleado una nueva metodología para equilibrar el término fuente y el vector de flujo evitando así oscilaciones espurias y movimientos no reales, y que modifica la pendiente de fondo en base a los parámetros del fluido y así detener su movimiento. La herramienta se ha probado en dos casos de estudio para analizar el comportamiento del fluido en función de los parámetros del modelo reológico.
Abstract
La creciente preocupación por los riesgos naturales, como las avalanchas de nieve, ha propiciado el desarrollo de modelos numéricos ad hoc como una herramienta [...]
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Hydrodynamic determination of the kinematic viscosity of waste brines
M. Sánchez-Juny, A. Triadú, A. Andreu, E. Bladé

ACS Omega (2019). Vol. 4 (25), pp. 20987-20999

Abstract
Wastewater from a potash mine in the central region of Catalonia is transported by means of a collector that runs more than 100 km, spilling into the sea on the Catalan central coast. To analyze the hydraulics of this infrastructure, the values of the basic parameters that condition the flow, such as the absolute roughness of poly(vinyl chloride) (PVC) pipes and the viscosity of the transported brine mixtures, must be characterized. There exists uncertainty about the value of absolute roughness of a PVC pipe as described in the literature; nevertheless, if the pipe is smooth, the influence of the absolute roughness in the hydraulic determination of viscosity will not be significant. In this work, an experimental procedure based on a hydraulic analysis was applied to estimate the kinematic viscosity of a brine mixture, depending on its temperature and concentrations of salts and fines. The results obtained were compared with the results from experiments using an Ostwald viscometer.
Abstract
Wastewater from a potash mine in the central region of Catalonia is transported by means of a collector that runs more than 100 km, spilling into the sea on the Catalan central [...]
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Characterization of wood-laden flows in rivers
V. Ruiz-Villanueva, E. Bladé, D. Ravazzolo, E. Wohl

Earth surface processes and landforms (2019). Vol. 44 (9), pp. 1694-1709

Abstract
Inorganic sediment is not the only solid-fraction component of river flows; flows may also carry significant amounts of large organic material (i.e., large wood), but the characteristics of these wood-laden flows (WLF) are not well understood yet. With the aim to shed light on these relatively unexamined phenomena, we collected home videos showing natural flows with wood as the main solid component. Analyses of these videos as well as the watersheds and streams where the videos were recorded allowed us to define for the first time WLF, describe the main characteristics of these flows and broaden the definition of wood transport regimes (adding a new regime called here hypercongested wood transport). According to our results, WLF may occur repeatedly, in a large range of catchment sizes, generally in steep, highly confined single thread channels in mountain areas. WLF are typically highly unsteady and the log motion is non-uniform, as described for other inorganic sediment-laden flows (e.g., debris flows). The conceptual integration of wood into our understanding of flow phenomena is illustrated by a novel classification defining the transition from clear water to hypercongested, wood and sediment-laden flows, according to the composition of the mixture (sediment, wood, and water). We define the relevant metrics for the quantification and modelling of WLF, including an exhaustive discussion of different modelling approaches (i.e., Voellmy, Bingham and Manning) and provide a first attempt to simulate WLF. We draw attention to WLF phenomena to encourage further field, theoretical, and experimental investigations that may contribute to a better understanding of flows river basins, leading to more accurate predictions, and better hazard mitigation and management strategies.
Abstract
Inorganic sediment is not the only solid-fraction component of river flows; flows may also carry significant amounts of large organic material (i.e., large wood), but the [...]
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IberHABITAT: evaluación de la Idoneidad del Hábitat Físico y del Hábitat Potencial Útil para peces. Aplicación en el río Eume
M. Sanz-Ramos, E. Bladé, A. Palau, D. Vericat, A. Ramos-Fuentes

Revista Iberoaméricana del Agua (2019). Vol. 6 (2), pp. 158-167

Abstract
La caracterización y cuantificación del Hábitat Potencial Útil (HPU) para peces fluviales a partir de modelos de simulación hidráulica se ha basado tradicionalmente en la utilización de modelos hidráulicos unidimensionales, los cuales han sido técnicamente superados, aunque no con igual robustez ni expansión de uso, por modelos bidimensionales (2D). Por ello, se ha desarrollado un nuevo módulo en la herramienta de simulación hidráulica 2D Iber para evaluar la Idoneidad del Hábitat Físico (IHF), que a su vez es una variable básica para la estimación del HPU. El modelo se aplicó a un caso de estudio consistente en la evaluación del hábitat disponible para la trucha común (Salmo trutta), para sus tres estadios de desarrollo, en función de dos variables hidráulicas (calado y velocidad). El desarrollo del modelo ha permitido evaluar de manera directa la IHF y el HPU, así como obtener relaciones Caudal-HPU.
Abstract
La caracterización y cuantificación del Hábitat Potencial Útil (HPU) para peces fluviales a partir de modelos de simulación hidráulica [...]
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Monitoring and modelling instream wood in rivers
V. Ruiz-Villanueva, L. Burki, N. Galatioto, E. Bladé, C. Gamberini, W. Bertoldi, M. Stoffel

Ingenieurbiologie (2019). Vol. 19 (1), pp. 14-20

Abstract
During the past three years the WoodFlow project (2015-2019), funded by the Federal Office for the Environment) aimed to develop the knowledge and methods to analyse instream large wood dynamics and to mitigate potential wood-related hazards in Swiss rivers. This paper presents part of the relevant results related to the analysis of the spatial and temporal variability of large wood transport, which was done by remote sensed monitoring and numerical modelling.
Abstract
During the past three years the WoodFlow project (2015-2019), funded by the Federal Office for the Environment) aimed to develop the knowledge and methods to analyse instream [...]
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Evaluación numérico-experimental del comportamiento histérico del coeficiente de rugosidad de los macrófitos
M. Sanz-Ramos, E. Bladé, D. Niñerola, A. Palau-Ibars

Ingeniería del agua (2018). Vol. 22 (3), pp. 109-124

Abstract
La problemática asociada al crecimiento masivo de macrófitos en el Bajo Ebro ha llevado a autoridades y gestores a examinar la posibilidad de paliar los efectos negativos que producen sobre el sistema hídrico, y los usos del agua, mediante la realización de avenidas controladas periódicas que provoquen su remoción. El presente trabajo tiene como objetivo principal evaluar el comportamiento hidráulico de los macrófitos mediante simulación numérica (modelo Iber) y su comparación con datos de campo, con el fin de explorar las mejores opciones posibles para diseñar avenidas controladas más eficaces. Para ello se han empleado diferentes relaciones entre el coeficiente de rugosidad y la altura de agua bajo tres enfoques distintos (constante, variable y variable con histéresis) a fin de calibrar el modelo numérico con los datos de campo. Se ha podido observar que el mejor ajuste se produce cuando dichas curvas son de tipo variable con histéresis (diferente rama de subida que de bajada).
Abstract
La problemática asociada al crecimiento masivo de macrófitos en el Bajo Ebro ha llevado a autoridades y gestores a examinar la posibilidad de paliar los efectos [...]
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Análisis numérico 3D de una rotura de presa utilizando el método VOF y el modelo de turbulencia LES
E. Sánchez-Cordero, J. Boix, M. Gómez, E. Bladé

Ingeniería del agua (2018). Vol. 22 (3), pp. 167-176

Abstract
El presente trabajo muestra un análisis numérico 3D del comportamiento del flujo de agua en una rotura de presa a escala de laboratorio. La simulación se realizó utilizando el software de dinámica de fluidos computacional (CFD) basado en el método de volúmenes finitos (FVM) – OpenFOAM. En el modelo numérico la turbulencia es tratada con la metodología LES (Large Eddy Simulation) y el método VOF (Volume of Fluid) es usado para la captura de la superficie libre del agua. Los resultados numéricos obtenidos se comparan con datos experimentales publicados haciendo uso de las variables de calado y presión. Los resultados muestran que la configuración del código numérico 3D es capaz de reproducir satisfactoriamente la variación temporal de las variables en estudio, con tendencias correctas y una alta correlación con los valores experimentales.
Abstract
El presente trabajo muestra un análisis numérico 3D del comportamiento del flujo de agua en una rotura de presa a escala de laboratorio. La simulación [...]
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Modelación numérica de flujo mixto en conductos cerrados con esquemas en volúmenes finitos
J. Aragón, E. Bladé

Tecnología y ciencias del agua (2017). Vol. VIII (3), pp. 127-142 (Preprint)

Abstract
En este trabajo se presenta un modelo numérico para la simulación de flujo mixto (flujo en lámina libre y flujo en presión) en conductos cerrados a través de las ecuaciones de Saint Venant en una dimensión para flujo en lámina libre y con el método de la ranura de Preissmann para considerar el flujo en presión. Para la resolución de las ecuaciones se emplea el método de los volúmenes finitos con un esquema de alta resolución. El esquema utilizado es el método de Godunov con el Riemann solver de Roe de primer orden de precisión, más unas correcciones de segundo orden para obtener el esquema de alta resolución WAF-TVD. El flujo mixto es un fenómeno bastante común en colectores pluviales, túneles, tuberías de obras de toma de instalaciones hidroeléctricas, llenado/vaciado de tuberías, colectores de almacenamiento, etcétera. La entrada en carga en los conductos se puede generar desde el extremo aguas abajo, desde el extremo aguas arriba y por ambos extremos simultáneamente, siendo la primera la más común y, por lo tanto, la evaluada en este trabajo. Para demostrar la actuación del modelo, éste se aplica a un caso de referencia y a dos ensayos de laboratorio existentes en la literatura técnica. Los resultados obtenidos muestran que el modelo numérico es capaz de reproducir los experimentos con buena precisión. Con ello se demuestra que el modelo es idóneo para simular flujo en lámina libre en régimen lento, rápido y transcrítico (de lento a rápido y de rápido a lento), y flujo mixto.
Abstract
En este trabajo se presenta un modelo numérico para la simulación de flujo mixto (flujo en lámina libre y flujo en presión) en conductos cerrados [...]
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Improvements in speed for explicit, transient compressible flow solvers
R. Lohner, H. Luo, J. Baum, D. Rice

Int. J. Numer. Meth. Fluids (2008). Vol. 56 (12), pp. 2229-2244

Abstract
Several explicit high‐resolution schemes for transient compressible flows with moving shocks are combined in such a way so as to achieve the highest possible speed without compromising accuracy. The main algorithmic changes considered comprise the following: replacing limiting and approximate Riemann solvers by simpler schemes during the initial stages of Runge–Kutta solvers, and only using limiting and approximate Riemann solvers for the last stage; automatically switching to simpler schemes for smooth flow regions; automatic deactivation of quiescent regions; and unstructured grids with cartesian cores or embedded cartesian grids. The results from several examples demonstrate that speedup factors of 1:4 are attainable without compromising the accuracy of the traditional FCT schemes.
Abstract
Several explicit high‐resolution schemes for transient compressible flows with moving shocks are combined in such a way so as to achieve the highest possible speed without [...]
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Comparison of body‐fitted, embedded and immersed solutions of low Reynolds‐number 3‐D incompressible flows
R. Lohner, S. Appanaboyina, J. Cebral

Int. J. Numer. Meth. Fluids (2007). Vol. 57 (1), pp. 13-30

Abstract
The solutions obtained for low Reynolds‐number incompressible flows using the same flow solver and solution technique on body‐fitted, embedded surface and immersed body grids of similar size are compared. The cases considered are a sphere at Re = 100 and an idealized stented aneurysm. It is found that the solutions using all these techniques converge to the same grid‐independent solution. On coarser grids, the effect of higher‐order boundary conditions is noticeable. Therefore, if the manual labor required to set up a body‐fitted domain is excessive (as is often the case for patient‐specific geometries with medical devices), and/or computing resources are plentiful, the embedded surface and immersed body approaches become very attractive options.
Abstract
The solutions obtained for low Reynolds‐number incompressible flows using the same flow solver and solution technique on body‐fitted, embedded surface and immersed body [...]
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Computational fluid dynamics of stented intracranial aneurysms using adaptive embedded unstructured grids
S. Appanaboyina, F. Mut, R. Lohner, C. Putman, J. Cebral

Int. J. Numer. Meth. Fluids (2008). Vol. 57 (5), pp. 475-493

Abstract
Recently, there has been increased interest in the use of stents as flow diverters in the endovascular treatment of cerebral aneurysms as an alternative to surgical clipping or endovascular embolization with coils. The aim of aneurysm stenting is to block the flow into the aneurysm in order to clot the blood inside the aneurysm and effectively isolate it from the circulation and prevent bleeding from the aneurysm. A hybrid meshing approach that combines body‐fitted grids for the vessels and adaptive embedded grids for the stents is proposed and analyzed. This strategy simplifies considerably the geometry modeling problem and allows accurate patient‐specific hemodynamic simulations with endovascular devices. This approach is compared with the traditional body‐fitted approach in the case of the flow around a circular cylinder at representative Reynolds number and an idealized aneurysm model with a stent. A novel technique to map different stent designs to a given patient‐specific anatomical model is presented. The methodology is demonstrated on a patient‐specific hemodynamic model of an aneurysm of the internal carotid artery constructed from a 3D rotational angiogram and stented with two different stent designs. The results show that the methodology can be successfully used to model patient‐specific anatomies with different stents thereby making it possible to explore different stent designs
Abstract
Recently, there has been increased interest in the use of stents as flow diverters in the endovascular treatment of cerebral aneurysms as an alternative to surgical clipping [...]
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A discontinuous Galerkin method based on a Taylor basis for the compressible flows on arbitrary grids
H. Luo, J. Baum, R. Lohner

Journal of Computational Physics (2008). Vol. 227 (20), pp. 8875-8893

Abstract
A discontinuous Galerkin method based on a Taylor basis is presented for the solution of the compressible Euler equations on arbitrary grids. Unlike the traditional discontinuous Galerkin methods, where either standard Lagrange finite element or hierarchical node-based basis functions are used to represent numerical polynomial solutions in each element, this DG method represents the numerical polynomial solutions using a Taylor series expansion at the centroid of the cell. Consequently, this formulation is able to provide a unified framework, where both cell-centered and vertex-centered finite volume schemes can be viewed as special cases of this discontinuous Galerkin method by choosing reconstruction schemes to compute the derivatives, offer the insight why the DG methods are a better approach than the finite volume methods based on either TVD/MUSCL reconstruction or essentially non-oscillatory (ENO)/weighted essentially non-oscillatory (WENO) reconstruction, and has a number of distinct, desirable, and attractive features, which can be effectively used to address some of shortcomings of the DG methods. The developed method is used to compute a variety of both steady-state and time-accurate flow problems on arbitrary grids. The numerical results demonstrated the superior accuracy of this discontinuous Galerkin method in comparison with a second order finite volume method and a third-order WENO method, indicating its promise and potential to become not just a competitive but simply a superior approach than its finite volume and ENO/WENO counterparts for solving flow problems of scientific and industrial interest.
Abstract
A discontinuous Galerkin method based on a Taylor basis is presented for the solution of the compressible Euler equations on arbitrary grids. Unlike [...]
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Deflated preconditioned conjugate gradient solvers for the Pressure–Poisson equation
R. Aubry, F. Mut, R. Lohner, J. Cebral

Journal of Computational Physics (2008). Vol. 227 (24), pp. 10196-10208

Abstract
A deflated preconditioned conjugate gradient technique has been developed for the solution of the Pressure–Poisson equation within an incompressible flow solver. The deflation is done using a region-based decomposition of the unknowns, making it extremely simple to implement. The procedure has shown a considerable reduction in the number of iterations. For grids with large graph-depth the savings exceed an order of magnitude. Furthermore, the technique has shown a remarkable insensitivity to the number of groups/regions chosen, and to the way the groups are formed.
Abstract
A deflated preconditioned conjugate gradient technique has been developed for the solution of the Pressure–Poisson equation within an incompressible flow solver. [...]
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A discontinuous Galerkin method using Taylor basis for computing shock waves on arbitrary grids
H. Luo, J. Baum, R. Lohner

(2009). Shock Waves. Springer, Berlin, Heidelberg

Abstract
The discontinuous Galerkin methods [1] (DGM) have recently become popular for the solution of systems of conservation laws to arbitrary order of accuracy. The DGM combine two advantageous features commonly associated to finite element and finite volume methods. As in classical finite element methods, accuracy is obtained by means of high-order polynomial approximation within an element rather than by wide stencils as in the case of finite volume methods. The physics of wave propagation is, however, accounted for by solving the Riemann problems that arise from the discontinuous representation of the solution at element interfaces.
Abstract
The discontinuous Galerkin methods [1] (DGM) have recently become popular for the solution of systems of conservation laws to arbitrary order of accuracy. The DGM combine [...]
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A Hybrid Building-Block and Gridless Method for Computing Shock Waves
H. Luo, J. Baum, R. Lohner

(2009). Computational Fluid Dynamics 2006. Springer, Berlin, Heidelberg

Abstract
The numerical methods for the solution of the compressible Euler and Navier-Stokes equations can be classified by the mesh they use as structured grid methods, unstructured grid methods, Cartesian grid methods, and gridless methods. Each of these methods, advocated, promoted, developed, and used by their respective supporters, has its own advantages and disadvantages. The structured grid methods have a disadvantage in mesh generation for complex geometries. The main advantage of the unstructured grid methods is the ease of grid generation for complex configurations. However, the computational costs and memory requirements are generally higher than their structured grid counterparts. The advantages of the Cartesian grid methods include ease of grid generation, lower computational storage requirements, and significantly less operational count per cell.
Abstract
The numerical methods for the solution of the compressible Euler and Navier-Stokes equations can be classified by the mesh they use as structured grid methods, unstructured [...]
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On 3D Anisotropic Local Remeshing for Surface, Volume and Boundary Layers
A. Loseille, R. Lohner

(2009). Proceedings of the 18th International Meshing Roundtable. Springer, Berlin, Heidelberg

Abstract
A simple strategy for generating anisotropic meshes is introduced. The approach belongs to the class of metric-based mesh adaptation procedures where a field of metric tensors governs the adaptation. This development is motivated by the need of generating anisotropic meshes for complex geometries and complex flows. The procedure may be used advantageously for cases where global remeshing techniques become either unfeasible or unreliable. Each of the local operations used is checked in a variety of ways by taking into account both the volume and the surface mesh. This strategy is illustrated with surface mesh adaptation and with the generation of meshes suited for boundary layers analysis. Two simple mesh operators are used to recursively modify the mesh: edge collapse and point insertion on edge. It is shown that using these operators jointly with a quality function allows to quickly produce an quality anisotropic mesh. Each adaptation entity, ie surface, volume or boundary layers, relies on a specific metric tensor field. The metric-based surface estimate is used to control the deviation to the surface and to adapt the surface mesh. The volume estimate aims at controlling the interpolation error of a specific field of the flow. The boundary layers metric-based estimate is deduced from a level-set distance function.
Abstract
A simple strategy for generating anisotropic meshes is introduced. The approach belongs to the class of metric-based mesh adaptation procedures where a field of metric tensors [...]
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On the Computation of Steady-State Compressible Flows Using a DG Method
H. Luo, J. Baum, R. Lohner

(2006). Computational Fluid Dynamics 2006. Springer, Berlin, Heidelberg

Abstract
Most efforts in the development of the discontinuous Galerkin methods (DGM) in computational fluid dynamics are primarily focused on the time accurate compressible Euler and Navier-Stokes equations. Its accuracy, efficiency, capability, robustness for steady state flow problems are relatively unexplored. In order for DGM to become a viable, attractive, probably even better alternative to the more traditional, more elaborate, well established finite volume methods (FVM), and finite element methods (FEM) for steady state computations, the following three issues have to be addressed: 1) Lack of efficient flow solver for steady state computations: Most efforts in the development of the discontinuous Galerkin methods are primarily focused on the spatial discretization.
Abstract
Most efforts in the development of the discontinuous Galerkin methods (DGM) in computational fluid dynamics are primarily focused on the time accurate compressible Euler and [...]
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Image-based analysis of blood flow modification in stented aneurysms
J. Cebral, F. Mut, S. Appanaboyina, R. Lohner, C. Miranda, E. Escrivano, P. Lylyk, C. Putman

(2009). Proceedings Volume 7262, Medical Imaging 2009: Biomedical Applications in Molecular, Structural, and Functional Imaging;

Abstract
Currently there is increased interest in the use of stents as flow diverters for the treatment of intracranial aneurysms, especially wide necked aneurysms that are difficult to treat by coil embolization or surgical clipping. This paper presents image-based patient-specific computational models of the hemodynamics in cerebral aneurysms before and after treatment with a stent alone, with the goal of better understanding the hemodynamic effects of these devices and their relation to the outcome of the procedures. Stenting of cerebral aneurysms is a feasible endovascular treatment option for aneurysms with wide necks that are difficult to treat with coils or by surgical clipping. However, this requires stents that are capable of substantially modifying the intra-aneurysmal flow pattern in order to cause thrombosis of the aneurysm. The results presented in this paper show that the studied stent was able to change significantly the hemodynamic characteristics of the aneurysm. In addition, it was shown that patient-specific computational models constructed from medical images are capable of realistically representing the in vivo hemodynamic characteristics observed during conventional angiography examinations before and after stenting. This indicates that these models can be used to better understand the effects of different stent designs and to predict the alteration in the hemodynamic pattern of a given aneurysm produced by a given flow diverter. This is important for improving current design of flow diverting devices and patient treatment plans.
Abstract
Currently there is increased interest in the use of stents as flow diverters for the treatment of intracranial aneurysms, especially wide necked aneurysms that are difficult [...]
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Calculating Blast Loads for Civil Engineering Structures
R. Lohner, J. Baum

(2009). 100 Volumes of ‘Notes on Numerical Fluid Mechanics’. Notes on Numerical Fluid Mechanics and Multidisciplinary Design, vol 100. Springer, Berlin, Heidelberg

Abstract
A brief overview of the state of the art of computing blast loads on civil engineering structures is given. The general problem setting, requirements, main physical phenomena and timescales, as well as suitable numerical methods are described. Several examples show the power of blast loads calculations for civil engineering structures.
Abstract
A brief overview of the state of the art of computing blast loads on civil engineering structures is given. The general problem setting, requirements, main physical phenomena [...]
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Numerical simulation of H2/air detonation using unstructured mesh
F. Togashi, R. Lohner, N. Tsuboi

Shock Waves (2009). Vol. 19 (2), pp. 151-162

Abstract
To explore the capability of unstructured mesh to simulate detonation wave propagation phenomena, numerical simulation of H2/air detonation using unstructured mesh was conducted. The unstructured mesh has several adv- antages such as easy mesh adaptation and flexibility to the complicated configurations. To examine the resolution dependency of the unstructured mesh, several simulations varying the mesh size were conducted and compared with a computed result using a structured mesh. The results show that the unstructured mesh solution captures the detailed structure of detonation wave, as well as the structured mesh solution. To capture the detailed detonation cell structure, the unstructured mesh simulations required at least twice, ideally 5times the resolution of structured mesh solution.
Abstract
To explore the capability of unstructured mesh to simulate detonation wave propagation phenomena, numerical simulation of H2/air detonation using unstructured mesh was conducted. [...]
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Adjoint-Based Design of Shock Mitigation Devices
A. Stück, F. Camelli, R. Lohner

Int. J. Numer. Meth. Fluids (2009). Vol. 64 (4), pp. 443-472

Abstract
Unsteady Euler and adjoint Euler solvers have been combined in order to aid in the design of shock mitigation devices. The flowfield is integrated forward in time and stored. The adjoint is then integrated going backwards in time, restoring and interpolating the saved Euler solution to the current point in time. The gradient is obtained from a surface integral formulation during the adjoint run. Comparisons of adjoint‐based and finite‐differencing gradients for different verification cases show less than 10% deviation. The results obtained indicate that this is a very cost‐effective way to obtain the gradients of an objective function with respect to surface design changes. Moreover, as the sensitivity information is determined over a complete surface, the procedure provides considerable insight, and can efficiently facilitate the design of shock mitigation devices such as architecturally appealing blast walls.
Abstract
Unsteady Euler and adjoint Euler solvers have been combined in order to aid in the design of shock mitigation devices. The flowfield is integrated forward in time and stored. [...]
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Large-Eddy Simulations of a Supersonic Jet and Its Near-Field Acoustic Properties
J. Liu, K. Kailasanath, R. Ramamurti, R. Lohner, D. Munday, E. Gutmark

AIAA Journal (2009). Vol. 47 (8), pp. 1849-1864

Abstract
Large-eddy simulations of imperfectly expanded jet flows from a convergent-divergent nozzle with a sharp contraction at the nozzle throat have been carried out. The flowfield and near-field acoustics for various total pressure ratios from overexpanded to underexpanded jet flow conditions have been investigated. The location and spacing of the shock cells are in good agreement with experimental data and previous theoretical results. The velocity profiles are also in good agreement with data from experimental measurements. A Mach disk is observed immediately downstream of the nozzle exit for overexpanded jet conditions with nozzle pressure ratios much lower than the fully expanded value. It is found that this type of nozzle with a sharp turning throat does not have a shock- free condition for supersonic jet flows. The near-field intensities of pressure fluctuations show wavy structures for cases in which screech tones are observed. The large-eddy simulations predictions of the near-field noise intensities show good agreement with those obtained from experimental measurements. This good agreement shows that large- eddy simulations and measurements can play complementary roles in the investigation of the noise generation from supersonic jet flows.
Abstract
Large-eddy simulations of imperfectly expanded jet flows from a convergent-divergent nozzle with a sharp contraction at the nozzle throat have been carried out. The flowfield [...]
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Simulation of intracranial aneurysm stenting: Techniques and challenges
S. Appanaboyina, F. Mut, R. Lohner, C. Putman, J. Cebral

Comput. Methods Appl. Mech. Engrg., (2009). Vol. 198 (45-46), pp. 3567-3582

Abstract
Recently, there has been considerable interest in the use of stents as endovascular flow diverters for the treatment of intracranial aneurysms. Simulating this novel method of treatment is essential for understanding the intra-aneurysmal hemodynamics in order to design better stents and to personalize and optimize the endovascular stenting procedures. This paper describes a methodology based on unstructured embedded grids for patient-specific modeling of stented cerebral aneurysms, demonstrates how the methodology can be used to address specific clinical questions, and discusses remaining technical issues. In particular, simulations are presented on a number of patient-specific models constructed from medical images and using different stent designs and treatment alternatives. Preliminary sensitivity analyses with respect to stent positioning and truncation of the stent model are presented. The results show that these simulations provide useful and valuable information that can be used during the planning phase of endovascular stenting interventions for the treatment of intracranial aneurysms.
Abstract
Recently, there has been considerable interest in the use of stents as endovascular flow diverters for the treatment of intracranial aneurysms. Simulating this novel method [...]
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The journal Communications in Numerical Methods in Engineering with Biomedical Applications becomes the International Journal for Numerical Methods in Biomedical Engineering (IJNMBE) from 1st January 2010
P. Nithiarasu, R. Lohner

Int. J. Numer. Meth. Fluids (2009). Vol. 61 (10), pp. 1179-1180

Abstract
Communications in Numerical Methods in Engineering , founded by Roland W. Lewis in 1985, will change its title to the ‘International Journal for Numerical Methods in Biomedical Engineering ’ and has a revised Aims and Scope.
Abstract
Communications in Numerical Methods in Engineering , founded by Roland W. Lewis in 1985, will change its title to the ‘International Journal for Numerical Methods [...]
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Fast numerical solutions of patient‐specific blood flows in 3D arterial systems
F. Mut, R. Aubry, R. Lohner, J. Cebral

Int. J. Numer. Meth. Bio. Engng. (2010). Vol. 26 (1), pp. 73-86

Abstract
The study of hemodynamics in arterial models constructed from patient‐specific medical images requires the solution of the incompressible flow equations in geometries characterized by complex branching tubular structures. The main challenge with this kind of geometries is that the convergence rate of the pressure Poisson solver is dominated by the graph depth of the computational grid. This paper presents a deflated preconditioned conjugate gradients (DPCG) algorithm for accelerating the pressure Poisson solver. A subspace deflation technique is used to approximate the lowest eigenvalues along the tubular domains. This methodology was tested with an idealized cylindrical model and three patient‐specific models of cerebral arteries and aneurysms constructed from medical images. For these cases, the number of iterations decreased by up to a factor of 16, while the total CPU time was reduced by up to 4 times when compared with the standard PCG solver.
Abstract
The study of hemodynamics in arterial models constructed from patient‐specific medical images requires the solution of the incompressible flow equations in geometries characterized [...]
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On the modeling of pedestrian motion
R. Lohner

Applied Mathematics Modelling (2010). Vol. 34 (2), pp. 366-382

Abstract
A model for the simulation of pedestrian flows and crowd dynamics has been developed. The model is based on a series of forces, such as: will forces (the desire to reach a place at a certain time), pedestrian collision avoidance forces, obstacle/wall avoidance forces; pedestrian contact forces, and obstacle/wall contact forces. Except for the will force, it is assumed that for any given pedestrian these forces are the result of only local (nearest neighbour) situations. The near-neighbour search problem is solved by an efficient incremental Delaunay triangulation that is updated at every timestep. In order to allow for general geometries a so-called background triangulation is used to carry all geographic information. At any given time the location of any given pedestrian is updated on this mesh. The results obtained to date show that the model performs well for standard benchmarks, and allows for typical crowd dynamics, such as lane forming, overtaking, avoidance of obstacles and panic behaviour.
Abstract
A model for the simulation of pedestrian flows and crowd dynamics has been developed. The model is based on a series of forces, such as: will forces (the desire to reach a [...]
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An efficient fluid–solid coupled finite element scheme for weapon fragmentation simulations
O. Soto, J. Baum, R. Lohner

Engineering Fracture Mechanics (2010). Vol. 77 (3), pp. 549-564

Abstract
An efficient finite element (FE) scheme to deal with a class of coupled fluid–solid problems is presented. The main ingredients of such methodology are: an accurate Q1/P0 solid element (trilinear in velocities and constant piecewise-discontinuous pressures); a large deformation plasticity model; an algorithm to deal with material failure, cracking propagation and fragment formation; and a fragment rigidization methodology to avoid the possible numerical instabilities that may produce pieces of material flying away from the main solid body. All the mentioned schemes have been fully parallelized and coupled using a loose-embedded procedure with a well-established and validated computational fluid dynamics (CFD) code (FEFLO). A CSD and a CFD/CSD coupled case are presented and analyzed.
Abstract
An efficient finite element (FE) scheme to deal with a class of coupled fluid–solid problems is presented. The main ingredients of such methodology are: an accurate [...]
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Cache‐efficient renumbering for vectorization
R. Lohner

Int. J. Numer. Meth. Bio. Engng. (2008). Vol. 26 (5), pp. 628-636

Abstract
A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring techniques, the new renumbering strategy reduces the spread in point‐data access for edge‐based solvers by more than an order of magnitude. The technique is particularly suited for multicore, cache‐based machines that allow for vectorization or pipelining.
Abstract
A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring [...]
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Selective edge removal for unstructured grids with Cartesian cores
R. Lohner, H. Luo, J. Baum

Journal of Computational Physics (2005). Vol. 26 (1), pp. 208-226

Abstract
Several rules for redistributing geometric edge-coefficient obtained for grids of linear elements derived from the subdivision of rectangles, cubes or prisms are presented. By redistributing the geometric edge-coefficient, no work is carried out on approximately half of all the edges of such grids. The redistribution rule for triangles obtained from rectangles is generalized to arbitrary situations in 3-D, and implemented in a typical 3-D edge-based flow solver. The results indicate that without degradation of accuracy, CPU requirements can be cut considerably for typical large-scale grids. This allows a seamless integration of unstructured grids near boundaries with efficient Cartesian grids in the core regions of the domain.
Abstract
Several rules for redistributing geometric edge-coefficient obtained for grids of linear elements derived from the subdivision of rectangles, cubes or prisms are presented. [...]
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Numerical Simulation of Long-Duration Blast Wave Evolution in Confined Facilities
F. Togashi, J. Baum, E. Mestreau, R. Lohner, D. Sunshine

Shock Waves (2010). Vol. 20 /5(, pp. 409-424

Abstract
The objective of this research effort was to investigate the quasi-steady flow field produced by explosives in confined facilities. In this effort we modeled tests in which a high explosive (HE) cylindrical charge was hung in the center of a room and detonated. The HEs used for the tests were C-4 and AFX 757. While C-4 is just slightly under-oxidized and is typically modeled as an ideal explosive, AFX 757 includes a significant percentage of aluminum particles, so long-time afterburning and energy release must be considered. The Lawrence Livermore National Laboratory (LLNL)-produced thermo-chemical equilibrium algorithm, “Cheetah”, was used to estimate the remaining burnable detonation products. From these remaining species, the afterburning energy was computed and added to the flow field. Computations of the detonation and afterburn of two HEs in the confined multi-room facility were performed. The results demonstrate excellent agreement with available experimental data in terms of blast wave time of arrival, peak shock amplitude, reverberation, and total impulse (and hence, total energy release, via either the detonation or afterburn processes. KeywordsDetonation-Blast wave-EOS-After burning-CFD
Abstract
The objective of this research effort was to investigate the quasi-steady flow field produced by explosives in confined facilities. In this effort we modeled tests in which [...]
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Hemodynamic Analysis of Intracranial Aneurysms with Moving Parent Arteries: Basilar Tip Aneurysms
D. Sforza, R. Lohner, C. Putman, J. Cebral

Int. J. Numer. Meth. Bio. Engng. (2010). Vol. 26 (10), pp. 1219-1227

Abstract
The effects of parent artery motion on the hemodynamics of basilar tip saccular aneurysms and its potential effect on aneurysm rupture were studied. The aneurysm and parent artery motions in two patients were determined from cine loops of dynamic angiographies. The oscillatory motion amplitude was quantified by registering the frames. Patient‐specific computational fluid dynamics (CFD) models of both aneurysms were constructed from 3D rotational angiography images. Two CFD calculations were performed for each patient, corresponding to static and moving models. The motion estimated from the dynamic images was used to move the surface grid points in the moving model. Visualizations from the simulations were compared for wall shear stress (WSS), velocity profiles, and streamlines. In both patients, a rigid oscillation of the aneurysm and basilar artery in the anterio‐posterior direction was observed and measured. The distribution of WSS was nearly identical between the models of each patient, as well as major intra‐aneurysmal flow structures, inflow jets, and regions of impingement. The motion observed in pulsating intracranial vasculature does not have a major impact on intra‐aneurysmal hemodynamic variables. Parent artery motion is unlikely to be a risk factor for increased risk of aneurysmal rupture.
Abstract
The effects of parent artery motion on the hemodynamics of basilar tip saccular aneurysms and its potential effect on aneurysm rupture were studied. The aneurysm and parent [...]
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On the simulation of highly nonlinear wave-breakwater interactions
C. Yang, H. Lu, R. Lohner

Journal of Hydrodynamics (2010). Vol. 22 (5), pp. 975-981

Abstract
A numerical time domain simulation model has been developed to study the highly nonlinear interactions between waves and rubble mound breakwaters. In this model, a volume of fluid (VOF) technique is used to capture the violent free surface motion. The incompressible Euler/Navier-Stokes equations, written in an arbitrary Lagrangian-Eulerian (ALE) frame, are solved using projection schemes and a finite element method on unstructured grids. A general advancing front technique for filling space with arbitrary separated objects is developed to model the rubbles that are laid down on the sloped surface of the breakwater in a random way. Three case studies are performed to study the effects of rubbles and rubble types on the wave dissipation and wave overtopping.
Abstract
A numerical time domain simulation model has been developed to study the highly nonlinear interactions between waves and rubble mound breakwaters. In this model, a volume [...]
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A Hybrid Grid Generation Method for Complex Geometries
H. Luo, S. Spiegel, R. Lohner

AIAA Journal (2010). Vol. 48 (11), pp. 2639-2647

Abstract
A hybrid mesh generation method is described to discretize complex geometries. The idea behind this hybrid method is to combine the orthogonality and directionality of a structured grid, the efficiency and simplicity of a Cartesian grid, and the flexibility and ease of an unstructured grid in an attempt to develop an automatic, robust, and fast hybrid mesh generation method for configurations of engineering interest. A semistructured quadrilateral grid is first generated on the wetted surfaces. A background Cartesian grid, covering the domain of interest, is then constructed using a Quadtree-based Cartesian Method. Those Cartesian cells overlapping with the semistructured grids or locating outside of computational domain are then removed using an Alternating Digital Tree method. Finally, an unstructured grid generation method is used to generate unstructured triangular cells to till all empty regions in the domain as a result of the trimming process. The automatic placement of sources at the geometrical irregularities is developed to render these regions isotropic, thus effectively overcoming the difficulty of generating highly stretched good-quality elements in these regions. The self-dividing of the exposed semistructured elements with high aspect ratio and the adaptation of the background mesh using the cell size information from the exposed semistructured elements for generating Cartesian cells are introduced to improve the quality of unstructured triangular elements and guarantee the success of the unstructured grid generation in the void regions. The developed hybrid grid generation method is used to generate a hybrid grid for a number of test cases, demonstrating its ability and robustness to mesh complex configurations.
Abstract
A hybrid mesh generation method is described to discretize complex geometries. The idea behind this hybrid method is to combine the orthogonality and directionality of a structured [...]
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An assessment of architecturally appealing, semi-open shock mitigation devices
R. Lohner, J. Baum

Engineering Computations (2010). Vol. 29 (1), pp. 19-30

Abstract
Purpose – Limitations in space and city planning constraints have led to the search for alternative shock mitigation devices that are architecturally appealing. The purpose of this paper is to consider a compromise solution which consists of partially open, thick, bending-resistant shapes made of acrylic material that may be Kevlar- or steel-reinforced. Seven different configurations were analyzed numerically. Design/methodology/approach – For the flow solver, the FEM-FCT scheme as implemented in FEFLO is used. The flowfields are initialized from the output of highly detailed 1-D (spherically symmetric) runs. Peak pressure and impulse are stored and compared. In total, seven different configurations were analyzed numerically. Findings – It is found that for some of these, the maximum pressure is comparable to usual, closed walls, and the maximum impulse approximately 50 percent higher. This would indicate that such designs offer a blast mitigation device eminently suitable for built-up city environments. Research limitations/implications – Future work will consider fully coupled fluid-structure runs for the more appealing designs, in order to assess whether such devices can be manufactured from commonly available materials such as acrylics or other poly-carbonates. Practical implications – This would indicate that such designs offer a blast mitigation device eminently suitable for built-up city environments. Originality/value – This is the first time such a semi-open blastwall approach has been tried and analyzed.
Abstract
Purpose – Limitations in space and city planning constraints have led to the search for alternative shock mitigation devices that are architecturally appealing. The [...]
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Numerical modeling of blast wave propagation in a generic facility
F. Togashi, J. Baum, O. Soto, R. Lohner

(2011). Proceedings of the 9th International Conference on Shock and Impact Loads on Structures. pp. 659-667.

Abstract
Accurate evaluation of interior wall response to blast in a building requires the use of a coupled fluid-structure dynamics methodology. The reason is that the walls respond to the blast loading on the time-scale of the blast reverberation within the building. When the wall fails due to blast loading, the debris as well as the airblast propagate into adjacent rooms to load the next layer of walls. To provide better understanding this phenomenon, a program has been initiated which combines experimental and computational efforts. The program objective is to improve the understandings of internal blast damage and fragment dispersion phenomena under the transient/quasi-static pressure condition, so as to improbé the modeling of these scenarios in fast-running codes. The paper will describe the numerical simulations conducted, in which we modeled CMU walls response to blast in a generic facility. The results show that the numerical code was able to accurately predict wall breach due to nearby charge, the complete wall movement, and the loading of the second wall first by the airblast, then by the high-inertia slower-moving wall. Comparison of pressure time histories between the measured and predicte data show good agreement both in detonation room as well as the adjacent bay.
Abstract
Accurate evaluation of interior wall response to blast in a building requires the use of a coupled fluid-structure dynamics methodology. The reason is that the walls respond [...]
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Semi-automatic porting of a general fortran CFD code to GPUS: The difficult modules
R. Lohner, A. Corrigan

Aerospace Research Central (2011). Vol. 20, pp. 1-19

Abstract
Over the last year, a considerable portion of FEFLO, a general-purpose legacy CFD code operating on unstructured grids, was ported to run on GPUs. Like so many other legacy codes, FEFLO is an adaptive, edge-based finite element code for the solution of compressible and incompressible flow, which was primarily written in Fortran 77 and has previously been ported to vector, shared memory parallel and distributed memory parallel machines. Due to the large size of FEFLO and the likelihood of human error in porting, as well as the desire for continued development within a single codebase, a specialized Python script, based on FParser (Peterson, 2009), was written to perform automated translation from the OpenMP-parallelized loops to GPU kernels implemented in CUDA, along with GPU memory management, while integrating with the existing framework for distributed memory parallelism via MPI. The present paper describes extensions of the script and algorithmic techniques that enable the efficient running on GPUs of the modules that are not straightforward to port. In particular, we consider LU-SGS algorithms, linelet preconditioning and particle-mesh algorithms.
Abstract
Over the last year, a considerable portion of FEFLO, a general-purpose legacy CFD code operating on unstructured grids, was ported to run on GPUs. Like so many other legacy [...]
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Generation of Unstructured Grids Suitable for Rans Calculations
R. Lohner

(2011). Computational Aerosciences in the 21st Century. ICASE LaRC Interdisciplinary Series in Science and Engineering, vol 8. Springer, Dordrecht

Abstract
The present Conference was intended to provide a forum to discuss future trends and developments that may affect the way Computational Aerosciences are conducted. One way to venture statements about the future is via exptrapolations from historical trends. In each of the individual disciplines that comprise the aerosciences - aerodynamics, structures, propulsion, control, stealth, etc. - one observes the typical bottom-up trend. Starting from sufficiently simple geometries and equations to have an impact and be identified as `computational’ (e.g., potential flow in 2-D for aerodynamics), more and more realism is added at the geometrical and PDE level. While the engineering process (Figure 1) follows the line: project, objectives, optimization goals, discipline, problem definition, gridding, solution of the PDE and evaluation, the developments in the Computational Sciences tend to run in the opposite direction: solvers, mesh generators, preprocessors, multi-disciplinary links, complete database. At present (1998), for commercial software we are at the threshold mesh generators/ preprocessors, so we can estimate that sometime during the next decade we will see the widespread use of multi-disciplinary links and towards the end of the decade the appearance of integrated analysis and simulation databases. At the research level, we are already entering the complete database approach.
Abstract
The present Conference was intended to provide a forum to discuss future trends and developments that may affect the way Computational Aerosciences [...]
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Validation of the computational fluid–structure interaction simulation at real-scale tests of a flexible 29 m umbrella in natural wind flow
A. Michalski, P. Kermel, E. Haug, R. Lohner, R. Wüchner, K. Bletzingerm

Journal of Wind Engineering and Industrial Aerodynamics (2011). Vol. 99 (4), pp. 400-413

Abstract
The sensitivity of membrane structures to wind loads due to their flexibility and small inertial masses raises the question of their behavior under natural wind conditions. Particularly transient wind loads could lead to dynamic amplification of the structural response. The assessment of the dynamic response of membrane structures is complex due to their special load carrying behavior, their material properties, and their distinct structural interaction with flow induced effects. Computationally intensive fluid–structure interaction simulation could overcome simplifications and limitations of existing approaches, especially small scale wind tunnel tests, and allow the assessment of all relevant structural and fluid phenomena. This paper outlines a virtual design methodology for lightweight flexible membrane structures under the impact of fluctuating wind loads and provides results on the unique validation of the method at real-scale tests of a highly flexible 29 m umbrella.
Abstract
The sensitivity of membrane structures to wind loads due to their flexibility and small inertial masses raises the question of their behavior under natural wind conditions. [...]
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Running unstructured grid‐based CFD solvers on modern graphics hardware
A. Corrigan, F. Camelli, R. Lohner, J. Wallin

Int. J. Numer. Meth. Fluids (2011). Vol. 66 (2), pp. 221-229

Abstract
Techniques used to implement an unstructured grid solver on modern graphics hardware are described. The three‐dimensional Euler equations for inviscid, compressible flow are considered. Effective memory bandwidth is improved by reducing total global memory access and overlapping redundant computation, as well as using an appropriate numbering scheme and data layout. The applicability of per‐block shared memory is also considered. The performance of the solver is demonstrated on two benchmark cases: a NACA0012 wing and a missile. For a variety of mesh sizes, an average speed‐up factor of roughly 9.5 × is observed over the equivalent parallelized OpenMP code running on a quad‐core CPU, and roughly 33 × over the equivalent code running in serial.
Abstract
Techniques used to implement an unstructured grid solver on modern graphics hardware are described. The three‐dimensional Euler equations for inviscid, compressible flow [...]
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Computational Hemodynamics Framework for the Analysis of Cerebral Aneurysms
F. Mut, R. Lohner, A. Chien, S. Tateshima, F. Viñuela, C. Putman, J. Cebral

Int. J. Numer. Meth. Bio. Engng. (2010). Vol. 27 (6), pp. 822-839

Abstract
Assessing the risk of rupture of intracranial aneurysms is important for clinicians because the natural rupture risk can be exceeded by the small but significant risk carried by current treatments. To this end numerous investigators have used image‐based computational fluid dynamics models to extract patient‐specific hemodynamics information, but there is no consensus on which variables or hemodynamic characteristics are the most important. This paper describes a computational framework to study and characterize the hemodynamic environment of cerebral aneurysms in order to relate it to clinical events, such as growth or rupture. In particular, a number of hemodynamic quantities are proposed to describe the most salient features of these hemodynamic environments. Application to a patient population indicates that ruptured aneurysms tend to have concentrated inflows, concentrated wall shear stress distributions, high maximal wall shear stress, and smaller viscous dissipation ratios than unruptured aneurysms. Furthermore, these statistical associations are largely unaffected by the choice of physiologic flow conditions. This confirms the notion that hemodynamic information derived from image‐based computational models can be used to assess aneurysm rupture risk, to test hypotheses about the mechanisms responsible for aneurysm formation, progression, and rupture, and to answer specific clinical questions.
Abstract
Assessing the risk of rupture of intracranial aneurysms is important for clinicians because the natural rupture risk can be exceeded by the small but significant risk carried [...]
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Clinical application of image‐based CFD for cerebral aneurysms
J. Cebral, F. Mut, D. Sforza, R. Lohner, E. Scrivano, P. Lylyk, C. Putman

Int. J. Numer. Meth. Bio. Engng. (2011). Vol. 27 (7), pp. 977-992

Abstract
During the last decade, the convergence of medical imaging and computational modeling technologies has enabled tremendous progress in the development and application of image‐based computational fluid dynamics modeling of patient‐specific blood flows. These techniques have been used for studying the basic mechanisms involved in the initiation and progression of vascular diseases, for studying possible ways to improve the diagnosis and evaluation of patients by incorporating hemodynamics information to the anatomical data typically available, and for the development of computational tools that can be used to improve surgical and endovascular treatment planning. However, before these technologies can have a significant impact on the routine clinical practice, it is still necessary to demonstrate the connection between the extra information provided by the models and the natural progression of vascular diseases and the outcome of interventions. This paper summarizes some of our contributions in this direction, focusing in particular on cerebral aneurysms.
Abstract
During the last decade, the convergence of medical imaging and computational modeling technologies has enabled tremendous progress in the development and application of image‐based [...]
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Deflated preconditioned conjugate gradient solvers for the pressure‐Poisson equation: Extensions and improvements
R. Lohner, F. Mut, J. Cebral, R. Aubry, G. Houzeaux

Int. J. Numer. Meth. Engng. (2011). Vol. 87 (1-5), pp. 2-14

Abstract
Extensions and improvements to a deflated preconditioned conjugate gradient technique for the solution of the pressure‐Poisson equation within an incompressible flow solver are described. In particular, the use of the technique for embedded grids, for cases where volume of fluid or level set schemes are required and its implementation on parallel machines are considered. Several examples are included that demonstrate a considerable reduction in the number of iterations and a remarkable insensitivity to the number of groups/ regions chosen and/or to the way the groups are formed.
Abstract
Extensions and improvements to a deflated preconditioned conjugate gradient technique for the solution of the pressure‐Poisson equation within an incompressible flow solver [...]
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Deflated preconditioned conjugate gradient solvers for linear elasticity
R. Aubry, F. Mut, S. Dey, R. Lohner

Int. J. Numer. Meth. Engng. (2011). Vol. 88 (11), pp. 1112-1127

Abstract
Extensions of deflation techniques previously developed for the Poisson equation to static elasticity are presented. Compared to the (scalar) Poisson equation (J. Comput. Phys. 2008; 227 (24):10196–10208; Int. J. Numer. Meth. Engng 2010; DOI: 10.1002/nme.2932; Int. J. Numer. Meth. Biomed. Engng 2010; 26 (1):73–85), the elasticity equations represent a system of equations, giving rise to more complex low‐frequency modes (Multigrid . Elsevier: Amsterdam, 2000). In particular, the straightforward extension from the scalar case does not provide generally satisfactory convergence. However, a simple modification allows to recover the remarkable acceleration in convergence and CPU time reached in the scalar case. Numerous examples and timings are provided in a serial and a parallel context and show the dramatic improvements of up to two orders of magnitude in CPU time for grids with moderate graph depths compared to the non‐deflated version. Furthermore, a monotonic decrease of iterations with increasing subdomains, as well as a remarkable acceleration for very few subdomains are also observed if all the rigid body modes are included.
Abstract
Extensions of deflation techniques previously developed for the Poisson equation to static elasticity are presented. Compared to the (scalar) Poisson equation (J. Comput. [...]
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Combinatorial Aspects/Algorithms in Computational Fluid Dynamics
R. Lohner

(2012). Combinatorial Scientific Computing, Edited ByUwe Naumann, Olaf Schenk, Chapter 9, 24 p.

Abstract
Most of the equations used to describe the behaviour of continua are of the form: u,t +∇ · (Fa − Fv) = S(u) , (9.1) where u,Fa,Fv,S(u) denote the vector of unknowns, advective and diffusive flux tensors, and source-terms respectively. In the case of compressible gases, we have u = (ρ ; ρvi ; ρe) , Faj = (ρvj ; ρvivj + pδij ; vj(ρe+ p)) , Fvj = (0 ; σij ; vlσlj + kT,j) . (9.2) Here ρ, p, e, T, k, vi denote the density, pressure, specific total energy, temperature, conductivity and fluid velocity in direction xi respectively. This set of equations is closed by providing an equation of state, e.g., for a polytropic gas: p = (γ − 1)ρ[e− 1 2 vjvj ] , T = cv[e − 1 2 vjvj ] , (9.3 a, b) where γ, cv are the ratio of specific heats and the specific heat at constant volume respectively. Furthermore, the relationship between the stress tensor σij and the deformation rate must be supplied. For water and almost all gases, Newton’s hypothesis σij = µ( ∂vi ∂xj + ∂vj ∂xi ) + λ ∂vk ∂xk δij (9.4) complemented with Stokes’ hypothesis λ = −2µ 3 (9.5) is an excellent approximation. The compressible Euler equations are obtained by neglecting the viscous fluxes, i.e., setting Fv = 0. The incompressible Euler or Navier-Stokes equations are obtained by assuming that the density is constant and that pressure does not depend on temperature. The Maxwell equations of electromagnetics, the heat conduction equations of solids, and the equations describing elastic solids undergoing small deformation can readily be written in the form given by Equation (9.1).
Abstract
Most of the equations used to describe the behaviour of continua are of the form: u,t +∇ · (Fa − Fv) = S(u) , (9.1) where u,Fa,Fv,S(u) denote the vector [...]
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The computation of the eddy along the upper wall in the three‐dimensional flow over a backward‐facing step
N. Malamataris, R. Lohner

Int. J. Numer. Meth. Fluids (2011). Vol. 68 (9), pp. 1102-1125

Abstract
A three‐dimensional laminar flow over a backward‐facing step is studied as a numerical experiment by solving the steady‐state, isothermal and incompressible Navier–Stokes equations using two different finite element codes. The Reynolds number ranges from 100 to 1050. The expansion ratio is 1:1.94, and the aspect ratio is 1:36.7. The numerical experiment reveals both eddies along the lower and upper walls downstream of the step. Results of computations regarding positions of detachment of the eddy along the upper wall and positions of reattachments of the eddies along both the lower and upper walls are tabulated along with positions and magnitudes of global extrema of shear rate within the eddies. The wall effects are shown by calculating streamlines along planes parallel/normal to the lateral walls of the domain and depicting how the streamlines are distorted close to the walls and how they assume a two‐dimensional configuration in the plane of symmetry. Comparisons are made with available numerical results and laboratory measurements.
Abstract
A three‐dimensional laminar flow over a backward‐facing step is studied as a numerical experiment by solving the steady‐state, isothermal and incompressible Navier–Stokes [...]
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Generating seamless surfaces for transport and dispersion modeling in GIS
F. Camelli, J. Lien, D. Shen, D. Wong, M. Rice, R. Lohner, C. Yang

GeoInformatica (2012). Vol. 16, pp. 307-327

Abstract
A standard use of triangulation in GIS is to model terrain surface using TIN. In many simulation models of physical phenomena, triangulation is often used to depict the entire spatial domain, which may include buildings, landmarks and other surface objects in addition to the terrain surface. Creating a seamless surface of complex building structures together with the terrain is challenging and existing approaches are laborious, time-consuming and error-prone. We propose an efficient and robust procedure using computational geometry techniques to derive triangulated building surfaces from 2D polygon data with a height attribute. We also propose a new method to merge the resultant building surfaces with the triangulated terrain surface to produce a seamless surface for the entire study area. Using Oklahoma City data, we demonstrate the proposed method. The resultant surface is used as the input data for a simulated transport and dispersion event in Oklahoma City. The proposed method can produce the seamless surface data to be used for various types of physical models in a fraction of the time required by previous methods.
Abstract
A standard use of triangulation in GIS is to model terrain surface using TIN. In many simulation models of physical phenomena, triangulation is often used to depict the entire [...]
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Semi‐automatic porting of a large‐scale Fortran CFD code to GPUs
A. Corrigan, F. Camelli, R. Lohner, F. Mut

Int. J. Numer. Meth. Fluids (2011). Vol. 69 (2), pp. 314-331

Abstract
The development of automatic techniques to port a substantial portion of FEFLO, a general‐purpose legacy CFD code operating on unstructured grids, to run on GPUs is described. FEFLO is a typical adaptive, edge‐based finite element code for the solution of compressible and incompressible flows, which is primarily written in Fortran 77 and has previously been ported to vector, shared memory parallel and distributed memory parallel machines. Owing to the large size of FEFLO and the likelihood of human error in porting, as well as the desire for continued development within a single codebase, a specialized Python script, based on FParser (Int. J. Comput. Sci. Eng. 2009; 4 :296–305), was written to perform automated translation from the OpenMP‐parallelized edge and point loops to GPU kernels implemented in CUDA, along with GPU memory management. The results of verification benchmarks and performance indicate that performances achieved by such a translator can rival those of codes rewritten by specialists. The approach should be of general interest, as how best to run on GPUs is being presently considered for many so‐called legacy codes.
Abstract
The development of automatic techniques to port a substantial portion of FEFLO, a general‐purpose legacy CFD code operating on unstructured grids, to run on GPUs is described. [...]
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Numerical Studies of Green Water Impact on Fixed and Moving Bodies
H. Lu, C. Yang, R. Lohner

International Journal of Offshore and Polar Engineering (2012). Vol. 22 (2), pp. 123-132

Abstract
A numerical time domain simulation model has been developed to study green water phenomena and its impact loading on structures. In this model, a volume-of-fluid (VOF) technique is used to capture the violent free-surface motion. The incompressible Euler/Navier-Stokes equations, written in an arbitrary Lagrangian-Eulerian (ALE) frame, are solved using projection schemes and a finite element method on unstructured grids. Numerical simulations of green water problems carried out in this study include: green water overtopping a fixed 2D deck; green water impact on a fixed 3D body with or without a vertical wall on the deck; and green water impact on the deck and deckhouse of a moving floating production, storage and offloading (FPSO) model. Numerical results obtained using the present model are compared with experimental measurements for each case; and very good qualitative and relatively good quantitative agreements are obtained. The present numerical model can be used for simulating green water effects.
Abstract
A numerical time domain simulation model has been developed to study green water phenomena and its impact loading on structures. In this model, a volume-of-fluid (VOF) technique [...]
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