Scipedia: Documents published in 2021

Geometry Dependence On the Behaviour of Masonry Clay Blocks at High Temperatures

Scipedia content — Thu, 11 Mar 2021 17:38:22 +0100

The development of different constructive techniques is somehow conditioned to the investment in scientific research, aiming at comprehend the behaviour of materials; not only as individuals, but also as a whole compound. One area that has growth, for its importance in structural integrity analysis, is the analysis of masonry structures in fire situations. However, there exists an inherent difficulty on performing real standard tests under fire situations. The present work has as its main objective the study of the thermal behaviour of clay blocks at high temperatures, having the geometry of internal cavities as the main reference parameter. For this purpose, four different geometric configurations were analyzed. The thermal analysis was accomplished by using the commercial software ABAQUS. It was considered radiation, convection and conduction as heat transfer mechanisms. Radiation and convection was accounted for heat transfer between fire and the fire-exposed face of the wall, and between the wall and the ambient. As a simplification, it was considered air as solid for the cavity representation. Thus, heat conduction was considered within the internal cavities. The numerical simulations were purely thermal, aiming at analyzing the dependence of the internal geometry of the cavities. Numerical results show that thermal behavior not only depends on the empty spaces, but also on their distribution along the block interior.

Comparison Criteria for Ice Accretion On An Aerofoil Surface

Scipedia content — Thu, 11 Mar 2021 17:38:13 +0100

Ice accretion on aerofoils has been researched for many years for aircraft, rotorcraft and wind turbine blades using experimental and numerical techniques. While each method has some distinct advantages, it is the combination of the two strategies that is suggested to be the best way to understand and combat the icing challenge. However, both experimental and numerical techniques have certain limitations which can affect the droplet behavior and the resulting ice profiles. Therefore, when comparing numerically predicted ice shape with experimental data for validation purposes, it is important to account for the limitations of both techniques and identify how the two profiles were compared and deemed acceptable even when the two ice shapes appear to have geometrical discrepancies. Although most of the studies highlight the sources of error in their data, almost none of them explain the strategy employed to validate and verify their results. Furthermore, while the anti/de-icing is involved, energy consumption will be one key factor to consider. There should be a criteria to compare ice shapes from the same icing wind tunnel or results from different test facilities which can further help with comparison of numerical and experimental ice shapes. It would aid to set a standard to improve the quality of our research and make progress in the industry. The aim of the present study is to propose criteria that could be used to compare experimental and numerical ice profiles on an aerofoil surface. Therefore, thorough, and meaningful discussion will be conducted in academic world and relevant industries. The main features of the article are outlined as follows. A brief review on some of the experimental and numerical studies on ice accretion have been presented to discuss the methods used, highlight the technological advancements, and discuss key results and sources of error. Research shows that, in addition to the geometrical comparison, the performance of iced aerofoil must also be considered as a comparison strategy. Based on this analysis, the aim of comparing the ice shapes will be discussed. Following this, numerical results from current study are compared with experimental results to illustrate the necessity and importance of having the criteria. Next section will be devoted to proposing the preliminary criteria. Finally, the conclusion will be provided.

On a Proper Tensorial Subgrid Heat Flux Model

Scipedia content — Thu, 11 Mar 2021 17:38:05 +0100

In this work, we aim to shed light to the following research question: can we find a nonlinear tensorial subgrid-scale (SGS) heat flux model with good physical and numerical properties, such that we can obtain satisfactory predictions for buoyancy-driven turbulent flows?This is motivated by our findings showing that the classical (linear) eddy-diffusivity assumption, qeddy T , fails to provide a reasonable approximation for the actual SGS heat flux, q= uT uT : namely, a priori analysis for airfilled Rayleigh-Bénard convection (RBC) clearly shows a strong misalignment. In the quest for more accurate models, we firstly study and confirm the suitability of the eddy-viscosity assumption for RBC carrying out a posteriori tests for different models at very low Prandtl numbers (liquid sodium, Pr= 0.005) where no heat flux SGS activity is expected. Then, different (nonlinear) tensor-diffusivity SGS heat flux models are studied a priori using DNS data of an air-filled (Pr= 0.7) RBC at Rayleigh numbers up to 1011. Apart from having good alignment trends with the actual SGS heat flux, we also restrict ourselves to models that are numerically stable per se and have the proper cubic near-wall behavior. This analysis leads to a new family of SGS heat flux models based on the symmetric positive semi-definite tensor GGTwhere G u, i.e. q GGTT , and the invariants of the GGTtensor. Finally, relevant numerical aspects regarding the implementation of this type of models are discussed in detail. A list of physical and numerical properties is identified and subsequently imposed, leading to a symmetrypreserving discretization that is based on discrete operators already available in any CFD code.

On the Interpolation Problem for the Poisson Equation on Collocated Meshes

Scipedia content — Thu, 11 Mar 2021 17:37:57 +0100

The appearence of unphysical velocities in highly distorted meshes is a common problem in many simulations. In collocated meshes, this problem arises from the interpolation of the pressure gradient from faces to cells. Using an algebraic form for the classical incompressible Navier-Stokes equations, this problem is adressed. Starting from the work of F. X. Trias et. al. [FX.Trias et al. JCP 258: 246-267, 2014], a new approach for studying the Poisson equation obtained using the Fractional Step Method is found, such as a new interpolator is proposed in order to found a stable solution, which avoid the appearence of these unpleasant velocities. The stability provided by the interpolator is formally proved for cartesian meshes and its rotations, using fully-explicit time discretizations. The construction of the Poisson equation is supported on mimicking the symmetry properties of the differential operators and the Fractional Step Method. Then it is reinterpreted using a recursive application of the Fractional Step Method in order to study the system as an stationary iterative solver. Furthermore, a numerical analysis for unstructured mesh is also provided.

Symmetry-Preserving Discretization of Navier-Stokes on Unstructured Grids: Collocated Vs Staggered

Scipedia content — Thu, 11 Mar 2021 17:37:49 +0100

The essence of turbulence are the smallest scales of motion. They result from a subtle balance between convective transport and diffusive dissipation. Mathematically, these terms are governed by two differential operators differing in symmetry: the convective operator is skew-symmetric, whereas the diffusive is symmetric and positive-definite. On the other hand, accuracy and stability need to be reconciled for numerical simulations of turbulent flows around complex configurations. With this in mind, a fully-conservative discretization method for general unstructured grids was proposed [Trias et al., J.Comp.Phys. 258, 246-267, 2014]: it exactly preserves the symmetries of the underlying differential operators on a collocated mesh. However, any pressure-correction method on collocated grids suffer from the same drawbacks: the cell-centered velocity field is not exactly incompressible and some artificial dissipation is inevitable introduced. On the other hand, for staggered velocity fields, the projection onto a divergence-free space is a well-posed problem: given a velocity field, it can be uniquely decomposed into a solenoidal vector and the gradient of a scalar (pressure) field. This can be easily done without introducing any dissipation as it should be from a physical point-of-view. In this work, we explore the possibility to build up staggered formulations based on collocated discrete operators.

A Legendre-Based Displacement Field for Two-Dimensional Digital Image Correlation

Scipedia content — Thu, 11 Mar 2021 17:37:40 +0100

Two-dimensional digital image correlation (DIC) is one of the most commonly used measurement methods for displacement and deformation of specimen surfaces in the field of experimental mechanics. Since its presentation, DIC has been evolving with many new ideas and assets. This paper proposes a new displacement field, based on Legendre polynomials and compares it with conventional second-order polynomial, commonly used in DIC's method.

Adapting Complex and Clumsy CFD Code to Rapidly Changing Supercomputing Realities

Scipedia content — Thu, 11 Mar 2021 17:37:33 +0100

This work is devoted to acceleration and upgrade of the CFD code NOISEtte for scale-resolving simulations of compressible turbulent flows using edge-based high-accuracy methods on unstructured hybrid meshes. Attempts to extend the baseline multilevel MPI+OpenMP parallelization towards GPU-based hybrid systems have faced the problem: the code is too complex. It is an in-house research code with plenty of numerical methods, schemes, models, most of which are experimental and are not used in practical simulations. This chaotic zoo leads to excessive conditional branches, switches, redundant functional calls that slow down computations. Although the parallel algorithm is fully adapted to the stream processing paradigm, such an immense amount of code is too difficult to port efficiently to OpenCL or CUDA and maintain it in consistency with the CPU version. An approach to survive in the process of adaptation to hybrid systems has been elaborated. It consists of various components, such as creation of a simplified configurations, combining different stages of the algorithm in order to reduce memory traffic, collapsing multiple functions in one function without branches and switches, mixing single and double precision, etc. As a result, the upgraded code is about twice as fast on CPUs and can use GPUs from different manufacturers AMD, NVIDIA, Intel through the OpenCL standard.

A Topological Derivative-Based Hydro-Mechanical Fracture Model

Scipedia content — Thu, 11 Mar 2021 17:37:25 +0100

The present work aims to introduce a hydraulic fracture model based on the concept of the topological derivative. This well stimulating technique also known as fracking was proposed by [1] in 1949 and has been widely used in oil and gas industry [2]. Overall, the fracking technique consists in significantly increase the production surface area of a porous medium from a pre-existing fault in such a way that the gas trapped in the rock formation might be collected in the surface. Furthermore, the fracking process has been the subject of many recent works due to its environmental impacts and economical aspects. In this context, we present an extension of the hydro-mechanical fracturing model proposed by [3] in pursuit of a scenario closer to reality by taking into account the initial stress state (in situ stress) and the fluid inertia during the propagation process. The present model results from adapting the Francfort-Marigo damage model [4] to the context of hydraulic fracture together with Biot's theory [5]. We consider a two dimensional idealization in which the fracturing process is activated by a non constant pressure field distributed over the whole domain. A shape functional given by the sum of the total potential energy of the system with a Griffith-type dissipation term is minimized with respect to a set of ball-shaped inclusions by using the topological derivative concept. Then the associated topological derivative is used to construct a topology optimization algorithm designed to simulate the nucleation and propagation process. Finally, some numerical examples depict the role of the in situ stress in the fracture propagation, specific crack path growth (allowing kinking and bifurcations) and also the applicability of the methodology here proposed.

Penalization in Density-Based Topology Optimization: A Mathematical Perspective

Scipedia content — Thu, 11 Mar 2021 17:37:17 +0100

Most real-life engineering optimization problems are non-convex by nature. In a topology optimization context, this non-convexity is even exacerbated by the extra restrictions imposed during the optimization process to enforce mesh-independent black/white manufacturable solutions. Such restrictions include intermediate density penalization, as well as external regulation techniques imposed to tackle some numerical instabilities such as checkerboarding and mesh dependence, in addition to various design constraints. This non-convexity gives rise to the problem of local minima, where the converged solution is greatly affected by the algorithmic parameters as well as the initial guess. To overcome this non-convexity, it's often advised to use continuation methods, that is to introduce non-convexification gradually between iterations. In this article, we present a comprehensive treatment of the sources of nonconvexity in density-based topology optimization problems, with a special emphasis on linear elastic compliance minimization. This is in an attempt to better bridge the gap between the mathematical and the engineering/physical aspects of the topic.

Least-Squares Methods for Linear Elasticity: Refined Error Estimates

Scipedia content — Thu, 11 Mar 2021 17:37:10 +0100

We consider the linear elasticity problems and compare the approximations obtained by the Least-Squares finite element method with the approximations obtained by the standard conforming finite element method and the mixed finite element method. The main result is that the H1-conforming displacement approximations (least-squares finite element and standard finite element) as well as the H(div)-conforming stress approximations are higher-order pertubations of each other. This leads to refined a priori bounds and superconvergence results. Numerical experiments illustrate the theory.

A Class of Examination of Fiber Orientation Dependency and Failure with Multi-Scale Simulation

Scipedia content — Thu, 11 Mar 2021 17:37:02 +0100

This paper examines the mechanical behavior of the short fiber reinforced plastic (SFRP) considering the microstructure using multi-scale simulation. The SFRP, which the microstructure consists of resin and fiver, has a rate, thermal, and fiber orientation dependency. The mechanical property is predicted by matrix and fiber properties. However, the properties by material testing are different from the prediction. Thus, this paper decides each microscopic material parameter on microstructure by the least-square method using the experimental data. Comparing the macro simulation results using homogenized values and the experimental results shows microstructure influences the macroscopic behavior. Moreover, the numerical examples compare the results under two types of tensile speed and three fiber orientation types. The present work completes the multi-scale simulation using a simple microstructure considering fiber orientation and strain rate.

Meshless Collocation Methods Applied to Problems with Material Discontinuities

Scipedia content — Thu, 11 Mar 2021 17:36:56 +0100

The paper deals with the use of a kind of meshless method to solve the problem with
material discontinuity on an interface. Such a problem is described by a differential equation
with discontinuous coefficients. To solve the problem, the abovementioned method is
associated with the subdomain approach that divides the whole domain into subdomains, in
which the problem is continuous. To accurately address the analyzed problem, proper continuity
conditions are imposed on the interface. The Gaussian kernel, which belongs to the family of
infinitely smooth radial basis functions, is taken into consideration as the basis function for the
method. It is known that this type of method can provide very high rate of convergence and
high accuracy but it suffers from instability. To avoid the instability, some recent advances in
kernel methods, based on Mercer’s theorem, are involved in the present paper. The usefulness
of the approaches are shown by benchmark problems described by ordinary as well as partial
differential equations.

Failure of Myocardial Tissue: Simulation of Blood Perfusion

Scipedia content — Thu, 11 Mar 2021 17:36:48 +0100

Technological advances in computational mechanics allow to simulate failure of any part of airplanes, pumps, compressors, electric power stations, foundations, bridges, etc. Billions of dollars have been invested to calculate 3D stress configuration in engineering structures. Because the criterion of failure in engineering structures is generally exceeds stress. Failure of the human heart is responsible for 6 million deaths per year worldwide. Models of cardiac mechanics have been developed to analyse cardiac pump function from tissue to organ scale. All of these models focus on stress and strain. However, heart failure is not associated with fracture. Failure of a heart is usually induced by a mismatch between blood perfusion and metabolic needs of the cardiomyocytes. Because failure is associated with blood perfusion and present day models do not address this failure mechanism, there is an urgent need for a computational strategy for blood perfusion in deforming myocardial tissue. Upscaling of the vessel trees to a continuum opens the way to computation of coronary blood flow in a multi compartment poro-mechanical model of the beating heart. Arterial, arteriolar, capillary, venular and venous blood are treated as separate compartments. As a result, the supply of oxygen to the tissue is modelled. The present interest in tissue engineering as means to support heart function, and the great difficulties associated with angiogenesis in myocardial tissue engineering, calls for a virtual environment for testing cardiac interventions, so that the time to market of newly designed devices and therapies can be shortened substantially.

Automated Generation of Material Laws at Large Strains Using a Genetic Algorithm

Scipedia content — Thu, 11 Mar 2021 17:36:40 +0100

This work presents an approach for automatically generating material laws at large strain. To this end, a concept of material modelling based on directly connected rheological elements is applied. This concept allows the implementation of any material law defined by a rheological model by recursive evaluation of connection relations. Therewith a tree encoding for the rheological model is applied. The search for suitable material laws fitting experimental data is realized by a genetic algorithm, which generates rheological models and evaluates their quality with respect to the given measurements. The central concepts and the genetic operators are presented in this contribution. Finally, the method is verified with synthetic data and utilized to real experimental data.

Geometrical Imperfections in Lattice Structures: a Simulation Strategy to Predict Strength Variability

Scipedia content — Thu, 11 Mar 2021 17:36:32 +0100

The additive manufacturing process (i.e. Selective Laser Melting) allow us to produce lattice structures which have less weight, higher impact absorption capacity and better thermal exchange property compared to the classical structures. Unfortunately, geometrical imperfections in the lattice structures are by-product results of the manufacturing process. These imperfections decrease the lifetime and the strength of the lattice structures and alterate their mechanical responses. The objective of the paper is to present a simulation strategy which allows us to take into account the effect of the geometrical imperfections on the mechanical response of the lattice structure. In the first part, an identification method of geometrical parameters of the lattice structure based on tomography measurements is presented. In the second part, a finite element model for the lattice structure with the simplified geometrical imperfections is obtained. In what follows, based on experimental tests, distributions of geometrical imperfections are proposed. Based on these distributions, a mathematical approach is presented to propagate the effect of uncertainties of the geometrical imperfections on the strength variability of the lattice structure.

Swelling-Induced Instabilities of Polymeric Hydrogels with Periodic Microstructures

Scipedia content — Thu, 11 Mar 2021 17:36:25 +0100

Hydrogels are soft, hydrophilic materials which can absorb a large volume of solvent and undergo finite volumetric deformations known as swelling. The swelling of a hydrogel can be a driving mechanism for complex material responses such as pattern transformation which lead to change of periodicity as a result of a microscopic instability in periodic materials. In the present contribution, we deal with the computational analysis of swelling-induced instabilities in periodic hydrogels. The stability analysis based on the Bloch-Floquet theory is carried out within a transient two-field minimization-type variational principle. The presented formulation and methodology for the stability analysis are computationally efficient, since the computations are carried out on the smallest representative volume element of the microstructure. Within this framework, we study swelling-induced microscopic instabilities for various perforated hydrogels. Our findings are consistent with experimental observations and show that the so-called diamond plate patterns are the critical buckling mode for voided microstructures. Moreover, we observe long-wavelength instabilities for certain volume fractions of voids.

Advantages and Disadvantages of Various Uncertainty Assessment Methods in Material and Technological Predictive Models

Scipedia content — Thu, 11 Mar 2021 17:36:17 +0100

This article reviews known approaches to determining the uncertainty of predictive models: probabilistic analytical, probabilistic simulation and fuzzy. The main elements determining the specificity of a given approach are shown. The advantages and disadvantages are compared. Finally, the application guidelines are listed.

Energy-Momentum Scheme For Nonlinear Thermo-Electro-Elastodynamics

Scipedia content — Thu, 11 Mar 2021 17:36:10 +0100

The present contribution aims at the consistent discretisation of nonlinear, coupled thermoelectro-elastodynamics. In that regard, a new one-step implicit and thermodynamically consistent energymomentum integration scheme for the simulation of thermo-electro-elastic processes undergoing large deformations will be presented. The consideration is based upon polyconvexity inspired, constitutive models and a new tensor cross product algebra, which facilitate the design of the so-called discrete derivatives (for more information it is referred to the pioneering works [3, 2]). The discrete derivatives are fundamental for the algorithmic evaluation of stresses and other derived variables like entropy density or the absolute temperature leading to a structure preserving integration scheme. In particular, recently published works of the authors concerning consistent time integration of large deformation thermo-elastodynamics (see [6]) and electro-elastodynamics (see [11]) are combined to a unified integration scheme. Numerical computations demonstrate the stability and conservation properties of the proposed energy-momentum scheme.

Thermal Analysis of a Metal Structure Using the Ansys Software

Scipedia content — Thu, 11 Mar 2021 17:36:02 +0100

Metallic structures are an alternative in civil construction in terms of optimization of time and efficiency, about the construction process, besides being more versatile, durable, resistant and sustainable compared to other structures. Therefore, self-supporting metal towers have become indispensable in the expansion of large urban centers, due to the need for more slender, efficient and durable structures to the impacts caused by both man and the environment. However, its structures suffer daily from long sun exposure and contact with the atmosphere, which carries chemical elements that react with the material and cause pathologies such as thermal wear and chemical corrosion. Thus, the proposal of the work consists of analysing the behaviour and thermal performance of a metal tower with different metal alloys: ASTM A36, ASTM A570, and ASTM B338, thus determining density, capacity, and thermal conductivity. The model of the self-supporting tower used, is 36 meters high, opening the base of 15 meters, weighing throughout 209,730 kg and with a total of 1777 connecting elements distributed throughout the structure, shown in (Figure 1).

A Coupled VEM-BEM Approach for Computational Homogenization of Heterogeneous Materials

Scipedia content — Thu, 11 Mar 2021 17:35:55 +0100

The Boundary Element Method (BEM) is well established as an accurate and powerful numerical technique in continuum mechanics. Due to its intrinsic feature of reducing the problem's dimensionality, which allows reducing the modelling effort without compromising on the solution accuracy, the BEM has been successfully employed for the computational homogenization of materials with complex morphologies. The Virtual Element Method (VEM) has recently emerged as a powerful and robust technique, capable of handling very general polygonal/polyhedral mesh elements. Such a property is of interest in treating problems whose analysis domain presents complex geometric features, as it simplifies the data preparation stage of the analysis. In this work, we use a coupled VEM-BEM approach for computational homogenization of heterogeneous materials whose microstructure is characterized by inclusions of irregular shapes embedded in a surrounding matrix.

An Efficient Design of Experiments Using RBD-Fast

Scipedia content — Thu, 11 Mar 2021 17:35:47 +0100

This research work deals with the efficient Design of Experiments (DoE) for the hybrid variant of the sensitivity analysis method RBD-FAST called Hyprid FAST RBD (HFR). RBD-FAST is a combination of the Random Balance Design (RBD) and the Fourier Amplitude Sensitivity analysis Test (FAST). Commonly, sensitivity analyses are used to determine whether input parameters have an influence on a target value or not. Currently, there is only little research to be found for the HFR method. The HFR method separates the input parameters into groups. Different constraints must be met for an optimal grouping. Theoretically, for every square number an optimal grouping exists, but only for squares of primes an optimal grouping is known to exist. An experiment with any number of input parameters needs as many samples as an experiment with the next higher square of a prime number as the number of input parameters. In this research work optimal groupings for squares of non-primes for the HFR method are found using a brute force algorithm.

Sample Preparation Methodology of the AL2O3 Surface Layers for Self-Lubricating Sliding Pair

Scipedia content — Thu, 11 Mar 2021 17:35:39 +0100

The article discusses the process of preparing samples with an anodic Al₂O₃ layer for tribological tests in the combination of sliding, reciprocating on the T17 tester. The method of cutting, grinding, drilling, threading, gluing, etching, and anodizing samples used in laboratory conditions was presented. As shown in the article, the number of factors influencing the surface quality of the oxide layer produced in terms of tribological associations is significant; therefore, the appropriate sample preparation methodology is so important.

Multigrid and Saddle-Point Preconditioners for Unfitted Finite Element Modelling of Inclusions

Scipedia content — Thu, 11 Mar 2021 17:35:32 +0100

In this work, we consider the modeling of inclusions in the material using an unfitted finite element method. In the unfitted methods, structured background meshes are used and only the underlying finite element space is modified to incorporate the discontinuities, such as inclusions. Hence, the unfitted methods provide a more flexible framework for modeling the materials with multiple inclusions. We employ the method of Lagrange multipliers for enforcing the interface conditions between the inclusions and matrix, this gives rise to the linear system of equations of saddle point type. We utilize the Uzawa method for solving the saddle point system and propose preconditioning strategies for primal and dual systems. For the dual systems, we review and compare the preconditioning strategies that are developed for FETI and SIMPLE methods. While for the primal system, we employ a tailored multigrid method specifically developed for the unfitted meshes. Lastly, the comparison between the proposed preconditioners is made through several numerical experiments.

A Low-Mach / Acoustic Solver for Fluid Structure Interaction Problems

Scipedia content — Thu, 11 Mar 2021 17:35:24 +0100

An all-Mach correction is applied to the HLLC scheme for computing liquid flows described by the Euler equations closed with the stiffened gas EoS. The accuracy provided by the corrected scheme is assessed on a series of model problems : Gresho vortex, low-Mach shock tube and a piston problem providing a simplified description of the bubble expansion occurring in some nuclear safety problems.

Bubble Expansion Computations at Low-Mach Number Regime

Scipedia content — Thu, 11 Mar 2021 17:35:16 +0100

A 1D bubble expansion problem is solved using standard compressible methods whose numerical solutions are compared with the exact low-Mach solution of the problem. The low Mach solution is proved to provide a very good approximation of the grid-converged compressible solution for a weak expansion and an envelope solution for the strong expansion. This motivates the design of a newly proposed Artificial Compressibility solver which will speed up low-Mach simulations of interest for nuclear safety with respect to standard compressible solvers.

Hybrid Eulerian-Lagrangian Approach for Dense Spray Simulations

Scipedia content — Thu, 11 Mar 2021 17:35:02 +0100

In this work, a hybrid Euler-Lagrangian solver for dense spray systems is developed specifically for cases where film creation by accumulation of liquid droplets at the walls plays a crucial role. EulerLagrangian solvers are commonly used to describe the spray with predefined spray characteristics. The Lagrangian particles represent liquid drops moving along the continuous gaseous phase. This approach assumes a small particle size compared to the cell size and it is unable to capture the breakup behavior of liquid jets in the presence of instabilities. VOF methods, on the other hand, are not a computationally feasible option when it comes to small droplet sizes as a result of liquid atomization because they have to be fully resolved by the computational mesh. Hence, multiscale simulations are required to bridge the gap between the two methods combining subgrid droplets in Lagrangian approaches and large liquid structures in VOF methods. In the present work, a multiscale approach is developed where Lagrangian particles representing small droplets are tracked through the continuous phase until they hit a wall or a liquid-gas interface represented by a continuous VOF field. At the time of impact, the Lagrangian particles are removed and the mass and momentum of these particles are transferred to the VOF field. This allows having a spray consisting of subgrid droplets computed with a Lagrangian particle tracking (LPT) approach and liquid films at the walls with VOF method. The method represents a one-way coupling, converting Lagrangian particles to Eulerian liquid phase (VOF) and has been implemented into the open-source CFD code OpenFOAM. Subsequently, the solver has been tested in different scenarios to ensure mass and momentum conservation. Positive test results encouraged its use to gain insight on the fluid flow in a real cylindrical compression-dissolution unit where the liquid is sprayed from the top while simultaneously the gas is compressed from the bottom. Dynamic mesh technique is used to account for the compression with a moving piston.

Multifidelity Approaches for Uncertainty Estimation in Wildfire Spread Simulators

Scipedia content — Thu, 11 Mar 2021 17:34:54 +0100

A variety of wildfire models are currently used for prescribed fire management, fire behaviour studies and decision support during wildfire emergencies, among other applications. All these applications are based on predictive analysis, and therefore require careful estimation of aleatoric and epistemic uncertainties such as weather conditions, vegetation properties and model parameters. However, the large computational cost of high-fidelity computaional fluid dynamics models prohibits the straightforward utilization of traditional Monte Carlo methods. Conversely, low-fidelity fire models are several orders of magnitude faster but they typically do not provide enough accuracy and they do not resolve all relevant phenomena. Multifidelity frameworks offer a viable solution to this limitation through the efficient combination of highand low-fidelity simulations. While high-fidelity models provide the required level of accuracy, low-fidelity simulations are used to economically improve the confidence on estimated uncertainty. In this work, we assessed the suitability of multifidelity methodologies to quantify uncertainty in wildfire simulations. A collection of different multifidelity strategies, including Multilevel and Control Variates Monte Carlo, were tested and their computational efficiency compared. Fire spread was predicted in a canonical scenario using popular simulators such as the Wildland-Urban Interface Fire Dynamics Simulator (WFDS) and FARSITE. Results show that multifidelity estimators allow speedups in the order of 100× to 1000× with respect to traditional Monte Carlo.

An Equi-Dimensional Finite Element Approach for Flow Problems in Fractured Porous Media

Scipedia content — Thu, 11 Mar 2021 17:34:45 +0100

We propose a novel approach for flow simulations in fractured porous media based on an equi-dimensional representation of the fractures and a standard continuous finite element (FE) method. We employ an adaptive mesh refinement strategy to automatically adjust an initial regular mesh to any fracture distribution with a desired accuracy. The proposed approach is easily implementable in any FE software, does not involve the coupling of different discretizations, and provides symmetric and positive definite stiffness matrices. We provide a systematic validation of our method and we show that it can be used to simulate fluid flow for fracture networks of realistic complexity.

Rectangular Isotropic Kirchhoff-Love Plate on an Elastic Foundation under the Action of Unsteady Elastic Diffusion Perturbations

Scipedia content — Thu, 11 Mar 2021 17:34:34 +0100

We study unsteady elastic diffusion vibrations of a freely supported rectangular isotropic Kirchhoff-Love plate on an elastic foundation, which is under the action of a distributed transverse load. A model that describes coupled elastic diffusion processes in multicomponent continuum is used for the mathematical problem formulation. The longitudinal and transverse vibrations equations of a rectangular isotropic Kirchhoff-Love plate with diffusion were obtained from the model using the d'Alembert variational principle. The problem solution of unsteady elastic diffusion plate vibrations is sought in integral form. The bulk Green's functions are the kernels of the integral representations. To find the Green's functions, we used the Laplace transform in time and the expansion into double trigonometric Fourier series in spatial coordinates. Green's functions in the image domain are represented in the form of rational functions depends on the Laplace transform parameter. The transition to the original domain is done analytically through residues and tables of operational calculus. The bulk Green's functions analytical expressions are obtained. Using a two-component continuum, a numerical study of unsteady mechanical and diffusion fields interaction is done for an isotropic plate. The solution is presented in analytical form, as well as in the form of three-dimensional graphs of the displacement fields and concentration increments on time and coordinates.

Investigation of the Response Variability of Base-A Isolated Building Equipped With Lead Rubber Bearings

Scipedia content — Thu, 11 Mar 2021 17:34:22 +0100

Nowadays analytical models of seismic isolators can fairly reproduce the force response of such devices, when implemented in a large variety of structural systems, such as buildings and bridges. Consequently, realistic hysteretic rules are available for the definition of the dynamic system for Non-Linear Time History Analyses, and earthquake simulations of the considered isolated structural systems can be computed. Such models are generally defined, according to mean values of mechanical properties of isolation devices, even though a certain variability has been experimentally assessed: precisely, statistical analyses of the outcomes of test database have outlined that the main response parameters of isolators should be considered as random variables, rather than as deterministic values. On the other hand, in the common practice both design and assessment procedures are mainly based on deterministic approaches, and bound analyses are ruled in just few standard codes. The present endeavor presents a wide parametric study on a case study structure, in order to assess the variability of the main response parameters, by accounting for random mechanical properties of isolation devices. Precisely, a combination of Lead Rubber Bearings and Flat Slider devices have been considered, and the spatial layout of isolators has been defined, according to a given performance point. The structural response of the case study building has been computed through Non-Linear Time History Analyses, by extracting 10'000 individual values of mechanical properties of devices. Presented results are related to the mean response of a spectrum-compatible set of natural records, in terms of displacement and force of both superstructure and isolation system.

Techniques to Reduce The Computational Cost of Finite Element Models of the Lower Limb Muscles

Scipedia content — Thu, 11 Mar 2021 17:34:14 +0100

Modeling the lower limb muscles using the Finite Element (FE) method is required for various applications including injury mechanisms or when stress/strain distribution in the muscle is of interest. When the muscles are represented with 3D FE models, the interaction between the muscles can be taken into account that has an effect on their force production.
However, the computational cost of such a method is considerably high. Besides, in many cases, a major part of this computational cost is committed to gain unnecessary information. For instance, when having two FE muscles in contact, both muscles need to get finely meshed to conserve their surface details even if having the stress/strain distribution inside one of the two muscles is not required. As a result, the current study aims to explore a model reduction technique based on mesh embedding to decrease the computational cost of such models. A combination of Computerized Tomography and Magnetic Resonance Imaging (MRI) data obtained from a volunteer subject was used to generate a musculoskeletal model of the quadriceps muscle group. The modeling process was performed in ArtiSynth which is an opensource 3D modeling platform supporting combined simulation of multibody and finite element models. This platform allows the attachment of a passive embedded mesh to a FE body so that it deforms in accordance with the motion of the FE body. Considering that the external forces applied to the passive mesh can be propagated back to the FE body attached to it, contact can be defined between the embedded mesh and any other structure. A full and a reduced model are generated and are used to simulate a passive deep knee flexion to test the reliability of the method. The kinematic outcomes are compared against data points obtained from MRI at different flexion angles. The results show that the proposed methodology can be considered as a substitute to fully FE models without a substantial sacrifice on the outcomes despite having lower computational cost.

Force Distribution Along Tree Branch – Static Analysis

Scipedia content — Thu, 11 Mar 2021 17:34:06 +0100

The standard approach of tree stability assessment assumes that wind force is applied in a unique point (centre of gravity). Such a description of the loading brings a significant simplification of the reality, where the tree stem is submitted to multiple forces distributed from higher-order branches to lower order one. The aim of this study is to describe the influence of the simplification by comparison of branch response (deflection curve, bending moment) in the case of singleand the multi-point loading. Four beam-like models were built in ANSYS APDL according to geometry (tapered beams with realistic elliptical cross-sections) and material properties (shear included) of real tested branches. The models were experimentally validated in the case of single-point loading and consequently used for simulations of various loading scenarios. The distribution of force along the branches, by weighted division of total wind force according to different properties of branches, was applied. The branch response differs between singleand multi-point loading in the case of deflection curve, whereas the bending moment at branch anchorage is comparable.

A GPU Accelerated Framework for Partitioned Solution of Fluid-Structure Interaction Problems

Scipedia content — Thu, 11 Mar 2021 17:33:58 +0100

We present a GPU-accelerated solver for the partitioned solution of fluid-structure interaction (FSI) problems. Independent scalable fluid and structure solvers are coupled by a library which handles the inter-code data communication, mapping and equation coupling. A coupling strategy is incorporated which allows accelerating expensive components of the coupled framework by offloading them to GPUs. To prove the efficiency of the proposed coupling strategy in conjunction with the offloading scheme, we present a numerical performance analysis for a complex test case in the filed of biomedical engineering. The numerical experiments demonstrate an excellent speed-up in the accelerated kernels (up to 133 times) which results in 6 to 8 times faster overall simulations. In addition, we observed a very good reduction in total simulation time by increasing the exploited compute nodes up to 8 (complete machine capacity).

Node Averaged Nodal Integral Method

Scipedia content — Thu, 11 Mar 2021 17:33:50 +0100

In this paper, a new variant of the nodal integral method (NIM) is proposed for the solution of 2D diffusion equation. In NIM, the scheme is derived using Transverse Integration Process (TIP) and analytical solutions of ODEs within nodes. This analytical pre-processing dramatically improve the quality of the solution, which is the main distinguishing feature of NIM. However, in this new approach, the TIP is performed at node center instead of node edge. Adapting this approach simplifies the scheme and make it easier to solve. To test the capabilities of this newly developed scheme, some benchmark problems using the diffusion equation are solved. It is found that the scheme is as accurate as traditional NIM, and the scheme is relatively easy to solve. It can be said that a simplified scheme is developed while retaining the accuracy of traditional NIM.

Development of a Thermomechanical Model for Prediction of Residual Stress during Laser Powder-Bed Fusion: Evaluation of Inherent Strain Approach

Scipedia content — Thu, 11 Mar 2021 17:33:42 +0100

The build-up of residual stresses in a part during laser powder-bed fusion (LPBF) provides a significant limitation to the adoption of this process. These residual stresses may cause a part to fail during a build or fall outside the specified tolerances after fabrication. Defectfree production of metallic parts using LPBF requires process optimization, as a crucial step, for effective usage of the process. Development of a numerical model to accurately predict the induced residual stresses and distortion during the LPBF process is of great interest as it allows to effectively investigate the influence of processing parameters on the quality of the parts. In this work, we developed a novel high-fidelity finite element (FE) model based on the inherent strain (local-global) approach to simulate the build process and calculate the residual stress and distortion for Hastelloy X specimens built with a continuous scan strategy. Conclusions from the thermomechanical simulations showed good agreement with X-ray diffraction measurements and 3D scanning data used to determine the residual stresses and distortions in the parts.

Local Reliability Based Sensitivity Analysis with the Moving Particles Method

Scipedia content — Thu, 11 Mar 2021 17:33:36 +0100

Local reliability sensitivity methods aim at determining the partial derivatives of the failure probability or the reliability index with respect to model parameters. For efficient local reliability based sensitivity analysis, it is important to avoid repeated evaluations of the performance function. To this end, an extension of the moving particles method to local reliability based sensitivity analysis is presented that is completely based on the already evaluated samples for the reliability estimate and thus avoids repeated evaluations of the performance function. In order to further reduce the variance of the estimator and to increase the efficiency, a multilevel variant of the estimator is proposed. The method is discussed in detail and illustrated by means of examples.

A Discussion on a Rate-Dependent Cohesive Law for Polymer Adhesion Inspired from Existing Constitutive Equations

Scipedia content — Thu, 11 Mar 2021 17:33:29 +0100

The failure of adhesion between two polymers is a complex phenomenon involving bulk dissipation (i.e., shear yielding, cavitation and crazing), surface adhesion (through Van der Waals interactions) and connector chain pull-out. This article is concerned with a cohesive model for polymer surface separation, which incorporates connector pull-out since it is attempted to adjust this model on coarse-grained molecular dynamics (MD) simulations. This model is made of a viscoplastic constitutive equation taking inspiration from the works by Anand and Gurtin (2003). The MD simulations performed and here analysed were done in close collaboration with Prof. Erik van der Giessen (MuMec, Zernike Institute for Advanced Materials, University of Groningen, Groningen, the Netherlands).

Vibration Control Using Piezoelectric Connected with Shunts Circuits

Scipedia content — Thu, 11 Mar 2021 17:33:21 +0100

The study analyzes the dynamic behaviour and vibration attenuation of an Euler Bernoulli beam coupled with a piezoelectric layer. The vibration attenuation is given by the shunt circuit connected to the piezoelectric. This paper shows different shunt circuit impedances and how they affect the system's frequency response (FRF) functions. The spectral element method is used to model the intelligent material under analysis and has its function based on the exact solution of the wave equation, therefore, a single element is necessary to model the structure. The estimated responses for each shunt configuration express a relationship with the passive components used in the respective circuits. In each case, the circuit has a particular effect on FRFs.

Transient Contact Interaction of Solid Bodies and Thin Shells

Scipedia content — Thu, 11 Mar 2021 17:33:13 +0100

A spatial transient contact problem with moving boundaries for a thin elastic cylindrical shell and a rigid indenter bounded by a smooth convex surface is considered. A closed mathematical formulation is given and a system of resolving equations is constructed. The main integral equation follows from the principle of superposition and contact conditions. The core of this equation is the transient function for the cylindrical shell. To a closed system of resolving equations, it is supplemented by a kinematic relation for determining the moving boundary of the contact region and the equation of motion of the indenter as a rigid body. An algorithm for solving the spatial non-stationary contact problem for an infinitely long cylindrical shell and rigid indenter in the case of a normal impact on the side surface of the shell is constructed and implemented. Examples of calculations are given.

CFD Analysis to Demonstrate in Vessel Retention Inducing a Bypass Flow

Scipedia content — Thu, 11 Mar 2021 17:33:05 +0100

The goal of this work is to design a nozzle system around the Reactor Pressure Vessel (RPV) of a VVER-1000 nuclear reactor which would ensure successful melt retention during a severe accident. Simulations were performed with a multiphase model in ANSYS Fluent 19.1 to determine this. The results suggest that an efficient cooling can be achieved by inducing a flow rising parallel to the RPV walls in the flooded reactor cavity. In order to do this, it is proposed to use one central nozzle below the RPV and a ring of 32 nozzles 0.5 m above the RPV bottom. Assuming a 2 bar pressure, injection of 750 m³/h water at 80^oC through the nozzles led to little steam production and a heat flux below the critical value.

A Contribution to Predict the Structural Dynamics of Mounted Short Glass Fiber Reinforced Thermoplastic Components

Scipedia content — Thu, 11 Mar 2021 17:32:58 +0100

At the state of art, current simulative methods modelling joint properties as fixed interaction between components or concentrate on the characterization of the joint stiffness and damping of coupled metallic structures. In the current work, the developed thin layer interaction (TLI) method was applied to study the influence of the joint stiffness and damping of plastic components coupled with metallic structures. The joint stiffness and damping of different coupled metallic-plastic joint structures was characterized experimentally and was used to fill the reduced transverse shear stiffness and damping tensors of the numerical TLI model. The TLI model was used to determine the structural dynamics behavior of a short glass fiber reinforced plastic engine bracket mounted to a steel block. The comparison of experimental and simulative results shows a better correlation under the usage of the TLI model compared to a fixed modeled joint behavior. As result, the TLI model allows a better prediction of the structural dynamics of coupled plastic components. Further researches focus on the accurate experimental estimation of the boundary conditions regarding excitation and mounting of the plastic components. Thus, a more realistic modelling of the numerical joint boundary conditions is possible for an improved simulation of the structural dynamics of plastic components.

Validation of An Energy-Based Fatigue Life Model for Fibre Reinforced Plastics Under Different Stress Ratios

Scipedia content — Thu, 11 Mar 2021 17:32:50 +0100

The energy-based fatigue model presented in this work overcomes different shortcomings of existing model approaches, such as the need of separated assumptions for constant life diagrams. By using the range of the normalised strain energy density and a probabilistic based mode interaction approach, a failure mode dependent fatigue model for CFRP is established for directly predicting constant life diagrams and calculating the fatigue life for multiaxial loads with constant amplitude. In this contribution, the ply-based model and some of its main features, such as the consideration of residual stresses or of mode interactions at general threedimensional stress states, are shortly summarised. The stepwise model validation on different literature datasets is considered in more detail, including prediction of SN-curves with scatter band and constant life diagrams.

Hierarchical Parallelism for Transient Solid Mechanics Simulations

Scipedia content — Thu, 11 Mar 2021 17:32:42 +0100

Software development for high-performance scientific computing continues to evolve in response to increased parallelism and the advent of on-node accelerators, in particular GPUs. While these hardware advancements have the potential to significantly reduce turnaround times, they also present implementation and design challenges for engineering codes. We investigate the use of two strategies to mitigate these challenges: the Kokkos library for performance portability across disparate architectures, and the DARMA/vt library for asynchronous many-task scheduling. We investigate the application of Kokkos within the NimbleSM finite element code and the LAME constitutive model library. We explore the performance of DARMA/vt applied to NimbleSM contact mechanics algorithms. Software engineering strategies are discussed, followed by performance analyses of relevant solid mechanics simulations which demonstrate the promise of Kokkos and DARMA/vt for accelerated engineering simulators.

Base Isolated Buildings With Curved Surface Sliders Including Displacement Restraints

Scipedia content — Thu, 11 Mar 2021 17:32:33 +0100

Recent researches on seismic isolated structures are focused on the behaviour of isolation devices when extreme loading conditions are applied, with displacement demands higher than the design value [1]. Generally, the design displacement is considered as the collapse limit and no further capacity is taken into account [2]. This study investigates the effects of the presence of displacement restraints on the response of seismically isolated buildings based on Double Curved Concave Surface Sliders (DCCSS) for displacements larger than the design values. Two residential buildings case studies are considered with moment resistant steel or reinforced concrete frames both isolated with DCCSS [3] [4]. The isolated structures satisfy prescriptions of the current Italian seismic code considering the same seismic actions for the site of L'Aquila. Pushover analyses are carried out in order to estimate the engineering demand parameter (Superstructure Global Drift) limit values used for the definition of the superstructure failure condition. The effects of the impact of the isolation systems against retaining elements on the seismic response of isolated structures are highlighted. The results of multi-stripe analyses using twenty ground motions at ten different seismic intensities [5] are compared.

Isogeometric Analysis for Vehicle Model in Dynamic Explicit Method

Scipedia content — Thu, 11 Mar 2021 17:32:26 +0100

In Isogeometric Analysis, CAD format models can be used for analysis by using BSpline/NURBS, which is used for drawing shapes in CAD, as a shape function of the finite element method. This method can significantly reduce the time required to generate a mesh, which used to take much time. It is necessary to verify the performance of Isogeometric Analysis in the dynamic explicit method for application to automobile analysis. In this study, automobile structural parts were modeled by Isogeometric Analysis method and structural analysis was performed by the dynamic explicit method for evaluation.

Study On The Effects Of Adding Diagonal Springs In A Rigid Body Spring Model With Quadrilateral Elements

Scipedia content — Thu, 11 Mar 2021 17:32:18 +0100

An improved Rigid Body-Spring model (RBSM) for in-plane analyses is here proposed. The approach has been developed starting from the model presented by Casolo [4], according to which a continuum plate is discretized in an assembly of rigid quadrilateral elements connected by elasticplastic springs. In the proposed model, not only the elements that share one edge are connected, as it usually happens in RBSMs, but also the elements that share only one node, that are the elements on the diagonals. These elements are connected through an axial spring fixed to their centers of gravity. The spring stiffnesses have been obtained through the equivalence of the stored strain energy for a plane stress state. These, for an isotropic material, results to be function of the Young's modulus (E) and of the Poisson's ratio (). Thanks to the addition of the diagonal springs, this quadrilateral RBSM is capable of modeling isotropic materials with a Poisson's ratio different from zero. The effectiveness of the proposed computational method was proved comparing the elastic solution with a Finite element code for an uniaxial tensile test. About the failure response, on one hand, the addition of the diagonal springs introduces a new direction in which the failure condition is verified, reducing the intrinsic anisotropy of the failure response, typical of the discrete approaches [5, 14]. On the other hand, the new springs increase the complexity of the model and the spring constitutive behavior is no longer directly related to the material uni-axial behavior. In this work, the spring post elastic response was defined considering the mode I fracture energy of the material. In the end, the model was applied to the study of a concrete notched beam in a three point bending test. The results have been compared with the experimental one find in the literature. The analyses were executed with a dynamic explicit solver and different tensile spring behaviors were compared to highlight the importance of considering a bi-linear softening law for modeling the failure response of quasi-brittle materials like concrete.

Cad-Based Adaptive Shape Parametrisation for Aerodynamic Shape Optimisation

Scipedia content — Thu, 11 Mar 2021 17:32:10 +0100

Non-Uniform Rational B-Splines (NURBS) have become the industrial standard to represent and exchange a CAD geometry between CAD/CAE systems. CAD-based shape parameterisation uses parameters of a CAD model to modify the shape which allows to integrate a CAD model into the design loop. However, feature-trees of typical commercial CAD systems are not open and obtaining exact derivatives for gradient-based optimisation methods is not possible. Using the CAD-based NSPCC approach a designer can deform multiple NURBS patches in the design loop without violating geometric and/or thickness constraints. The NSPCC approach takes CAD descriptions as input and perturbs the control points of the NURBS boundary representation to modify the shape. In this work, an adaptive NSPCC method is proposed where the optimisation begins with a coarser design space and adapts to finer parametrisation during the design process where more shape control is needed. The refinement sensor is based on a comparison of smoothed node-based sensitivity compared to its projection onto the shape modes of the current parametrisation. Both static and adaptive parametrisation methods are coupled in the adjoint-based shape optimisation process to reduce the total pressure loss of a turbine blade internal cooling channel. The discrete adjoint flow solver STAMPS is used to compute the flow fields and their derivatives w.r.t. surface node displacements. The shape derivatives for gradient-based optimisation are obtained by application of reverse mode AD to the NSPCC CAD kernel. Since a CAD model is kept inside the design loop, the resulting optimal shape is directly available in CAD for further analysis or manufacturing. Based on the analysis regarding quality of the optima and rate of convergence of the design process adaptive NSPCC method outperforms static NSPCC approach.

Penetration Problem for Infrastructe Stability in Conditions of Offshore Fields Development

Scipedia content — Thu, 11 Mar 2021 17:32:02 +0100

The paper is generally devoted to the problem of offshore oil and gas fields' development. The main focus of the study is mechanical behavior of seafloor sediments and stability of infrastructure objects' supporting constructions. We study the problem of estimating the effective mechanic properties of seafloor sediments. To solve the problem, we propose the usage of gravity corer of bottom sediments. These corers make it possible to study dynamic elastic properties at the ship laboratory conditions. We study the sampling process itself as a tool to estimate the rheological properties of seafloor sediments. In the current study, we propose a specific analysis of the samples and sampling process. The sampling corer is equipped with tools providing an opportunity to measure its acceleration at each moment during the sampling process. This acceleration depends on controllable sampling process conditions and mechanical properties of seafloor sediments being sampled. A corresponding contact problem is considered using the finite-element method. It is shown that there is an opportunity to evaluate some (but not all) parameters for visco-elasto-plastic rheology of the sediments from a known acceleration of the sampling tube. The obtained results make it possible to improve the quality of model of mechanical properties of the seafloor sediments. This improvement provides the corresponding increase in mechanical modeling of infrastructure stability and decreases the risks accompanying offshore field development.

Development of a Nitridation Gas-Surface Boundary Condition for High-Fidelity Hypersonic Simulations

Scipedia content — Thu, 11 Mar 2021 17:31:54 +0100

Gas-surface interaction phenomena have a strong impact on the heat flux experienced by atmospheric entry bodies in the hypersonic regime. Numerically, they can be expressed as a boundary condition to be imposed to the Navier-Stokes equations to achieve predictive engineering simulations. The mass and energy conservation can be abstracted in a thin layer containing both the solid and the gas phases. Such a balance was implemented in the open source MUTATION++ library. It is convenient to easily plug verified models in any type of CFD solver to model the response of material surfaces. We have extended the library to accommodate a state-of-the-art nitridation and nitrogen recombination mechanisms derived from beam experiments. MUTATION++ was coupled to US3D, a high-fidelity finite-volume flow solver, to simulate an experimental campaign conducted in the VKI Plasmatron facility. The experiment consists in applying a subsonic high-enthalpy nitrogen flow over an axi-symmetric ablative material sample. The simulation results on the stagnation line were compared to those obtained using a one-dimensional solver. Both results showed good agreement, verifying the implementation of the boundary condition. The computational model predicts a lower mass blowing rate than the experimental value. The catalytic behaviour of the mechanism, in agreement with the beam experiment predictions, induces higher heat flux values than those expected for the testing conditions of the Plasmatron facility.

General Case of Movement of Solid System with Slider-Crank Mechanisms As Internal Movers On Rough Surface with Friction

Scipedia content — Thu, 11 Mar 2021 17:31:47 +0100

The equations of motion in the mathematical model of the solid system with slidercrank mechanisms as internal movers are obtained in the general case of translational-rotational motion on a rough plane with dry friction. Viscous friction with linear dissipation is also considered in the model.

Some Remarks on the a Posteriori Error Analysis of the Mixed Laplace Eigenvalue Problem

Scipedia content — Thu, 11 Mar 2021 17:31:39 +0100

In this note we consider the a posteriori error analysis of mixed finite element approximations to the Laplace eigenvalue problem based on local postprocessing. The estimator makes use of an improved L2approximation for the Raviart-Thomas (RT) and Brezzi-Douglas-Marini (BDM) finite element methods. For the BDM method we also obtain improved eigenvalue convergence for postprocessed eigenvalues. We verify the theoretical results in several numerical examples.

Knockdown Factors For Two Kinds of Stiffened Cylinders Under Axial Compression

Scipedia content — Thu, 11 Mar 2021 17:31:31 +0100

Stiffened cylinder is widely used in launch vehicle structures because its stiffener can prevent buckling wave to spread in the skin. However, it is still sensitive to shape imperfection more or less. Knockdown factors of stiffened cylinders have been researched all the time. However, comparison among different grid types is seldom. In this paper, both orthogrid stiffened cylinder and isogrid stiffened cylinder under axial compression are studied. Linear eigenvalue method and nonlinear implicit method are adopted to calculate the bearing capacities of the cylinders with ABAQUS. The sinusoidal function is used to model the imperfection. Given different imperfection parameters, the sensitive of bearing capacities to the shape imperfection is researched for two kinds of cylinders. Axial compression experiments are also implemented for the stiffened cylinders, and the results agree with numerical result. Knockdown factor of orthogrid cylinder is about 0.5~0.7. Meanwhile, it proves that when the stiffener is strong enough, the effects of shape imperfection on load capacity can be ignored for orthotropic grid, and its knockdown factor is nearly 1.

An Finite Volume Based Multigrid Preconditioner for DG-SEM for Convection-Diffusion

Scipedia content — Thu, 11 Mar 2021 17:31:23 +0100

time integration. We are motivated by three-dimensional unsteady compressible flow applications, which often result in large stiff systems. Implicit time integrators overcome the impact upon restrictive CFL conditions on explicit ones but leave the problem to solve huge nonlinear systems. In this paper we consider a multigrid preconditioning strategy for Jacobian-free Newton-Krylov (JFNK) methods for the solution of algebraic equation systems arising from implicit Discontinuous Galerkin (DG) discretizations. The preconditioner is defined by an auxiliary first order Finite Volume (FV) discretization that refines the original DG mesh, but can still be implemented algebraically. Different options exist to define the grid transfer between DG and FV. We suggest an ad hoc assignment of the unknowns as well as L₂ projections. We present new numerical results for the two-dimensional convection-diffusion equation in combination with the different transfer options, which demonstrate the quality and efficiency of the suggested preconditioner with regards to convergence speed up and CPU time. The suggested L₂ projection from this paper result in the best convergence speed up.

Comparison of Different Solvers and Geometry Representation Strategies for Dns of Rough Wall Channel Flow

Scipedia content — Thu, 11 Mar 2021 17:31:14 +0100

In the present study we investigate an incompressible turbulent channel flow with heat transfer at Re_τ = 180 with a deterministic surface topography consisting of truncated cones. Two solvers for each of the two boundary handling strategies are considered. With Nek5000 and OpenFOAM the influence of the roughness elements is directly accounted for by an unstructured body fitted mesh, whereas Xcompact3d and SIMSON utilize the immersed boundary method (IBM) to deal with the 3D geometry. The main focus of this work is on an evaluation of the usability of the IBM and a comparison of the parallel performance of the different solvers. Since usability is an ambiguous definition, various quantities are compared: global statistics like Nusselt number and friction coefficient, one-dimensional wall-normal profiles for first and second order statistics, as well as three-dimensional averages over roughness sections. In addition, the computational effort for each method is documented.

Efficiency and Accuracy of Simulated Microstructure Images Generated by Machine Learning

Scipedia content — Thu, 11 Mar 2021 17:31:07 +0100

Computer image analysis is a well-known method in material science, mechanical engineering and other branches of science and engineering. Application of the machine learning method in image processing delivers promising results, especially in images with a high level of noise and low contrast. Employing automatic classification, predictive models, simulation models deliver a huge benefit in research. The former model of science-based mainly on experiments slowly passing away. It is still an important part of research but, using advanced computer methods save time and allow significantly reduces the cost of studies. It begins new branch of material science and new research challenges, interdisciplinary filed of materials informatics, incorporating scientists exploring, mathematics, informatics and material science.

Topology Optimization of Elasto-Plastic Structures in Contact

Scipedia content — Thu, 11 Mar 2021 17:31:00 +0100

This paper is concerned with the analysis as well as the numerical solution of the structural optimization problem for bilateral frictional contact problem where the static elasto-plastic material model with linear kinematic hardening rather than elastic material model is assumed. The displacement and stress of the bodies in elasto-plastic contact with a given friction are governed by the system of the coupled variational inequalities. In these problems usually very high stress appear along the surfaces in contact. It leads to wear or fatigue of the contacting surfaces. Therefore the aim of the topological optimization is to find such distribution of the material filling the body in contact to minimize the contact stress. Using von Mises yield function as well as the regularization and penalization techniques the original system of variational inequalities is transformed into the system of coupled nonlinear equations. The derivative of the cost functional with respect to the perturbation of the domain occupied by the body in contact is calculated using the material derivative method. Finite element method is used as the discretization method. In the numerical computations generalized Newton method is used to solve this contact problem. The level set method is used to describe and govern the evolution of the domain shape in the design space where the direction of the evolution is determined based on the calculated shape derivative. The results of computation are provided and discussed.

A General Method to Compute Numerical Dispersion Error

Scipedia content — Thu, 11 Mar 2021 17:30:52 +0100

This article presents a new methodology to compute numerical dispersion error. The analysis here presented is not restricted to uniform structured meshes nor linear discrete operators as it does not rely on sinusoids to compute the associated error. When using uniform meshes, the results obtained with the present method collapse onto the obtained with the classic one via an easy change of basis. If non-uniform meshes are used, a new kind of results are obtained which shed some light onto the role stretching has on dispersion error.

Numerical Investigation of Oleo-Pneumatic Shock Absorber: Setup and Validation

Scipedia content — Thu, 11 Mar 2021 17:30:43 +0100

The simulation of an oleo-pneumatic shock absorber is discussed focusing on the solver validation and high fidelity case setup. The multi-physics nature of the problem is tackled by conducting a range of validation cases in the base areas expected to be of relevance. A dynamic system model of the shock absorber is used to generate physically consistent boundary conditions. In addition, steady RANS simulations provide a preliminary insight into the internal flow development and to assist in the design of higher resolution grids.

On The Mesh Divergence Of Inviscid Adjoint Solutions

Scipedia content — Thu, 11 Mar 2021 17:30:36 +0100

Certain two and three dimensional numerical solutions to the adjoint Euler equations have a value at and near the surface of wings and airfoils that depends strongly on the mesh density and which does not converge as the mesh is refined. The purpose of this paper is to characterize this problem and offer insights as to the possible explanation of this unusual behavior.

A Method of Parameters Identification for the Coupled Dry Friction Model for Pneumatic Tires

Scipedia content — Thu, 11 Mar 2021 17:30:28 +0100

The theory of multi-component dry friction accounting for the coupled kinematics of relative motion in finite contact spots is applied to the modelling of pneumatic tires. The analytical models of the combined dry friction accounting for the anisotropy of the dry friction factors as well as the distribution of the contact pressure close to the real one are introduced. The exact integral relationships between the dry friction force and torque and the generalized velocities, i. e. the speed of sliding and angular velocity of spinning, appears as a result of integration over the contact spot and are too complex to be implemented analytically into the engineering practice. The proposed approximate models are based on the fractional approximations and could be interpreted as rheological models with low number of constitutive constants. These constants could be identified after the solution of some specific inverse problems with input data given by the diagrams of the dry friction forces and torque obtained as a result of simple physical tests. Here these inverse problem are formulated and solved using the perturbed benchmark solutions of the corresponding direct problems with the factors obtained after the numerical solution of the contact problem for the heterogeneous pressurized tire. The possibility of stable solution for the inverse problems on the basis of the proposed approach is shown. The presented models as well as the method if constitutive constants identification could be applied for more detailed investigation of unsteady rolling regimes of pneumatics which are characterized by the non-vanishing sliding and spin.

Comparison of Lagrangian-Eulerian and Eulerian-Eulerian Approaches for Particle Laden Free Surface Flow by Means of Lattice Boltzmann Method

Scipedia content — Thu, 11 Mar 2021 17:30:20 +0100

The aim of this study is a comparison of Lagrangian-Eulerian and Eulerian-Eulerian numerical approach for the simulation of fluid-particles interaction. Within the study the immersed particles are restricted to have spherical shapes and are equal or smaller than the resolution of the computational mesh. The interaction between fluid and particles is performed using the immersed boundary method and the free surface flow of an incompressible fluid is simulated using the lattice Boltzmann method. Both approaches are compared within two test problems. Firstly, the swarm of particles falling in the fluid, and secondly, casting of the fluid with dispersed particles into a mold. Both tests showed good qualitative and quantitative agreement of mentioned approaches.

Vibration and Noise Identification of a Commercial Refrigerator

Scipedia content — Thu, 11 Mar 2021 17:30:12 +0100

The vibration transmission to the cooled goods within a refrigerator is a usually neglected field of interest. Previous studies however show that biochemical properties can be affected by continuous exposure to vibration. Optimal storage conditions necessitate protection against UV light and vibration to prevent the cooled good's quality being degraded by outside factors. Using operational transfer path analysis, we investigate different excitation sources that affect a commercial refrigerator and quantify the spectral input-distributions for all potential sources. Environmental influences, operational conditions and human interactions with both the refrigerator directly as well as indirectly via the environment were considered. Based on previous works, we developed a robust algorithm for the estimation of the varying transmission paths within the refrigerator and the close environment. Utilizing the measured and calculated data we investigate optimizations in design. We identified and charactarized the impact of different vibration-sources and used deflection shapes of the system and the results from the TPA to identify areas of interest. Finally, we investigate measures to shift resonance frequencies into spectral ranges with less excitation and to introduce additional damping.

Convolutional Neural Networks for Approximation of Internal Non-Newtonian Multiphase Flow Fields

Scipedia content — Thu, 11 Mar 2021 17:30:05 +0100

Neural networks (NNs) as an alternative method for universal approximation of differential equations have proven to be computationally efficient and still sufficiently accurate compared to established methods such as the finite volume method (FVM). Additionally, analysing weights and biases can give insights into the underlying physical laws. FVM and NNs are both based upon spacial discretisation. Since a Cartesian and equidistant grid is a raster graphics, image-to-image regression techniques can be used to predict phase velocity fields as well as particle and pressure distributions from simple mass flow boundary conditions. The impact of convolution layer depth and number of channels of a ConvolutionDeconvolution Regression Network (CDRN), on prediction performance of internal non-Newtownian multiphase flows is investigated. Parametric training data with 2055 sets is computed using FVM. To capture significant non-Newtownian effects of a particle-laden fluid (e.g. blood) flowing through small and non-straight channels, an Euler-Euler multiphase approach is used. The FVM results are normalized and mapped onto an equidistant grid as supervised learning target. The investigated NNs consist of n= {3, 5, 7} corresponding encoding/decoding blocks and different skip connections. Regardless of the convolution depth (i.e. number of blocks), the deepest spacial down-sampling via strided convolution is adjusted to result in a 1 × 1 × f · 2nfeature map, with f = {8, 16, 32}. The prediction performance expressed is as channel-averaged normalized root mean squared error (NRMSE). With a NRMSE of < 2 · 10-3, the best preforming NN has f = 32 initial feature maps, a kernel size of k = 4, n = 5 blocks and dense skip connections. Average inference time from this NN takes < 7 · 10-3s. Worst accuracy at NRMSE of approx 9 · 10-3is achieved without any skips, at k = 2, f = 16 and n = 3, but deployment takes only < 2 · 10-3s Given an adequate training, the prediction accuracy improves with convolution depth, where more features have higher impact on deeper NNs. Due to skip connections and batch normalisation, training is similarly efficient, regardless of the depth. This is further improved by blocks with dense connections, but at the price of a drastically larger model. Depending on geometrical complexity, spacial resolution is critical, as it increases the number of learnables and memory requirements massively.

Surrogate Modeling Based On Dynamic Numerical Simulation and Measurements for Fast Emulation

Scipedia content — Thu, 11 Mar 2021 17:29:57 +0100

Today, in many complex real-world systems, physics-based simulation models often provide sufficient precision but are computationally intensive. Machine learning surrogates, once trained, can achieve simulations by orders of magnitude faster than their original physical model without sacrificing much accuracy. In this paper, we present a surrogate model in form of a neural network that is fitted to a set of different time series. The time series data are generated partly by a physical model and partly by measurement. This is because a physical model is available only for a part of the entire state space that is to be modeled. This method is used to predict the flue gas temperature at the output of the evaporator in the heat recovery steam generator of a combined cycle power plant. For simulation we use a specialized in house tool for transient power plant processes, called 'Dynaplant'. The generated surrogate model is fast and captures the major dynamics. Consequently, the model can be used in applications where fast evaluation is required, e.g., in parallel to operation. One form of such usage is virtual sensors, whereby, physical detectors can be omitted, and thus costs are reduced. With this, we demonstrate a method that beneficially merges physical insight from simulation with reallife data and machine learning. Our findings are of interest to applications where either simulated or measured time series data or both of different operating points are available and a fast simulation model is required.

Numerical Simulation of Weakly Compressible Multiphase Flows with a Baer-Nunziato Type Model

Scipedia content — Thu, 11 Mar 2021 17:29:49 +0100

We present the results of the simulation of two-phase CO

Determination of Continuum Fracture Mechanics Parameters for Molecular Dynamics Simulations

Scipedia content — Thu, 11 Mar 2021 17:29:42 +0100

This study is aimed at determination of continuum fracture mechanics parameters such as stress intensity factors, T-stress and higher-order terms, from molecular dynamics simulation performed for copper plate with narrow diamond-like crack. For this purpose, an embedded atom potential (EAM) available in LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) molecular dynamics (MD) software is utilized to accurately describe mixed-mode crack growth. The stresses calculated from MD method using the virial theorem are used for over-deterministic method for extracting stress intensity factors, T-stress and higher order terms of Williams series expansion. It is shown that the algorithm of the over-deterministic method can be generalized for MD calculations and gives the reasonable value of continuum fracture mechanics parameters. The obtained value of stress intensity factors, T-stresses and higher order terms are compared with the theoretical values for an infinite plate with the central crack under Mode I loading and Mixed Mode loading conditions for several values of the mixity parameter which defines the type of Mixed Mode loading.

Analyses on Friction Stir Based Techniques to Join Lightweight Alloys to Thermoplastic Matrix Parts

Scipedia content — Thu, 11 Mar 2021 17:29:33 +0100

Nowadays, the manufacturing research efforts have to be conceived in such a way that the product performance criteria are achieved in a lightweighting design concept. Taking these extensions to their extreme, the material properties and the manufacturing solutions have to be considered together in a revolutionary body concept, which should result in an ideal sight to the use of the most performing material in the right place depending on the product requirements. Polymer matrix composites (PMCs) belong to this new material category. The development of joining techniques available to connect PMCs and lightweight alloys has been considered as a key enabling solution in making innovative and sustainable products. The goal of obtaining high joint efficiency must face two main problems, i.e. to deal with the polymeric matrices to get mechanical, physical and chemical compatibilities and to attain or preserve the integrity of reinforcements across the joints customizing the fiber distribution in the joining area. The understanding of current and emerging joining technologies, e.g. the friction stir based techniques, with an optimization of the process parameters needs performant numerical tools to be employed, efficiently. In the work herein proposed, a polymeric base plate was joined to an aluminum alloy part simulating the friction lap joint sequences. Numerical tests have been set by a commercial FE code (DEFORM 2DTM) and a DoE, generated using hypercube sampling, was defined to perform a sensitivity analysis of specific investigated variables on some process outputs.

Choosing the Subregions in Three-Level FROSch Preconditioners

Scipedia content — Thu, 11 Mar 2021 17:29:25 +0100

Different graph partitioning methods, i.e., linear partioning, parallel hypergraph (PHG) partioning, and two approaches using ParMETIS, are considered to generate an unstructured decomposition of the second-level coarse operator of three-level FROSch (Fast and Robust Overlapping Schwarz) preconditioners in the Trilinos software library. In our context, the parallel hypergraph method shows the most consistent results.

Enhancement of the Mixing Efficiency for a Steam Boiler Premix Channel with a Surrogate Based Optimization

Scipedia content — Thu, 11 Mar 2021 17:29:17 +0100

Global warming and the ever-increasing pollutants in the atmosphere force many governments to limit emissions. The use of methane as a fuel is widespread in the boiler industry, due to the low pollutant levels in its exhaust gas products. Nevertheless, in the combustion process, nitrogen and oxygen bind giving rise to a series of molecular compounds called NO x, which are considered pollutants because they react in the atmosphere causing the production of acid rain and reducing the level of ozone [1]. The aim of this work is to improve the mixture quality between fresh air, methane and recirculated exhaust gases introduced within Ecovapor Boiler's Mixing-Channel and, as consequence, to increase the combustion quality and limit the pollution production. The geometry is parameterized within Ansys Space Claim CAD software [2], and gas mixture flow is computed with Ansys Fluent solver [3]. To achieve these goals an automated shape optimization is adopted, which couples the Ansys Workbench environment to Dakota software [4]. In particular, a multi-objective genetic algorithm (MOGA) [5] combined with the Kriging response surface method is used, while the geometries are evaluated by solving for a compressible mixture of non-reacting gases the steady-state Reynolds Average NavierStokes (RANS) equations coupled with the kRealizable turbulence model [6].

An Adjoint Method for the Optimal Boundary Control of Turbulent Flows Modeled with the Rans System

Scipedia content — Thu, 11 Mar 2021 17:29:09 +0100

In recent years, the optimal control in fluid dynamics has gained attention for the design and the optimization of engineering devices. One of the main challenges concerns the application of the optimal control theory to turbulent flows modeled by the Reynolds averaging Navier-Stokes equations. In this work we propose the implementation of an optimal boundary control problem for the ReynoldsAveraged Navier-Stokes system closed with a two-equations turbulence model. The optimal boundary velocity is sought in order to achieve several objectives such as the enhancement of turbulence or the matching of the velocity field over a well defined domain region. The boundary where the control acts can be the main inlet section or additional injection holes placed along the domain. By minimizing the augmented Lagrangian functional we obtain the optimality system comprising the state, the adjoint, and the control equations. Furthermore, we propose numerical strategies that allow to solve the optimality system in a robust way for such a large number of unknowns.

Higher-Order Theories for Doubly Curved Laminated Lattice and Honeycomb Structures

Scipedia content — Thu, 11 Mar 2021 17:29:02 +0100

A collocation model based on a Generalized Differential Quadrature Method (GDQM) is proposed for the dynamic analysis of anisotropic curved laminated structures with a central lattice core and different external constraints. The theory is based on the Equivalent Single Layer (ESL) approach, together with higher-order kinematic assumptions. The reliability of the proposed method is checked with respect to classical 3D FEM-based solutions, for different shell geometries, lamination schemes and unit cell configurations. Based on the numerical investigation, the proposed formulation reveals to be computationally performing even for complicated shapes and structural members, compared to more expensive commercial finite-element-based packages.

New Scheme to Overcome Local Optima in the Voxel-Based Topology Optimization of Sheet Metal Structures

Scipedia content — Thu, 11 Mar 2021 17:28:55 +0100

Results from topology optimization will only be used in industry if manufacturing considerations have been taken into account. Ideally, the manufacturing processes are already integrated in the optimization process. Dienemann et. al [1] developed a scheme to optimize sheet metal structures by using the deep drawing manufacturing constraints. To do so a surface is calculated that represents the current sheet design and used to penalize the sensitivities of the voxel finite elements depending on their distance to it. This manufacturing constraint can cause the optimization to become stuck in a local optimum far off the global optimum. To prevent this, new methods that change the optimization process have been developed.

Mechanical Properties of the Cellular Structures Based On the Isotruss Unit Cell

Scipedia content — Thu, 11 Mar 2021 17:28:47 +0100

The present work aims to investigate the mechanical properties of the cellular structure based on the Isotruss unit cell by implementing the finite element method (FEM) introducing a representative elementary volume (REV). As an orthotropic cellular structure, the three principal Young's modulus along the longitudinal and transversal directions are evaluated. The influence of the geometrical parameters on the elastic properties is studied in detail. Results reveal that Young's modulus in two transverse directions are almost close, showing a 12% difference. Besides, the proposed unit cell presents the ability to tune the elasticity along with the main directions. It is seen that increasing the angle of helical pitch, increases the longitudinal Young's modulus while decreases the transverse ones that suggest a considerable possibility for optimal designs. As an example, mimicking Young's modulus of the human cortical bone as an orthotropic material is explored and it is seen that the presented cellular structure based on the Isotruss unit cell can successfully fit.

Characteristic-Based Volume Penalization Method for Compressible Flow Simulations on Unstructured Meshes

Scipedia content — Thu, 11 Mar 2021 17:28:39 +0100

The Characteristic-Based Volume Penalization (CBVP) method for numerical simulations of compressible flow over solid obstacles on unstructured meshes is presented. The approach belongs to the class of immersed boundary methods and is not relying on body-fitted meshes. Characteristic penalization terms, added to the compressible Navier-Stokes equations, are used to impose Dirichlet and Neumann boundary conditions on solid-fluid interface with an a priori defined accuracy. The details of numerical implementation, utilizing hybrid finitevolume method with high order edge-based reconstruction schemes in the flow region and loworder finite-difference approximation inside of the obstacle, are discussed. The developed algorithm provides the ability to perform calculations on grids of arbitrary type, including fully unstructured meshes. The efficiency of the characteristic based volume penalization method and its numerical implementation is demonstrated for shock wave reflection, acoustic pulse reflection and Couette flow problems. The results of CBVP simulations are compared with the numerical solutions of the same problems using Brinkman volume penalization method.

Solution Verification Studies of a Pressure-Based Compressible Flow Solver

Scipedia content — Thu, 11 Mar 2021 17:28:31 +0100

Although considering the fluid to be incompressible is a common and valid approximation in most hydrodynamic simulations, certain phenomena like sloshing or slamming involve compressibility effects. In order to capture such effects, the maritime CFD code ReFRESCO is being extended with a compressible flow solver for the air in two-phase flow simulations. The compressible Navier-Stokes equations, discretized with a cell-centered, collocated finite volume method, are solved with a pressurebased SIMPLE algorithm that is compatible with the incompressible flow solver and enforces pressurevelocity-density coupling with a pressure-correction equation and an equation of state. In this paper, the compressible solver is tested for subsonic, transonic and supersonic flow of an inviscid perfect gas in a channel with an arc circular bump. We confirm that the pressure-based solver can indeed achieve iterative convergence to levels close to machine accuracy for all three regimes, with moderate decrease of convergence rate at higher Mach numbers and on finer grids. Grid refinement studies are performed to determine its accuracy and show observed orders of grid convergence between one and two for different quantities and different convection schemes, with lowest order for the supersonic regime, as expected. Finally, we notice that monotonic grid convergence can be attained if the grids are refined far beyond the levels typically reported in open literature.

Numerical Simulation of Sloshing in LNG Tank Including Irregular Tank Motion Using Moving Grid Technique

Scipedia content — Thu, 11 Mar 2021 17:28:22 +0100

Numerical simulations of the sloshing phenomena of the liquid inside of a tank with the regular and irregular tank motions are carried out. An in-house structured CFD solver which is capable the moving grid technique is used. The governing equations are 3D Navier-Stokes equations for the incompressible flow. An interface capturing method based on the single phase level-set approach is employed to capture free surface of the liquid. The LNG tank shape which has the experimental data is utilized. The sway motion of the LNG tank is taken account by the grid velocity which is introduced by the amount of the displacement of the computational grid based on the tank motion. First, the regular sway motion is enforced to examine the occurrence of the swirling phenomena which the liquid rotates inside of the tank, and the time histories of the pressure at the positions on the tank wall are validated. Next, the irregular tank motion which is based on the RAO of the ship motion in irregular waves and the power spectrum of the wave is enforced, and the difference of the hydrodynamic forces of the tank between the regular and irregular motions are examined.

Influence of Geometrical Properties for the Calculation of a Pressure-Free Whole Heart Geometry

Scipedia content — Thu, 11 Mar 2021 17:28:14 +0100

Individualized computer models of the geometry of the human heart are often based on magnetic resonance images (MRI) or computed tomography (CT) scans. The stress distribution in the imaged state cannot be measured but needs to be estimated from the segmented geometry, e.g. by an iterative algorithm. As the convergence of this algorithm depends on different geometrical conditions, we systematically studied their influence. Beside various shape alterations, we investigated the chamber volume, as well as the effect of material parameters. We found a marked influence of passive material parameters: increasing the model stiffness by a factor of ten halved the residual norm in the first iteration. Flat and concave areas led to a reduced robustness and convergence rate of the unloading algorithm. With this study, the geometric effects and modeling aspects governing the unloading algorithm's convergence are identified and can be used as a basis for further improvement.

Hierarchical Modeling and Finite Element Analysis of Piezoelectric Energy Harvester From Structure-Piezoelectric-Circuit Interaction

Scipedia content — Thu, 11 Mar 2021 17:28:05 +0100

The piezoelectric energy harvesting devices for the conversion of mechanical vibration into electric energy via a flexible piezoelectric energy harvesting (FPED) structure have gained greater attention. Here, the large deformation of the FPED structure causes a strong interaction with the electric field (direct-piezoelectric effect) and structural field (inverse-piezoelectric effect), and vice-versa. Also an electrical circuit is attached to the electrodes covering the piezoelectric layers. This becomes a three-way coupling of the structure, the electromechanical effect of the piezoelectric material, and the electrical circuit. A mathematical and numerical model of the complex physical system of the involved multiphysics coupling characteristics in order to predict the operational properties and to increase the performance is very important. The presentation will discuss a partitioned coupling algorithm based hierarchical decomposition using finite element method for piezoelectric energy harvesting from structurepiezoelectric-circuit interaction. Results obtained with the finite element analysis are compared with the experimental results of PEHDs with base excitation reported in the literature.

Structure-Preserving Discretization in the Framework of a Discrete Model Problem for Large-Strain Thermo-Viscoelasticity

Scipedia content — Thu, 11 Mar 2021 17:27:57 +0100

The design of structure-preserving integrators is dealt with in the framework of a discrete model problem for large-strain thermo-viscoelasticity. The evolution equations pertaining to the model problem are cast in the framework of GENERIC. The GENERIC formalism provides an ideal foundation for the design of energy-momentum-entropy (EME) consistent time-stepping schemes. An analogous approach can be applied to the full continuum model for large-strain thermo-viscoelasticity.

Towards Multifidelity Models with Calibration for Turbulent Flows

Scipedia content — Thu, 11 Mar 2021 17:27:50 +0100

High-fidelity scale-resolving simulations of turbulent flows can be prohibitively expensive, especially at high Reynolds numbers. Therefore, multifidelity models (MFM) can be highly relevant for constructing predictive models for flow quantities of interest (QoIs), uncertainty quantification, and optimization. For numerical simulation of turbulence, there is a hierarchy of methodologies. On the other hand, there are calibration parameters in each of these methods which control the predictive accuracy of the resulting outputs. Compatible with these, the hierarchical MFM strategy which allows for simultaneous calibration of the model parameters as developed by Goh et al. [7] within a Bayesian framework is considered in the present study. The multifidelity model is applied to two cases related to wall-bounded turbulent flows. The examples are the prediction of friction at different Reynolds numbers in turbulent channel flow, and the prediction of aerodynamic coefficients for a range of angles of attack of a standard airfoil. In both cases, based on a few high-fidelity datasets, the MFM leads to accurate predictions of the QoIs as well as an estimation of uncertainty in the predictions.

A Computational Approach To Predict Warp Of Sawn Lumber Due To Residual Growth Stress In A Log

Scipedia content — Thu, 11 Mar 2021 17:21:53 +0100

A tree generates a complicated stress distribution inside the stem during its formation, which is called 'residual growth stress (RGS)'. The RGS often induces warp in a sawn lumber such as bow, crook, and other deformation, which causes severe losses of materials and benefits in the sawmill industry. Generally, warp becomes more serious in the lumber 'without pith' than in the lumber 'with pith'. In Japan, 50% of conifer plantations are ready to produce large-diameter logs of which diameter is more than 30 cm. The use of those logs inevitably leads to the production of the lumber 'without the pith', so we are concerned about the problems due to lumber warp in future sawmill industry using largediameter logs. Based on those practical background, this study presents a computational procedure to predict warp of the squared lumber, such as a beam, a bearer, and so forth, when those lumbers are sawn from the log and/or the thick plank. Using the derived procedure, 2-D patterns of the RGSs in the sawn lumbers and their changes during sawing processes are simulated. Simulated results will be experimentally verified by measuring the warp of the sawn lumber, as well as by measuring the RGS distributions in a log.

Numerical Simulation of Coupled Thermo-Hydro-Mechanical Processes in Freezing Soils

Scipedia content — Thu, 11 Mar 2021 17:21:45 +0100

In the study a thermo-hydro-mechanical model of freezing of saturated soil is presented, with focus on numerical simulation of artificial ground freezing (AGF). Artificial freezing of saturated soils induces such process in the soils as water migration, frost heave and consolidation which can have an effect on the freezing process and surrounding areas. To take into account the important from geotechnical point of view processes the thermo-hydromechanical model was developed. The model is based on the fundamental balance equations of continuum media mechanics. The Clausius-Clapeyron equation and constitutive relations of poromechanics are used for describing a relationship between pore pressure, temperature, stress and strain fields. Also an inelastic strain is included accounting for an effect of frost heave. The equations of the model were implemented in Comsol Multiphysics® software and solved using the finite element method relative to variables of porosity temperature and displacement. Numerical simulation of artificial freezing of a soil stratum for a vertical shaft sinking was carried out. A mesh convergence of numerical solution was analyzed. Results of the simulation have shown the model enables to describe a frozen wall formation with a coupled change of porosity, water pore pressure, volumetric strain and mean stress.

Energy Preserving Multiphase Flows: Application to Falling Films

Scipedia content — Thu, 11 Mar 2021 17:21:37 +0100

The numerical simulation of multiphase flows presents several challenges, namely the transport of different phases within de domain and the inclusion of capillary effects. Here, these are approached by enforcing a discrete physics-compatible solution. Extending our previous work on the discretization of surface tension [N. Valle, F. X. Trias, and J. Castro, 'An energy-preserving level set method for multiphase flows,' J. Comput. Phys., vol. 400, p. 108991, 2020] with a consistent mass and momentum transfer a fully energy-preserving multiphase flow method is presented. This numerical technique is showcased within the simulation of a falling film under several working conditions related to the normal operation of LiBr absorption chillers.

Eulerian Pressure-Velocity/ Lagrangian Vorticity-Velocity Coupling Applied to Wake and Forces Calculation of Biofouled Tidal Turbines

Scipedia content — Thu, 11 Mar 2021 17:21:30 +0100

Marine tidal turbines are subject to the environment in which they are deployed. In the natural environment, they are gradually colonized by sessile species. These fouling organisms modify the flow around the blades and in the wake of the tidal turbine. Unfortunately, they also complicate the numerical study of such tidal turbines by preventing the use of usual methods such as the Blade Element Method or the Lifting Line Theory. In this context, we propose to use an alternative solution, which combines an Eulerian code to study the near field with a Lagrangian code for the wake. After a short presentation of each code, the coupling method is detailed, and applied to the case of a tidal turbine with its own vertical axis. First results are shown and compared to a full Eulerian simulation. Although the data transmission between both codes works well, discrepancies were found due to abnormal increase of energy in the Lagrangian area. A solution is proposed and explained.

A Combination of the Scaled Boundary Finite Element Method with the Mortar Method

Scipedia content — Thu, 11 Mar 2021 17:21:22 +0100

Connecting different domains is one possibility to increase the performance of a numerical solution method. The Mortar Method is one of the well-established methods for this task. In this contribution, we focus on the solution of the elastodynamic wave equation by means of the scaled boundary finite element method and demonstrate that it is straightforward to connect different polygonal meshes by employing the Mortar Method in two dimensions. Examples show the stability for higher-order shape functions when performing h-refinement or p-refinement.

Resin Permeability Analysis For Woven Composites Using A Three-Scale Homogenization Method

Scipedia content — Thu, 11 Mar 2021 17:21:14 +0100

In this study, resin permeability of plain-woven composites is analyzed using a three-scale homogenization method. To this end, the three-scale homogenization method is developed to analyze the resin flow through both the structures of fiber bundles and the inside of fiber bundles, and also the mesoscopic and macroscopic resin permeability. To evaluate the validity of this method, resin permeability and characteristic flow velocity of plain-woven glass fiber-reinforced plastic (GFRP) are analyzed. The results show that the present method successfully analyzes the resin permeability of plain-woven composites.

Numerical Modelling of Pitting Corrosion in RC Structures

Scipedia content — Thu, 11 Mar 2021 17:21:06 +0100

During its life expectancy, a RC structure is exposed to corrosion. This phenomenon attacks
the reinforcement and lead to the creation of a third material that is rust, at the expense of steel.
This corrosion material takes more volume than the lost volume of steel and generates internal
stresses that lead to the deterioration of the steel-concrete interface and to the cracking of the
concrete cover. The distribution of the rust around the reinforcement for a natural corrosion is
non-uniform and irregular. This distribution is associated to corrosion “pits” that are localized
and concentrated in the regions exposed to corrosion. To better study and understand the
phenomenon of pitting corrosion, a 2D numerical model is adopted. This model associates the
corrosion of the reinforcement to a single pit, located at the top of the rebar. A model that take
into account the damaging of the concrete in compression and tension is used and an interface
between the two materials that models a tangential and normal contact is adopted.

Simulation of Helicopter Rotors On Unstructured Mixed Meshes Using Edge-Based Reconstruction Schemes

Scipedia content — Thu, 11 Mar 2021 17:20:58 +0100

The paper gives a detailed description of the developed numerical method for simulating the turbulent flow around a helicopter rotor and calculating its aerodynamic characteristics. The method is based on the original vertex-centered finite-volume EBR schemes. The features of these schemes are their higher accuracy that is achieved through the use of edge-based reconstruction of variables on extended quasi-one-dimensional stencils, and a moderate computational cost that allows for serial computations. When dealing with discontinuities or solutions with large gradients, a quasi-one-dimensional EBR-WENO scheme is used. The methods are implemented in the in-house code NOISEtte within the hybrid MPIOpenMP parallel model. Validation of the developed techniques is carried out by simulating several cases for hovering and forward-flight regimes of helicopter rotors.

Thermal Fluid Coupled Analysis of Hydrothermal Destruction Reactor

Scipedia content — Thu, 11 Mar 2021 17:20:49 +0100

Multi-regionally coupled analysis of thermal fluid flow and heat conduction of solid using OpenFOAM is carried out to clarify the behavior of hydrothermal oxidative destruction reactor of polychlorinated biphenyls (PCBs). Internal fluid of the reactor assumes a single-phase hot water without chemical reactions considering temperature dependence of thermophysical properties. Compressible Navier-Stokes equation with buoyancy force and energy equation with gravity term are alternately solved for the thermal fluid analysis. In order to consider conjugate heat transfer between the internal fluid and the reactor vessel, two-regionally coupled analysis of the fluid and vessel was executed by chtMultiRegionFoam solver in the OpenFOAM. To verify coupling effect, the multi-regionally coupled analyses results were compared with thermal fluid analysis of the internal fluid or heat conduction analysis of the vessel.

Fluid-Structure Interaction Analysis for Martian Exploration Parafoil with Deployable Structure by Coupling Library Precice

Scipedia content — Thu, 11 Mar 2021 17:20:40 +0100

trigger aerodynamic instability. Although instability has been reported in wind tunnel experiments and flight tests, the detailed mechanism of this possible instability has not been clarified. Therefore, it is necessary to elucidate the detailed instability mechanism by numerical analysis, as it cannot be realized experimentally. In addition, it is necessary to introduce fluid-structure interaction (FSI) analysis for flexible structures using a coupled method. The precise code interaction coupling environment (preCICE) coupling library is a powerful tool for the coupling analysis of fluid and structure solvers.
To evaluate the effect of coupling two physical fields, the analyses of the fluid and structure were conducted separately. The results of the analysis verified that the wing deformed under a fluid force, which indicates the effectiveness of the FSI analysis model developed. A comparison with the single-field analysis demonstrated that the structure-derived frequencies in the FSI analysis appeared in the wing surface deformation and aerodynamic forces. However, the aerodynamic
coefficients obtained by the FSI analysis converged to the same values as those obtained by the single fluid analysis, thus indicating that from a macroscopic perspective, structural deformation negligibly influences aerodynamic forces. Therefore, it is necessary to analyze a shape that is closer to the actual machine.

A Finite Element Method - Informed Neural Network For Uncertainty Quantification

Scipedia content — Thu, 11 Mar 2021 17:20:32 +0100

Sampling approaches for uncertainty quantification for real-world engineering problems are associated with large computational time and cost. This cost comes from the expensive deterministic simulation. Usage of surrogate models is a common way to overcome this issue in engineering applications. A conventional Neural Network (NN) can be used for building such surrogates. However, these neural networks are built based on input-output pairs. It is not possible to verify that the predicted output satisfies underlying physics. In this contribution, a physics-informed neural network based on a hybrid model of machine learning and classical Finite Element Method (FEM) is presented for forward propagation of uncertainty. The method uses FEM during both training and prediction stages. A surrogate model based on neural network for high dimensional problem is constructed by constraining the predictions of the neural network with the discretized partial differential equation of the system. During the training stage, the predicted solution from the FEM informed Neural Network(FEM-NN) is used to compute the residual using stiffness matrices and force vectors. This residual is used as a custom loss function from NN. This makes the whole training unsupervised as it does not require any output values. Hence, the need for expensive FEM solves is circumvented. The FEM-NN hybrid also gives an estimate of the accuracy of prediction by means of the calculated residual along with the prediction. The framework does not require mandatory expensive linear solves of the discretized equation instead substitutes the prediction from the neural network for computing the residual. This reduces the expensive training phase of the problem and can be applicable to real-world FEM simulations. The trained neural network is then sampled in a Monte Carlo (MC) manner to evaluate the statistics of the Quantities of Interest (QoI). The resulting FEM-NN hybrid is physics confirming and data-efficient. The efficacy of the framework is presented by a series of test case examples. The results are compared with classical MC results. The suitability of the method for the uncertainty quantification is studied and presented.

A Novel Inlet Model for Two-Phase Bubbly Axial Flow Through a Tube Bundle

Scipedia content — Thu, 11 Mar 2021 17:20:17 +0100

In numerical research about two-phase flow in tube bundles, mainly two trends are distinguishable. On the one hand, some studies start from the assumption that only relatively small bubbles occur in a continuous liquid flow, for which typically Eulerian-Lagrangian modelling is appropriate. This approach is not suitable when modelling flow-induced vibrations in tube bundles, as the most severe vibration conditions are achieved in intermittent, churn or slug flow. On the other hand, some studies focus on accurate numerical modelling of the bubble shape. In that case, it is customary to start from a bubble which is already present inside the domain. However, in order to simulate a sufficiently long flow time which is of interest in the current research, the bubbles should enter the domain through an inlet boundary. In this paper, a new inlet model is proposed which defines a transient inlet boundary condition to be applied in a subsequent Eulerian simulation, more specifically using the Volume-Of-Fluid method. In the first part of this paper, the inlet model is described. The model guarantees the introduction of a userspecified amount of gas during a set time-interval at locations in space and time that are chosen randomly. Subsequently, the model is tested on a 3x5 tube bundle subjected to an axially flowing air/water mixture. The computational time required to complete the inlet model is reported for different values of the most important model parameters. Finally, the computational effort of the new inlet model is compared to that of a simulation with a precursor-domain where bubbles are created by break-up of air jets imposed at the inlet.

The Impact of Laser Processing On the Performance Properties of Electro-Spark Coatings

Scipedia content — Thu, 11 Mar 2021 17:20:09 +0100

The main objective of the present work was to determine the influence of laser beam processing (LBP) on microstructure, microhardness, surface geometric structure, adhesion tests and tribological properties of coatings deposited on C45 carbon steel by the electro-spark deposition (ESD) process. The coatings were deposited by means of an EIL-8A. The laser processing was performed with an Nd:YAG, BLS 720 system. The studies were conducted using WC-Cu electrodes produced by the powder metallurgy route. The tests show the lasertreated electro-spark deposited WC-Cu coatings are characterized by higher adhesion and seizure resistance which come at the expense of lower microhardness. In addition, WC-Cu coatings after laser treatment had lower values of parameters of the surface geometric structure. The laser treatment process causes the homogenization of the chemical composition, structure refinement and healing of microcracks and pores of the electro-spark deposited coatings. Laser treated ESD coatings can be applied in sliding friction pairs and as protective coatings.

Parallel Numerical Solution of Two-Phase Flow in Porous Media On Non-Orthogonal Geometries: a Performance Study Using Different GPU Architectures

Scipedia content — Thu, 11 Mar 2021 17:20:01 +0100

A parallel numerical model for two phase flow (water and oil) in porous media on nonorthogonal geometries is solved by using different Graphics Processing Unit (GPU) architectures to carry out a comparison of the performance that can be reached by each of them. The mathematical model is based on the mass conservation transformed equations for water and oil phases, which results in two coupled non-linear partial differential equations (PDEs). The Finite Volume Method (FVM) is used to discretize the set of PDEs that govern this problem and the Newton-Raphson method is utilized to linearize and solve them simultaneously. Solution of the linear equations system is computationally expensive and requires a large amount of time as the number of unknowns increases. We take advantage of the current GPUs computing technology for constructing massive parallel numerical algorithms for modeling multi-phase flow in porous media [1, 2]. The construction of the Jacobian is directly done in the GPU, which reduces the information that needs to be exchanged between the CPU (Central Processing Unit) and the GPU. Libraries that include Krylov methods are used and tested. The numerical results indicate until 12x of speed up over a single CPU by applying the GPU parallelism with the different architectures tested in this study (Kepler, Pascal and Turing). Furthermore, this study also tries to identify which of these architectures is the best option according to our computing needs.

Mechanical Properties of Hip Capsule Tissue After a Hip Arthroplasty

Scipedia content — Thu, 11 Mar 2021 17:19:52 +0100

Total hip arthroplasty is a surgical procedure that replaces the hip joint by artificial materials. Here, the morphological and mechanical properties of the scar tissues that form around implants composed of either polymer and metal or ceramic are compared to native tissue removed during an initial total hip arthroplasty. Immuno-histological analyses of the samples showed different hierarchical structures of the tissues over three scales: the fiber, the fascicle and the tissue scales. At the tissue scale, micro-tensile tests were performed on millimetric samples and their non-linear elastic responses were identified by either an exponential law or an Ogden third-order constitutive model. At the fiber scale, a patient-specific micro-scale finite element model including the measured morphological parameters and the identified Ogden constitutive models for the fiber and for the matrix composed of a mixture of fibers in ground substance.

On the Modeling of Thin-Walled Member Assemblies Combining Shell and GBT-Based Beam Finite Elements: the Linear and Bifurcation Case

Scipedia content — Thu, 11 Mar 2021 17:19:44 +0100

In this paper, a general and efficient approach to model thin-walled members and frames with complex geometries (including tapered segments and holes). The approach combines shell and GBT-based (beam) finite elements, using each of them where it is most efficient: (i) shell elements in the plastic and geometrically complex zones, and (ii) GBT elements in the prismatic and elastic zones. To illustrate the capabilities and potential of the proposed approach, a set of numerical examples are presented, concerning linear, bifurcation (linear stability) and first-order plastic zone analyses. The examples analysed involve (i) members with tapered segments, (ii) members with holes and (iii) tapered beam-column assemblies. For validation and comparison purposes, full shell finite element solutions are provided and it is demonstrated that the proposed approach yields very accurate solutions in all cases, while involving much less DOFs.

Mixing and Combustion in Supersonic Near-Wall Shear Flows

Scipedia content — Thu, 11 Mar 2021 17:19:37 +0100

Mixing, ignition, and flame stabilization in a shear supersonic hydrogen-air flow is numerically studied with the RANS model of Spalart and Allmaras. A detailed chemical kinetics of hydrogen-air combustion is used in the calculations. The system of governing equations is solved with a hybrid explicit-implicit time marching scheme and the LUSGS method. Modelling of hydrogen injection into the M = 2.44 vitiated air flow in the model Burrows-Kurkov combustor is carried out for 2D and 3D cases. The numerical results obtained are compared with available experimental data.

Automatic and Sampling-Free Parametric Model Order Reduction of Vibro-Acoustic Systems

Scipedia content — Thu, 11 Mar 2021 17:19:29 +0100

Recently, a novel parametric model order reduction formulation has been derived for vibroacoustic systems that allows for the reduction of systems with low-rank parametric changes [1]. This scheme does not require sampling of the parameter space, in contrast to conventional parametric model reduction techniques. This means that a single reduction basis, obtained with conventional non-parametric model order reduction schemes, can be used for a wide range of parameter values. This is done by rewriting the system in a non-parametric form, in which the low-rank contributions act as inputs. A disadvantage of this scheme is that the size of the input matrix scales with the amount of chosen parameters, leading to a potentially large reduced basis when many parameters are considered. Therefore, in [2] an automatic Krylov reduction scheme has been proposed that utilizes the similarity in the reduced bases for inputs which are spaced closely together to still get a small reduced basis with a large number of inputs. This is done by using a combination of block second order Arnoldi with a singular value decomposition acting on the resulting basis. The algorithm includes an error estimator that uses a complementary approximation to calculate the error. The main advantages of this algorithm as compared to the commonly used iterative rational Krylov approach [3] are that only a small amount of system inversions are required and that the final reduced order model has the desired predefined relative error in the specified frequency band. In this paper the automatic Krylov reduction scheme and low-rank parametric model order reduction approach are combined and a suitable error estimator is derived, to arrive at compact but accurate parametric reduced order models. The effectiveness is shown with several examples.

Curvilinear Reconstructions for EBR Schemes On Semi-Structured Meshes

Scipedia content — Thu, 11 Mar 2021 17:19:21 +0100

The vertex-centered edge-based reconstruction (EBR) schemes for solving Eulertype equations are originally developed for tetrahedral unstructured meshes and imply the usage of quasi-one-dimensional straight-line reconstructions to calculate the flux. However, on the hybrid unstructured meshes with layers of highly anisotropic prismatic cells that are commonly used in simulations of turbulent flows with boundary layers, the straight-line reconstructions may lead to strongly irregular stencils, which cause both larger approximation errors and computational instabilities. To improve the accuracy and stability characteristics of EBR schemes on such meshes, we propose to use the curvilinear reconstructions for the flux computation. In this paper, we describe the principles and algorithms of constructing curvilinear reconstructions on the structured and semi-structured prismatic meshes. The modified EBR schemes are tested on the well-known NACA 0012 airfoil validation case for both two-dimensional and three-dimensional formulations, and the obtained numerical results are discussed in detail.

Effects of Span Morphing on Flutter Characteristics of Subsonic Wing

Scipedia content — Thu, 11 Mar 2021 17:19:13 +0100

This paper focuses on the dynamic behavior of variable-span morphing wings (VSMW) oscillating in pitch and plunge motions under subsonic flight conditions. The unswept cantilevered wing is modeled as three-stepped Euler-Bernoulli beam. The aerodynamic loads acting on the wing are represented by Theodorsen's unsteady aerodynamic theory. The differential equations of motion that describe the behavior of the dynamics of Euler-Bernoulli beam are derived through the Hamilton's principle. The differential transformation method (DTM) is implemented to equations of motion and boundary conditions. The solution of the aeroelastic system is obtained by the classical frequency domain solution, k-method. Goland wing and High-Altitude Long-Endurance (HALE) wing are used as the basis for this study. Prior to analyzing flutter characteristics of VSMW, validation cases are conducted to ensure that the developed algorithm works well. Furthermore, flutter speed and flutter frequency are analyzed for different elongation ratios of wing. There is a significant difference in flutter values of fully retracted and fully extended wing configurations. It can be concluded that both flutter speed and flutter frequency decrease dramatically as wing span extends. Another important finding is that the flutter speed and flutter frequency reductions are relatively high at the initial stages of wing span extension.