Abstract

In this paper a fluid-structure algorithm for numerical simulation of wind tunnel aeroelastic analyses is proposed. The flow equations are resolved using the artificial compressibility approach and the explicit two-step Taylor-Galerkin scheme in the context of the Finite Element Method (FEM), where eight-point hexahedrons with one-point quadrature and hourglass control are adopted. Turbulence is analyzed using Large Eddy Simulation (LES) with the classical Smagorinsky’s sub-grid scale model. A partitioned coupling model is adopted for fluid-structure interaction considering the three-dimensional rigid body motion approach on the structural field. Numerical results obtained here are compared with wind tunnel predictions on the CAARC tall building model and bridge sectional models to validate the present scheme.

Abstract

In the present work, an investigation on the Characteristic Based Split (CBS) method applied to the analysis of incompressible, transient and turbulent flows is proposed aiming at the evaluation of computational efficiency and accuracy of the present formulation considering problems typically found in Computational Wind Engineering (CWE). The flow fundamental equations are discretized here using the CBS method in the context of the Finite Element Method (FEM), where triangular and linear tetrahedral elements are employed. A fully explicit scheme is obtained applying the artificial compressibility method and a semi-implicit scheme is also tested, where the momentum equations are solved explicitly while the mass equation is solved implicitly through a Poisson equation. For the analysis of turbulent flows Large Eddy Simulation (LES) is adopted with the classic Smagorinsky’s model for sub-grid scales. Some classical Computational Fluid Dynamics (CDF) problems are analyzed first for code verification and evaluation. Subsequently, the numerical model is validated considering typical CWE applications, such as the wind action on a large-span bridge deck and the boundary-layer flow over a hill model. It is concluded that the proposed numerical tool is able to obtain results consistent with the phenomena involved and more efficiently than other stabilization methods available.

Abstract

In this work we study the performance of some variational multiscale models (VMS) in the large eddy simulation (LES) of turbulent flows. We consider VMS models obtained by different subgrid scale approximations which include either static or dynamic subscales, linear or nonlinear multiscale splitting, and different choices of the subscale space. After a brief review of these models, we discuss some implementation aspects particularly relevant to the simulation of turbulent flows, namely the use of a skew symmetric form of the convective term and the computation of projections when orthogonal subscales are used. We analyze the energy conservation (and numerical dissipation) of the alternative VMS formulations, which is numerically evaluated. In the numerical study, we have considered three well known problems: the decay of homogeneous isotropic turbulence, the Taylor–Green vortex problem and the turbulent flow in a channel. We compare the results obtained using different VMS models, paying special attention to the effect of using orthogonal subscale spaces. The VMS results are also compared against classical LES scheme based on filtering and the dynamic Smagorinsky closure. Altogether, our results show the tremendous potential of VMS for the numerical simulation of turbulence. Further, we study the sensitivity of VMS to the algorithmic constants and analyze the behavior in the small time step limit. We have also carried out a computational cost comparison of the different formulations. Out of these experiments, we can state that the numerical results obtained with the different VMS formulations (as far as they converge) are quite similar. However, some choices are prone to instabilities and the results obtained in terms of computational cost are certainly different. The dynamic orthogonal subscales model turns out to be best in terms of efficiency and robustness.

Abstract

A numerical approximation for the one‐dimensional Burgers equation is proposed by means of the orthogonal subgrid scales–variational multiscale (OSGS‐VMS) method. We evaluate the role of the variational subscales in describing the Burgers “turbulence” phenomena. Particularly, we seek to clarify the interaction between the subscales and the resolved scales when the former are defined to be orthogonal to the finite‐dimensional space. Direct numerical simulation is used to evaluate the resulting OSGS‐VMS energy spectra. The comparison against a large eddy simulation model is presented for numerical discretizations in which the grid is not capable of solving the small scales. An accurate approximation to the phenomena of turbulence is obtained with the addition of the purely dissipative numerical terms given by the OSGS‐VMS method without any modification of the continuous problem.

Abstract

A sibilant fricative /s/ is generated when the turbulent jet in the narrow channel between the tongue blade and the hard palate is deflected downwards through the space between the upper and lower incisors and then impinges the space between the lower incisors and the lower lip. The flow eddies in that region become a source of direct aerodynamic sound, which is also diffracted by the speech articulators and radiated outwards. The numerical simulation of these phenomena is complex. The spectrum of an /s/ typically peaks between 4 and 10 kHz, which implies that very fine computational meshes are needed to capture the eddies producing such high frequencies. In this work, a large‐scale computation of the aeroacoustics of /s/ has been performed for a realistic vocal tract geometry, resorting to two different acoustic analogies. A stabilized finite element method that acts as a large eddy simulation model has been adopted to solve the flow dynamics. Also, a numerical strategy has been implemented that allows the determination, in a single computational run, of the separate contribution of the sound diffracted by the upper incisors from the overall radiated sound. Results are presented for points located close to the lip opening showing the relative influence of the upper teeth depending on frequency.

Abstract

The variational multiscale method thought as an implicit large eddy simulation model for turbulent flows has been shown to be an alternative to the widely used physical-based models. This method is traditionally combined with equal-order velocity–pressure pairs, since it provides pressure stabilization. In this work, we consider a different approach, based on inf–sup stable elements and convection-only stabilization. In order to do so, we consider a symmetric projection stabilization of the convective term using an orthogonal subscale decomposition. The accuracy and efficiency of this method compared with residual-based algebraic subgrid scales and orthogonal subscales methods for equal-order interpolation is assessed in this paper. Moreover, when inf–sup stable elements are used, the grad–div stabilization term has been shown to be essential to guarantee accurate solutions. Hence, a study of the influence of such term in the large eddy simulation of turbulent incompressible flows is also performed. Furthermore, a recursive block preconditioning strategy has been considered for the resolution of the problem with an implicit treatment of the projection terms. Two different benchmark tests have been solved: the Taylor–Green Vortex flow with Re=1600, and the Turbulent Channel Flow at Reτ=395 and Reτ=590.