LES in complex geometries using boundary body forces

AIAA paper, 2002

A computational study of unsteady, separated, low-Reynolds number fluid flow is made using a second-order accurate, cell-centered finite-volume method on unstructured grids. The three-dimensional simulations include largeeddy simulation turbulence modeling using a fixed-coefficient Smagorinsky subgrid-scale model. The flow about a three-dimensional circular cylinder at Reynolds number 3900 and Mach number 0.2 was studied using a standard Navier-Stokes formulation with and without the subgrid-scale modeling. A third case was also tested using a modified form of the Navier-Stokes equations with subgrid-scale modeling. Comparisons are made with experiment and selected three-dimensional numerical calculations performed on structured grids. The frequency of the unsteady vortex shedding was in good agreement with experiment, however, the numerical dissipation inherent in the upwind flux calculations affected the details of the time-averaged flowfield. The results showed an increased base pressure, shortened recirculation zones in the cylinder wake, and excessive drag compared to experiment for all cases.

2014

International Journal of Heat and Fluid Flow, 1998

In the present work the turbulent¯ow past a circular cylinder (Re 3900) was computed by large eddy simulation (LES). The objective was not to investigate the physical phenomena of this¯ow in detail but to study numerical as well as modeling aspects which in¯uence the quality of LES solutions. Concerning the numerical method the most important component is the discretization of the non-linear convective¯uxes. Five dierent schemes were investigated. Moreover, the in¯uence of dierent grid resolutions was examined. On the modeling side two aspects play an important role, namely the near-wall model and the subgrid scale (SGS) model. Owing to the restriction to a low Reynolds number in this study, no-slip boundary conditions were used at solid walls. Hence only the second aspect was taken into account. Two dierent subgrid scale models (Smagorinsky and dynamic model) were applied. Additionally, LES computations without any subgrid scale modeling were carried out in order to prove the performance of the above mentioned SGS models. Ó 1998 Elsevier Science Inc. All rights reserved.

International Journal for Numerical Methods in Fluids, 2004

Large scale unsteady motions in many practical engineering ows play a very important role and it is very unlikely that these unsteady ow features can be captured within the framework of Reynolds averaged Navier-Stokes approach. Large-eddy simulation (LES) has become, arguably, the only practical numerical tool for predicting those ows more accurately since it is still not realistic to apply DNS to practical engineering ows with the current and near future available computing power. Numerical methods for the LES of turbulent ows in complex geometry have been developed and applied to predict practical engineering ows successfully. The method is based on body-ÿtted curvilinear coordinates with the contravariant velocity components of the general Navier-Stokes equations discretized on a staggered orthogonal mesh. For incompressible ow simulations the main source of computational expense is due to the solution of a Poisson equation for pressure. This is especially true for ows in complex geometry. A multigrid 3D pressure solver is developed to speed up the solution. In addition, the Poisson equation for pressure takes a simpler form with no cross-derivatives when orthogonal mesh is used and hence resulting in increased convergence rate and producing more accurate solutions. Copyright ? 2004 John Wiley & Sons, Ltd.

1999

In this computation, Smagorinsky's eddy viscosity model combined with a truncated deductive model is used to represent the SGS stress fields in quadrilateral/hexahedral structured/unstructured grid system. The truncated deductive model considers the eddies immediately below the filter size, ∆, which presumably lies within the inertial range, but not those near Kolmogorov scale, η, such that dissipation is not properly accounted for. Therefore it is assumed in this study that the Smagorinsky model describes the dissipation correctly, and the truncated deductive model ensures a smooth cascade process. To show the applicability of the proposed SGS model to the structured/unstructured grid system, two cases of the flow about a yawed cylinder and a cavity flow were numerically tested and compared with experimental data. The present results of the yawed cylinder were produced at a Reynolds number of 1.3×10 4 in the sub-critical range. As a second example of validating the code, three configurations of cavity (L/D=1.5, 2.0, 10.0) were used to simulate quasi-periodic flow patterns at a Reynolds number of 1.0×10 4. The sound generated from turbulent flows around the yawed cylinder and in the cavity was also computed using Lighthill's acoustic analogy. The LES results are compared to experimental data in terms of local characteristics such as base pressure coefficients, wall pressure and far-field noise spectra, which were measured in an acoustic wind-tunnel facility.

A grid-free numerical method based on the Lagrangian random vortex element and boundary element methods has been developed for the simulation of unsteady flow in three-dimensional domains with moving boundaries of the type observed in IC engines. For this purpose, the computational domain is decomposed into a thin region near the boundary and a remaining domain interior. The evolution of vorticity in the interior is obtained by the Lagrangian vortex element method. The vorticity in the boundary region i s discretized using thin vortex tiles, and its evolution is obtained b y a Prandtl-like approximation of the governing equations. Simulations using this approach have thus far been limited to flow in simple geometries, because tile-tile interactions were formulated for rectangular tiles only. In this paper, we present an extended tile generation and evolution mechanism which allows inter-tile interaction for any polygon shape for the tiles. This development sets the stage for obtaining grid-free simulations of flow i n complex engine geometries. We demonstrate the feasibility of the new formulation using an example problem of intake flow inside a cylinder connected to a harmonically driven piston and an offcentered, tilted port at its ends -with and without a valve.

2009

This paper deals with the three-dimensional simulation of the unsteady flow around a stationary 1:5 rectangular cylinder at zero-degree angle of attack, low Mach number and relatively high Reynolds number. The computations have been performed using the DLR-Tau code, a non-commercial finite-volume code developed at the German Aerospace Center, and results obtained with a hybrid mesh are validated against the available experimental data, showing good agreement. Detached-Eddy Simulation (DES), that is a hybrid method combining Large-Eddy Simulation (LES) and Raynolds-Averaged Navier-Stokes (RANS) approaches, has been adopted as strategy of turbulence modelling. The comparison with respect to unsteady RANS results, although obtained with advanced turbulence models, show the improvement that can be expected with the DES technique for this type of massively separated bluff-body flows. The paper also clarifies the key role played in LES, and therefore also in DES simulations, by the artificial dissipation characterizing the numerical scheme used to discretize the filtered Navier-Stokes equations. Finally, the discussed results highlight the effects of the spanwise extension of the computational domain. A distance between the periodic boundary planes equal to the width of the cylinder is not enough to allow the natural loss of correlation of pressures and the free development of large-scale turbulent structures. In contrast, a span equal to the double of the cylinder width fulfill much better this requirement.

Int. J. Numer. Meth. Fluids, 2011

Large eddy simulation (LES) methodology in curvilinear coordinates is presented in this study, where filtering is performed prior to the transformation in contrast to the alternate approach proposed by Jordan [J. Comput. Phys. 1999; 148:322-340], where filtering is done after the transformation. The main objective of the present study is to elaborate on the problems mentioned by Jordan with the conventional approach, and its consequences for the overall accuracy. The paper also provides remedies to the problems associated with the conventional approach. The present method treats the Leonard term in physical space rather than in computational space. Some practical difficulties associated with the alternate approach are also considered. Main advantages with the present conventional approach are: first an existing Reynolds averaged Navier-Stokes code can easily be extended for LES and second a wide range of existing advanced subgrid stress model can be used without any modifications. Filtering in physical space introduces a commutation error between filtering and differentiation due to non-uniform meshes. The commutation filters in generalized coordinates are presented. LESs of four standard test cases are carried out to validate the present methodology. The four standard test cases are: (i) a lid-driven cavity at Reynolds number (Re) of 12 000, (ii) a pipe flow at a friction Reynolds number Re of 360, (iii) a backward facing step at Re = 5100, and (iv) an axisymmetric confined dump combustor at Re = 11 000. The results are validated against the reference data (direct numerical simulation or experiments). The mean velocity profiles and the turbulence intensities compared satisfactorily with the reference data. unsteady turbulent flow at comparatively reduced costs. In LES, the large geometrically dependent, energy-carrying eddies are resolved on grid scales, whereas the dynamics of the smaller, more universal scales are modeled using a subgrid stress (SGS) model.

2002

Orthogonal, structured grids allow flow simulations in simple geometries with high efficiency and accuracy. In contrast, complex and realistic flow problems have traditionally required the use of curvilinear or unstructured meshes, which require large computational costs and reduced accuracy due to limited grid smoothness and orthogonality. In recent years an alternative approach which combines the advantages of simple Cartesian grids with the ability to deal with complex geometries has been developed.

International Journal of Offshore and Polar Engineering, 2015

This study focuses on the numerical simulations of the flow around tandem cylinders placed in the vicinity of a rigid, horizontal plane wall. The cylinders are immersed close to a plane wall in a steady current with a logarithmic boundary layer profile at an intermediate, subcritical Reynolds number (Re = 1 31 × 10 4). The distance between the cylinder centers is L/D = 2 and 5, and the gap between the cylinders and the wall is G/D = 1. Three-dimensional large eddy simulations (LES) with a Smagorinsky subgrid scale model are performed using the open-source code OpenFOAM. The present results are analyzed through the values of drag and lift coefficients, as well as by the details of the flow fields in the near wake of the cylinders. The results are qualitatively compared to the results of the flow around tandem cylinders for both L/D = 2 and 5, as well as to the case of a single cylinder near a plane wall.