Direct and Large-Eddy Simulation VII

"LES Meets FSI – Important Numerical and Modeling Aspects (p. 245-246)

M. Breuer1,2 and M. Münsch2

1 Dept. of Fluid Mechanics, Institute of Mechanics, Helmut-Schmidt-University Hamburg, Holstenhofweg 85, D-22043 Hamburg, Germany, breuer@hsu-hh.de 2 Institute of Fluid Mechanics, University of Erlangen-N¨urnberg, Cauerstr. 4, D-1058 Erlangen, Germany, mmuensch@lstm.uni-erlangen.de

Abstract The paper is concerned with two main aspects, which should be considered when large–eddy simulation (LES) is married to ?uid–structure interaction (FSI). First, the in?uence of moving grids leading to temporally varying ?lter widths and thus additional commutation errors on the quality of the predicted results is thoroughly investigated. Second, a new partitioned coupling method based on the predictor–corrector scheme often used for LES is evaluated. A strongly coupled but nevertheless still explicit time–stepping algorithm results, which is very e?cient in the LES–FSI context. This new scheme is evaluated in detail based simulations around elastically supported cylindrical structures and a swiveling ?at plate.

1 Introduction

Fluid–structure interaction (FSI) plays a dominant role in many technical applications such as suspension bridges, o?-shore platforms or even vocal folds. Therefore, a strong need for appropriate numerical simulation tools exists for such coupled problems. In previous studies, FSI applications in the regime of laminar ?ows as well as turbulent ?ows using the RANS approach [5, 6] were numerically investigated. For that purpose, a partitioned fully implicit scheme was applied which coupled a three-dimensional ?nite-volume based multi-block ?ow solver for incompressible ?uids with a ?nite-element code for the structural problem.

This coupling scheme works e?ciently for large time step sizes typically used for implicit time-stepping schemes within RANS predictions. However, ?ow problems involving large-scale ?ow structures such as vortex shedding or instantaneous separation and reattachment are often not reliably predicted by RANS and more advanced techniques such as largeeddy simulation (LES) are required. To resolve the turbulent ?ow ?eld in time, LES uses small time steps.

Thus, in general explicit time-marching schemes are favored, especially predictor–corrector schemes [1,2]. Furthermore, for FSI applications the solution domain changes in time due to the displacement of the boundaries linked to the structure. Thus moving grids have to be used which has a direct in?uence on the ?ltering approach in LES. Thus the paper addresses the aspects of additional errors introduced (e.g., commutation errors) and code coupling, which should be considered when LES is married to FSI.

2 Important steps for joining LES and FSI

2.1 LES on moving grids

Within an FSI application the ?uid forces acting on the structure lead to the displacement or deformation of the structure. Thus the computational domain is no longer ?xed but changes in time. Besides other numerical techniques to account for these variations, the most popular one is the so-called Arbitrary Lagrangian–Eulerian (ALE) formulation. Here the conservation equations for mass, momentum (and energy) are re-formulated for a temporally varying domain."