Computational Fluid Dynamics and Aeroacoustics

Large Eddy Simulation (LES)

Vortex breakdown on a delta wing at angle of attack.
(click to enlarge)

Large Eddy Simulation (LES) is a promising technique for the prediction and analysis of unsteady flows.

This technique, contrary to Direct Numerical Simulation (DNS) that represents all the vortices present in a flow without approximation, is based on a separation of scales. Thus the vortices larger than an arbitrarily set limit are determined by the calculation, whereas the others are taken into account by means of a statistical model called the subgrid scale model. The quality of the results is conditioned by that of the subgrid scale model and by the numerical schema's error, which must be checked.

As the use of LES for simulating realistic vortex flows (geometry, Reynolds number, etc.) often proves to be too costly in computing time, the aim of current research is to reduce the cost of this approach. The work is centered on optimizing the numerical methods (hybrid spatial schema, implicit temporal integration, adaptative meshing, etc.), on developing physical models adapted to the compressible, incompressible and multi-physics type flows, on extending towards very large scale simulation (RANS/LES coupling) and on modeling limit conditions (unsteady vortex conditions, wall model).

In the current state of methods, models and supercomputers, LES is already a complementary tool to experimental measurement techniques. With it, we have been able gain a better understanding of certain fundamental aerodynamic phenomena: vortex breakdown (delta wing), unsteady transition for turbine engines (see animation below), vortex wakes, aero acoustics, aero-optics, active control, among others.

Numerical simulation of the transitional flow
around a fixed blade of a low pressure turbine
influenced by the wake of an upstream rotor.
Animations: mpeg 265 ko, gif 740 ko

Last Update: 13 October 2006 - © ONERA 2009 - Terms of use