Detached-Eddy Simulation (DES) is a hybrid turbulence modeling strategy, combining Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES), and attempts to take advantage of both techniques in regions where each is accurate and feasible. While DES has been well understood in massively separated flows, incorrect behavior can be encountered in flows with thick boundary layers and shallow separations. This motivated a detailed study of the grid sensitivity of the solutions in the flow over an Aerospatiale A-airfoil at an angle-of-attack of 13.3 degrees, and at a Reynolds number of 2.1 million. A new version of DES, which modifies the model length-scale to overcome errors arising from the interface between the RANS and LES regions was tested for the aforesaid flow and also for the flow over a circular cylinder at a Reynolds number of 8 million.
The other thrust of the current research is the transition from RANS to LES in the mean flow direction. In this scenario, a DES solution is characterized by a reduction in modeled stress, requiring a corresponding increase in resolved stress near the transition layer in order to avoid degrading predictions of skin friction. Seeding the near-wall flow with fluctuations can quickly generate the lacking unsteady content. In this light, a scheme that generates velocity fluctuations by the addition of a stochastic force to the momentum equations and subsequent tuning of the amplifications based on a prescribed shear stress profile has been developed. An adaptive control scheme has been used to shape the velocity fluctuations to mimic realistic flow features. The tests have been conducted using computations of turbulent channel flow at friction-velocity Reynolds numbers of 400 and 5000. Simulation results show that the method proposed is effective in achieving the desired results with the resolved shear stress reaching target levels at control planes. Simulations conducted also showed that the use of finer grids (with wall-parallel grid spacings 1/20th of the boundary layer thickness) reduced the error in the wall stress to less than 10% for a channel with the controllers taking up approximately 30% of the streamwise domain extent.
|School:||Arizona State University|
|School Location:||United States -- Arizona|
|Source:||DAI-B 69/07, Dissertation Abstracts International|
|Keywords:||Airfoil flow, Detached eddy simulation, Reynolds-averaged Navier-Stokes, Turbulence modeling|
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