Two Non-zonal Approaches to Accelerate RANS to LES Transition of Free Shear Layers in DES
We present two novel approaches to improve the behaviour of DES in the region where an attached boundary layer (handled with RANS) flows into a separated shear layer (to be resolved using LES). The approaches aim to be generally-applicable and retain the non-zonal nature of DES. Furthermore, the formulations are local and can be readily implemented in general-purpose solvers. One approach introduces an adaptive grid scale definition, sensitised to the local vorticity orientation. The second approach, which can be combined with the first, involves the incorporation of alternative SGS model formulations that discern between quasi 2D and developed 3D flow states. Both modifications lead to a strong reduction of eddy viscosity in the early shear layer. Consequently, a significant acceleration of RANS to LES transition is demonstrated for a plane shear layer, a backwards-facing step and a round jet, with results from two different flow solvers shown. The greatest improvement is seen when the approaches are applied in combination.
KeywordsShear Layer Eddy Viscosity Isotropic Turbulence Smagorinsky Model RANS Model
The research leading to these results has received funding from the European Union Seventh Framework Programme FP7/2007-2013 within the project Go4Hybrid (“Grey Area Mitigation for Hybrid RANS-LES Methods”) under grant agreement no. 605361. The authors from St. Petersburg acknowledge support from Boeing Commercial Airplanes and the Russian Scientific Foundation (Grant 14-11-00060). Fruitful discussions with Dr. J. Kok (NLR), including the recommendation of Ref. , are acknowledged with thanks.
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