Improved turbulence generation techniques for hybrid RANS/LES calculations

Hybrid turbulence simulations that use a combination of Reynolds-averaged Navier–Stokes (RANS) equations and large-eddy simulation (LES) are becoming increasingly popular to increase predictive accuracy in complex flow situations without the cost of full large-eddy simulations. In a typical application, the RANS equations are solved in the equilibrium or near-equilibrium regions of the flow, while LES equations are used in regions where the flow is away from equilibrium. A difficulty in performing hybrid simulations stems from the mismatch between resolved turbulent scales between the RANS and LES regions: in the RANS region all turbulent scales are modeled, while in the LES region the energy containing scales are resolved. Interfacing these two regions requires the generation of turbulent eddies capable of supporting the Reynolds stresses in the LES field. Previous work by the authors proposed a combination of synthetic turbulence and a controlled forcing method that was capable of quickly generating these eddies. In this paper, we extend this earlier work by investigating the physical meaning of the model parameters to understand their effect on the flow development. We find that by matching the time-averaging window to the local integral time scale of the flow improved results can be obtained compared with previous work. We also determined the range of the controller parameters that give robust results. The method was applied to non-equilibrium boundary-layer flows far from the calibration case and was found to perform well. This technique gives best results if an outer-flow perturbation drives the flow; when the inner-layer dynamics are critical, less rapid establishment of the turbulence is achieved. Even in the worst result, however, the controller was able to establish a realistic flow within five boundary layer thicknesses of the end of the controlled region.