Assessment and Improvement of Zonal Grey Area Mitigation Methods - Towards a faster RANS-to-LES transition

Abstract

Hybrid RANS-LES methods have become a popular numerical approach for a wide variety of flows. This is due to dissatisfaction with the RANS modelling paradigm in separated flows along with the prohibitive computational cost of pure LES, especially in wall-bounded flows at high Reynolds numbers. However, these methods are susceptible to the grey area problem, where the modeling approach is neither RANS nor LES: rather it is a region with an ambiguous modeling approach. In zonal approaches that function as embedded wall-modeled LES (WMLES), the transition from RANS to LES can be accelerated by improving the synthetic turbulence and its injection into the flow. In this work, a systemic assessment of the two aspects of zonal grey area mitigation methods was carried out. The synthetic turbulence was generated by the synthetic turbulence generator (STG) and injected into the flow using two different forcing terms. To ensure accurate second-order statistics of synthetic turbulence, a priori estimations of the bias error associated with a specific realization of a random number set were implemented and used. This resulted in smaller deviations between the statistics of the synthetic turbulence and the target Reynolds stresses. Furthermore, a modified synthetic turbulence forcing that ensures more accurate estimation of the total shear stress in close proximity to the RANS-LES interface was proposed. Moreover, a dynamic forcing that selectively enhances the production of underestimated Reynolds stresses was implemented and evaluated. These aspects resulted in a faster transition from RANS to LES in terms of both skin friction coefficients and Reynolds stresses. In addition, the WMLES capabilities of the subgrid length scale together with the subgrid-scale sigma-model were explored. This work revealed that this combination is troublesome when used as embedded WMLES with synthetic turbulence, especially in stable flows. This is due to excessively decreased levels of eddy viscosity in the near-wall RANS region.