Hybrid RANS-LES modeling of high-Re turbulent flows

Typically in hybrid methods, a Reynolds-Averaged Navier-Stokes (RANS) simulation is performed near the wall and a large-eddy simulation (LES) is performed elsewhere. In a zonal formulation of hybrid methods, two separate eddy viscosity transport equations---one for RANS and one for LES---are solved. The RANS and LES fields are then combined at a pre-determined near-wall location by matching a secondary quantity such as the wall shear stress or eddy viscosity. In a non-zonal method, the same eddy viscosity transport equation is used for both RANS and LES and the switch between them is effected by modifying the length scale of the destruction term in the eddy viscosity transport equation. An important example for the non-zonal method is the detached eddy simulation (DES) and this will be the one of the major focuses of this study. Just as in zonal methods, the original version of the DES also implemented a pre-determined single point transition between the RANS and LES. Single point transitions are now generally accepted to be ineffective in momentum transport. Furthermore, they create an area near the interface, where the total Reynolds stress is neither supported by the model nor by the resolution.;During the initial phase of this study, we focussed on improving this deficiency of DES. Our attempts were aimed at redefining the DES coefficient so that the RANS/LES interface evolves with the flow. However, while this work was in progress a new method known as delayed DES (DDES) was published with the same idea in principle as our work but without the use of any ad hoc criteria that restricted the universal usage of our method. We also devised a new improvement to DES known as the buffered interface method. This method was successfully applied to several flows.;In the final phase of this study, we concentrated on a very different approach to hybrid simulation. In this method, a hybrid filter H is formed by additively blending a RANS (E) and an LES ( F) operator using a blending function k as: H = kF + (1 -- k) E. After performing an a priori test to assess the model we concentrated on the full numerical simulation of it, for channel flows. Towards this end, we modified the governing Hybrid filtered Navier-Stokes equations suitably, reformulated the fractional step method and devised appropriate blending functions. The results of the full simulation reinforced many of the advantages seen in the a priori tests.