Development of a coupled hybrid RANS-LES solver for wall-bounded turbulent flows

Most hybrid RANS-LES turbulence models suffer from mean stress depletion at the interface of the RANS and LES zones while simulating turbulent wall bounded flows. This issue in turn can lead to wrong estimation of skin friction distribution at the wall, and also adversely affect resolved turbulent stress profiles. In this work, we formulate a new hybid coupled RANS-LES (CRL) model, in which both LES equations and RANS equations are solved simultaneously on the same computational grid. Three zones, namely RANS zone, LES zone and hybrid zone, are defined in the flow domain, based on the distance from the wall. The hybrid zone is sandwiched between the near-wall RANS zone and free stream-LES zone. Effective eddy viscosity (i.e. from resolved+modeled turbulent stress) in the RANS and LES zones is calculated from the modeled and resolved stresses respectively. In the hybrid zone, a novel interpolation scheme is used to obtain the effective eddy viscosity. The mean stress obtained from the net effective eddy viscosity is then imposed in in the mean momentum equation the RANS and hybrid zones. This method essentially allows inputs from both LES and RANS to be blended smoothly in the near-wall hybrid zone, without the need for an extra zonal grid for RANS (e.g. unlike WMLES). Issues related to mean stress depletion are also successfully avoided in this method. We implement a flow solver based on this formulation in OpenFOAM and conduct simulations for turbulent channel flow and backward facing step. Comparison of the results against existing DNS/LES data indicates that the CRL solver is able to predict more accurate mean velocity profiles and resolved turbulent stress profiles compared to DES solvers.