HYBRID RANS-LES MODELLING OF TURBULENT FLOWS WITH ADAPTIVE DOWNSTREAM ANISOTROPIC EDDY VISCOSITY

Turbulent flow pervades many engineering applications, and its prediction remains a challenge. One promising predictive method combines large-eddy simulation (LES) with simulation based on Reynolds Averaged Navier-Stokes equations (RANS). This study presents two methods to overcome stability and accuracy issues associated with LES-RANS simulations. One method developed first extracts the anisotropic eddy viscosity from the upstream LES solution at the RANS-LES interface and then uses that information to improve the downstream RANS model by invoking the weak-equilibrium assumption. That method was evaluated via two test problems – flow in a channel and over a periodic hill. Since that method depends on the choice of the coordinate system, a second method was developed that involve extracting Reynolds stresses from the upstream LES solution and then using that information to reconstruct and convert the downstream linear RANS model with a scalar eddy viscosity to a nonlinear RANS model with an effective anisotropic eddy viscosity. The second method developed was evaluated by computing film cooling of a flat plate with the coolant injected through a row of circular holes. Results obtained by both methods show instabilities at the LES-to-RANS interface to be eliminated. Results obtained also show the LES-RANS method developed can yield solutions almost as accurate as those from LES, even though a significant portion of the flow is computed by the adapted anisotropic RANS model instead of LES, which greatly increases computational efficiency. Since the modification of the downstream RANS model is based on information extracted from the upstream LES solution, the method developed is adaptive to the problem being studied.