Performance of Reynolds Averaged Navier Stokes Models for Unsteady Separated flows

Unsteady separation is a phenomenon that occurs in many flows due to time-varying adverse pressure gradients, and results in increased drag, decreased lift, and loss of efficiency or failure in flow devices. Therefore, it is important to predict and analyze unsteady separation. RANS models are commonly used in the industrial design process due to their low computational cost; however, their performance in predicting separations is unsatisfactory. Our goal is to use high-fidelity Large-Eddy Simulation results carried out by Ambrogi et al. [J. Fluid Mech. 245, A10, 2022] to validate RANS models for predicting unsteady separation. By using an identical grid, numerical scheme and consistent boundary conditions to the LES calculations we are able to isolate modelling errors and modify terms accordingly. The Spalart-Allmaras eddy viscosity model predicts the unsteady behaviour of the flow with fair accuracy; the location of the separation region agrees with the LES results, hysteresis is captured and the mean velocity profiles match that of the LES results well. Calculations using the k-ε and k-Ω models will also be discussed.