RANS and Hybrid LES/RANS Simulation of Airfoil under Static and Dynamic Stall

The hybrid Large-Eddy/Reynolds-Averaged Navier-Stokes (LES/RANS) and RANS simulations are used to investigate the aerodynamic characteristics of subsonic flow over airfoils undergoing dynamic and static stall. Simulations of flow over the Aerospatiale A-Airfoil show that the Menter BSL/SST RANS models, along with the LES/RANS models of Choi and Gieseking, accurately capture the velocity and Reynolds-stress fields associated with incipient trailing-edge separation. The inclusion of the Menter-Langtry transition model enables the capturing of an initial region of laminar flow culminating in a laminar separation bubble, in accord with experimental results. However, the transition model also results in a general thinning of the boundary layer downstream of the peak skin friction location and the elimination of incipient separation near the trailing edge. In the simulations of NACA 0012 airfoil at static stall case, Menter's SST with and without the inclusion of Menter-Langtry transition model both predict an attached flow at the leading edge, whereas the Gieseking's LES/RANS model on a coarser mesh predicts a massively separated flow characterized by the stabilization of a detached leading edge vortex near the trailing edge. The predictions by Gieseking's model on a coarse mesh agree closely with PIV measurements of mean velocity, the Reynolds axial stress and the Reynolds normal stress, but over-predict the magnitude of the Reynolds shear stress. However, Gieseking's model on a fine mesh predicts a more attached flow because the under-resolved LES on the fine mesh (but not fine enough as required in a wall-resolved LES) fails to reproduce the cascade process at the smaller scale and results in an overly-energetic boundary layer near leading edge which resists and delays the separation. In 3D simulations of NACA 0012 dynamic stall case, Gieseking's model on a coarse mesh in spanwise direction correctly predicts response of the massive separation at static stall angle of 16.7° during downstroke pitching, but it also predicts some leading edge separation which is not present in the experiment during upstroke pitching. Mesh refinement in the spanwise direction helps reducing the level of leading edge separation during upstroke pitching, but results in an under-separated flow solution for downstroke response.