Large Eddy Simulations on a pitching airfoil: Analysis of the reduced frequency influence

Large Eddy Simulations (LES) have been performed on a pitching NACA0012 airfoil at Rec=2.104. The influence of the reduced frequency on the dynamic stall phenomenon is investigated. Special attention is given on the aerodynamic coefficients drop incidence delay in comparison with a static case. The analysis is based on vorticity field observations and the introduction of local lift and friction coefficients. The boundary layer events and the presence and the impact of a Leading-Edge Vortex (LEV) are investigated. The total stall delay is decomposed into two main phases: a delay in the boundary layer separation which occurs at higher incidence as the reduced frequency increases and the initiation and the growth of the LEV. For selected reduced frequencies, the influence of the LEV on the lift and drag coefficients is analysed as a function of the airfoil angle of incidence and then as a function of time. The LEV maintains a high level of lift and drag. Its lifetime on the airfoil suction side seems to be shortened by the beginning of the airfoil downstroke motion. Given the fact that the LEV appears at higher incidence as the reduced frequency grows, the LEV lifetime on the airfoil suction side decreases as the reduced frequency increases. However, the airfoil oscillation being faster, the LEV goes through larger incidence ranges during its lifetime and leads to a larger lift and drag drop incidence delay as the reduced frequency increases.