Understanding Abrupt Leading Edge Separation as a Mechanism for the Onset of Dynamic Stall

摘要

Large-eddy simulations of the dynamic stall process on a pitching NACA 0012 airfoil at Re_c = 1.0 x 10~6 have been performed. Using a constant-rate pitch-up maneuver it is shown that the onset of dynamic stall occurs due to bursting of a very small laminar separation bubble located near the airfoil leading edge. This bursting is initiated by direct interaction with the upstream propagation of the trailing edge turbulent boundary layer separation. Two vortex structures develop, the first from the bursting separation bubble and the second from the roll-up of the turbulent separation. These two vortex structures interact and then merge to form the dynamic stall vortex. A parametric study of pitching parameters is used to better understand the sensitivity of these interactions to airfoil motion. It is shown that the timing of dynamic stall onset is entirely dependent on the rate at which turbulent separation propagates upstream and the timing of the roll-up of the turbulent shear layer. The computations highlight the significant role the laminar separation bubble plays in the process despite its small size. Computations are also performed on the NACA 0009 airfoil to contrast against a case of standard pressure-gradient induced laminar separation bubble bursting without the presence of turbulent separation.