On the time scales governing dynamic stall at high Reynolds numbers

Dynamic stall refers to the time-resolved stall process of an airfoil that is rapidly pitched above its static stall angle. It involves the role-up of the suction-side boundary layer into a dynamic stall vortex, which causes a momentary lift overshoot before convecting downstream. Here, the non-dimensional time scales governing various stages of the dynamic stall process are elucidated. To this end, we experimentally investigate a NACA 0021 airfoil undergoing ramp motions from below to above the stall angle. The Reynolds number for all test cases is 3.0 x 10^6 and the reduced frequency is varied over 0.01 ≤ k ≤ 0.40. This parameter space is achieved using a high-pressure wind tunnel. The transient pressure field provides insight into the time-resolved stall behavior. It reveals that at sufficiently high reduced frequencies, there is a distinct point in time at which the stall process becomes independent of the airfoil kinematics and is instead purely convection-governed.