Effects of Initial Acceleration on the Flow Field Development Around Rapidly Pitching Airfoils

Abstract

Computational results are presented to show how the acceleration period at the start of nominally constant pitch rate trajectories affects the dynamic stall process. Large amplitude motions of an NACA 0012 airfoil pitching about the quarter-chord axis were studied using constant (ON/OFF) acceleration profiles with nondimensional acceleration periods ranging between 0.039 to 0.6, and large pitch rates (0.1 ≤ Ω* ≡ α˙ c/2U∞≤0.4). The initial acceleration is observed to affect the integrated loads, surface pressure distribution, and the evolution of reversed flow regions just above the surface of the airfoil only during the acceleration period and for a relatively short time δτ ≤ 0.25 afterwards; τ≡ tU∞/c. After that time, all of these quantities only depend on the instantaneous angle of attack for a given pitch rate. These results are consistent with and explain previous experimental flow visualization observations. The onset of leading edge separation at high and low pitch rates is shown to be characterized by different processes. At low pitch rates leading edge separation occurs after the reversed flow front originating at the trailing edge has reached the leading edge. At higher pitch rates leading edge separation and the upstream progression of the (trailing edge) reversed flow front develop independently.