Investigation of the effects of reynolds number on the unsteady flow physics of airfoil dynamic stall

A series of wind-tunnel experiments were conducted on a dynamically pitching airfoil at Re_c = 0.5 × 10⁶ and 1 × 10⁶ in order to understand the effects of Reynolds number on the unsteady flow physics associated with dynamic stall. An NACA 0012 airfoil was dynamically pitched about the quarter-chord axis following a linear ramp maneuver at ω⁺ = 0.05. A series of high-frequency unsteady surface pressure measurements were acquired in order to investigate the time-dependent behavior of the flow in the immediate vicinity of the airfoil at two transition Reynolds numbers. These surface pressure measurements actively displayed the evolution of the airfoil boundary layer, leading-edge laminar separation bubble, and the dynamic stall vortex at the corresponding Re_c. Detailed time-frequency analysis of surface pressure measurements was performed in order to identify the dominant frequencies associated with the unsteady laminar separation bubble. This analysis was also used to study the evolution of the length scales associated with flow structures near the airfoil leading edge, which emerged prior to the formation of the dynamic stall vortex. Finally, TR-PIV measurements were acquired to compare the evolution of the off-body flow field, with particular emphasis on understanding the development of the dynamic stall vortex at Re_c = 0.5 × 10⁶ and 1 × 10⁶.