Airfoil surface pressure measurements in unsteady flow conditions

The combination of high Reynolds numbers and unsteady flow conditions depict a challenge in experimental wind tunnel studies. Unsteady airfoil aerodynamics are commonly described by the reduced frequency k = ωc/2/U, where a range of 0 ≤ k ≤ 0.25 characterizes steady to highly unsteady flow conditions. 

The Reynolds number scales proportionally with the flow velocity U, whereas the reduced frequency scales inversely proportional. In regular wind tunnels, this leads to unrealistically high pitching frequencies in experimental attempts to achieve high Reynolds numbers simultaneously with high reduced frequencies. The present study takes advantage of a high-pressure flow facility, in which the density of compressed air promotes high Reynolds numbers, while low velocities below 10 m/s allow for unsteady flow conditions. 

A symmetric NACA0021 airfoil is equipped with surface pressure sensors to investigate distributed pressures and integrated forces at a constant Reynolds number of 3 million. The present study focuses on the effect of altered mean angle of attack α and varied pitching amplitudes Δα at a constant reduced frequency k.