Characterizing Dynamic Stall at High Reynolds number using a Variable Pressure Wind Tunnel*

The objective of this work is to elucidate the effect of chord Reynolds number, Re_c , and airfoil kinematics on the salient fluid structures generated by a NACA0021 airfoil undergoing dynamic stall at 2 × 10⁵ ≤ Re_c ≤ 8 × 10⁶. This is accomplished by pitching the airfoil in the High Reynolds number Test Facility (HRTF) at Princeton University. The HRTF has a working pressure of up to 24 MPa (238 bar), achieving high Re_c through low freestream velocities, U_∞ ≤ 10 m/s. By scaling pressure instead of velocity, the experiments bypass compressibility and time-scale challenges faced by studies of dynamic stall in traditional wind tunnels operating at high Re_c and atmospheric pressure. The results reveal unique relations between Re_c and the non-equilibrium response of the boundary layer - namely, the stall angle as well as the temporal signatures of the transition region, boundary layer separation point, and dynamic stall vortex, all of which indicate the type of dynamic stall being observed. These findings are pertinent to large and small-scale aeronautics, wind energy, and biological flows.