Extending the Reynolds Number Range of High-Frequency Control of Dynamic Stall

A recently proposed high-frequency control concept for dynamic-stall mitigation is demonstrated using large-eddy simulation ail pitching NACA 0012 airfoil at a chord Reynolds number of Re-c = 10(6). At this condition, dynamic stall occurs when the suction-surface laminar separation bubble (LSB) breaks down due to interaction with the turbulent separation as it propagates upstream. It is shown that when high-frequency forcing is introduced near the leading edge, targeting the most amplified frequency of the LSB, the LSB amplifies the disturbances, resulting in a more energetic turbulent boundary layer. For a ramp-type pitching motion this control approach impedes the upstream progression of the turbulent separation and reduces the severity of the leading-edge breakdown process when dynamic stall eventually occurs. Control is then demonstrated on a sinusoidal pitching motion where interaction between the turbulent separation and the LSB is avoided. Large reductions in drag and moment excursions are noted, and it is surmised that, for higher reduced frequency or lower maximum alpha, the effects of dynamic stall would be completely eliminated.