Experimental and numerical investigation of three-dimensional vortex structures of a pitching airfoil at a transitional Reynolds number

This research examines the vortex behaviors and aerodynamic forces in dynamic stall phenomena at a transitional Reynolds number (Re = 90000) using experimental and numerical approaches. Periodic sinusoidal pitching motion at two different reduced frequencies is used to achieve the dynamic stall of a NACA 0012 airfoil. Several leading edge vortices form and detach in the dynamic stall stage. The flow then quickly transitions to a full separation zone in the stall stage when the angle of attack starts to decrease. There is discrepancy between the phase-averaged and instantaneous flow field in that the small flow structures increased with angle of attack, which is a characteristic of the flow field at the transitional Reynolds number. The interaction between the streamwise vortices in the three-dimensional numerical results and the leading edge vortex are the main contribution to the turbulent flow. In addition, the leading edge vortex that supplies vortex lift is more stable at higher reduced frequency, which decreases the lift fluctuation in the dynamic stall stage. The leading edge vortex at higher reduced frequency is strong enough to stabilize the flow, even when the airfoil is in the down-stroke phase.