Critical evolution of leading edge suction during dynamic stall

Dynamic stall dominates the aerodynamic performance, robustness, and wake dynamics of aircrafts in gusts, or on rotating blades such as helicopter or wind turbine blades. To assess dynamic stall onset and its associated unsteady effects, we analyse the evolution of the leading edge suction on a sinusoidally pitching airfoil based on time-resolved surface pressure measurements and particle image velocimetry. During the dynamic stall development stage, we link the shear layer evolution with the evolution of the leading edge suction. The dynamic stall development prior to stall onset consists of two stages: a primary instability stage and a vortex formation stage. The transition between stages is marked by a maximum in the leading edge suction. During the primary instability stage, both the leading edge suction and the shear layer height with respect to the airfoil's surface increase linearly with convective time. During the vortex formation stage, the leading edge suction drops and decreases linearly in time while the shear layer rolls up and creates a dynamic stall vortex. The maximum leading edge suction increases with normalized effective unsteadiness of the pitching motion.