Dynamic Stall Due to Fluctuations of Velocity and Incidence

Abstract

Proper simulation of a retreating-blade stall requires considering incidence fluctuations combined withsimultaneous velocity oscillations of the incoming airstream. This paper reports experimental results obtained when simultaneous fluctuations of both velocity and incidence are generated on an airfoil executing oblique fore-and-aft translations. When dynamic stall occurs in out-of-phase or in-phase variations of incidence and velocity, strong unsteady effects are present. In the first case, an instantaneous lift overshoot and a time delay between incidences of dynamic and static stall are observed. In the second case, the main effect consists of an overshoot of the mean aerodynamic coefficients over the period, owing to a cyclic separation and reattachment process with a strong vortex shedding all along the upper surface during the separation phase.