Vortex Dynamics of a Separated Wing in a Time-Varying Freestream

Abstract

Unsteady flows threaten the performance and efficiency of systems operating in gusty environments. The impact of an unsteady freestream, generated in a closed test-section unsteady wind tunnel, on the flow separation and aerodynamic performance of a wing at a post-stall angle of attack is examined within the current study. Synchronized two-dimensional, high-speed particle image velocimetry and integrated surface pressure measurements were collected for a finite-span wing in a time-varying, spatially uniform freestream. Freestream accelerations impose additional unsteady pressure gradients within the wind tunnel that alter the behavior of shed vortical structures within the separated flow above the wing. Freestream acceleration and conservation of circulation determine the orientation and interaction of shed vortical structures, which alter the magnitude of the fluctuations in the lift force and pitching moment experienced by the wing. Specifically, fluctuations in the sectional lift and pitching moment coefficients are amplified during deceleration and attenuated during acceleration.