A Study of Unsteady Turbulent Boundary Layer Separation Under Conditions Relevant to Helicopter Rotor Dynamics

An experimental investigation focused on the study of the physics of unsteady turbulent boundary layer separation under conditions relevant to the dynamic stall process that occurs in helicopter rotors is presented. A flat boundary layer development plate allows for the growth of a turbulent boundary layer of thickness sufficient for high spatial resolution measurements. Downstream of the flat plate, a convex ramp section imposes a streamwise adverse pressure gradient that gives rise to boundary layer separation. In order to impose an unsteady pressure gradient, an airfoil equipped with leading edge plasma flow control is located above the ramp section. Plasma flow control is used to alternately attach and separate the airfoil flow which gives rise to unsteady turbulent boundary layer separation on the convex ramp. Measurements of the resulting unsteady turbulent boundary layer separation via phase-locked two-component PIV, unsteady surface pressure measurements, and high speed digital imaging capture and quantify the dynamics the separation process at the wall and throughout the unsteady boundary layer. Two-component LDA measurements are used to characterize the motions of ejection and sweep events within the unsteady boundary layer using a quadrant splitting technique. Large amplitude quadrant 4 sweep events are the most dynamically significant in the near wall region during the unsteady separation process. The adverse pressure gradient boundary layer profiles throughout the unsteady cycle collapse remarkably well when scaled with embedded shear layer parameters. The implications of the experimental results for the development of flow control strategies for unsteady boundary layer separation are discussed.