Pitch rate and Reynolds number effects on unsteady boundary layer transition and separation

Abstract

A NACA 0015 airfoil was pitched at a constant rate through static stall to elevated angles of attack. Shear stress measurements of high spatial and temporal resolution were performed near the airfoil leading edge, in the vicinity of subsequent dynamic stall vortex initiation. Using these data, unsteady boundary-layer reversal and transition were characterized for a range of nondimensional pitch rates and Reynolds numbers. Analyses revealed the independent influences of nondimensional pitch rate and Reynolds number upon unsteady boundary-layer reversal and transition. Temporal and spatial relationships between unsteady boundary-layer reversal and transition imply that unsteady boundary-layer reversal is a precursor and principal determinant in unsteady boundary-layer transition. Comprehension of these and other fundamental unsteady flow physics are crucial for the control of dynamically separated flows generated by maneuvering aircraft, rotorcraft, and wind energy machines