Particle Image Velocimetry Characterization of a Chordwise Bending Cascade Flowfield

Abstract

Experiments are performed to investigate and quantify the velocity e eld over a chordwise-bending oscillating airfoil in a cascade operating at both design and off-design conditions. Particle image velocimetry is used to obtain a time history of the velocity e eld over the oscillating airfoil through one complete cycle. These data are then compared with unsteady surface-pressure data to identify the e ow phenomena responsible for the cascade unsteady aerodynamic loading distributions. High leading-edge unsteady loading at zero mean incidence is attributed primarily to stagnation-point migration from upper to lower surfaces during the oscillation cycle. The unsteadiness is enhanced by the formation of a small leading-edge separation bubble that alternately grows and collapses on the upper and lower airfoil surfaces. The reduction in leading-edge unsteady loading as mean incidence is increased results in a greatly reduced stagnation-point motion that now remains on the pressure surface throughout the cycle. A substantial reduction in suction surface leading-edge velocity unsteadiness is also observed. Migration of the high-velocity region above the dynamic stall separation bubble results in increased unsteady loading between 10 and 40% chord.