Flow, Pressure, and Aerodynamic Loads on an Oscillating Airfoil Undergoing Dynamic Stall
Abstract
The flow and pressure fields around a harmonically oscillating NACA 0015 airfoil undergoing dynamic stall are investigated at varying reduced frequencies ([Formula: see text]) and Reynolds numbers ([Formula: see text]). Refractive index matching enables simultaneous Stereo Particle Image Velocimetry measurements on both sides of the transparent airfoil. Assuming two-dimensional flows, the pressure is estimated by spatial integration of the material acceleration. The lift, pressure (form) drag, and moments are calculated by integration of the surface pressure, providing a comprehensive picture of the relationship between the flow structure and loading. Results confirm that the migrating broad low-pressure region induced by the dynamic stall vortex along the suction surface delays the stall onset and increases the maximum lift and drag. The vortex detachment time is a constant fraction (0.3) of the oscillation period, and its migration speed is [Formula: see text] of the freestream velocity for all [Formula: see text] and [Formula: see text]. When time scales are shifted by the vortex detachment times, all the lift and drag curves collapse. Vortex fragmentation near the trailing edge weakens the lift and drag, but entrainment of vorticity originating from the pressure side creates a pressure minimum that increases the leading-edge moment. Low-amplitude heaving at low [Formula: see text] and [Formula: see text], mostly at the end of the downstroke, causes lift oscillations.