Effect of Reynolds number and reduced frequency on instantaneous pressure field and loads on a harmonically pitching airfoil

Experimental investigation of the effect of Reynolds number and reduced frequency on an airfoil undergoing dynamic stall were performed on a harmonically pitching NACA 0015 airfoil. The experiments were performed in a refractive index matched water tunnel, with the airfoil oscillating between 5^o and 25^o, the Reynolds number ranging from 13.6-91x10³, and the reduced frequency, between 0.047-1.57. Time-resolved stereo PIV data covering both sides of the airfoil simultaneously was acquired at mid span at frame rates up to 1250Hz.  Using in-house, GPU based, parallel-line, omni-directional code, the pressure field around the airfoil was computed by direct integration of the material acceleration calculated from the time- resolved velocity field. The lift and pitching moment were determined from integration of the surface pressure distribution. Data analysis examined the effects of Reynolds number and reduced frequencies on the phase lag between the airfoil orientation and the suction side flow structure, and the resulting variations in lift, pitching moments, and trailing edge velocity during upstroke and downstroke.  The mechanisms involved were elucidated and compared to prior publications, such as the roles of the leading-edge and dynamic stall vortices.