On the interplay between flow and pressure fields around an airfoil undergoing dynamic stall

The evolution of pressure distribution around a 50mm chord, NACA 0015 airfoil, harmonically pitched between 5^o to 25^o, hence undergoing dynamic stall, was studied at reduced frequency (k) varying between 0.047 to 1.57 and Reynolds number (Re) between 13,600 to 136,000. Time resolved stereo-PIV measurements in the JHU refractive index matched water tunnel facilitated simultaneous measurements on both sides of the airfoil. The pressure field was computed by spatial integration of the material acceleration, assuming a 2D flow, using a GPU-based, parallel line, omni directional method. The lift and moments were calculated by integration of the pressure along the surface. The pressure induced as the dynamic stall vortex (DSV) migrated along the suction side was a prominent contributor to the delay in stall onset, and reduced frequency-dependent location of pressure minimum and angle of maximum lift. Fragmentation of the DSV near the trailing edge (TE) caused a sudden drop in lift. Entrainment of the pressure side vorticity by the DSV created a low-pressure region near the TE, and a short-lived peak in the leading-edge moment. A DSV location-based time scale, determined from the surface pressure gradients, was used to collapse the lift curves for different Re and k.