Unsteady Aerodynamics of a Transient Plunging Airfoil

In this study lift, pitching moment and particle image velocimetry (PIV) measurements are presented for a NACA 0012 airfoil with a Reynolds number of 20,000 at a geometric angle of attack in the range 0° to 20° undergoing a transient plunging motion. The transient motion is defined by a Strouhal number based on the chord length up to 0.60 and effective plunge angle amplitude from -30° to 30°. The largest peak in both lift and pitching moment was observed during the motion for all cases. This was attributed primarily to circulatory effects for the lift and added-mass effects for pitching moment. PIV showed the presence of a coherent leading-edge vortex (LEV) at positive plunge angle amplitudes which had the biggest effect on the pitching moment during motion. Substantial peaks in lift and pitching moment were observed for post-stall geometric angles of attack after the end of motion. This is postulated to be due to strong interactions between the leading and trailing-edge vortices. An increase in plunge angle amplitude caused an increase in the magnitudes of lift/moment change. This shedding phenomena was found for both positive and negative plunge angles and showed remarkably similar peak relative timings, highlighting the insensitivity of the shedding process to the initial transient forcing.