Lift-Enhancing Vortex Flows Generated by Plunging Rectangular Wings with Small Amplitude

Experiments in a water tunnel have been carried out on low-aspect-ratio rectangular wings undergoing a small-amplitude harmonic plunge motion at Reynolds numbers 10,000 and 20,000. A series of measurement techniques have been used, including force measurements, hot film, particle image velocimetry, and volumetric velocimetry measurements, to study the lift enhancement as a function of forcing frequency. Multiple peaks in the time-averaged lift have been observed, occurring at frequencies in the order of natural vortex-shedding frequencies of the stationary wings. It is postulated that interaction between the leading-edge and trailing-edge vortices contributes to the selection of the optimal frequencies for the time-averaged lift. At a specific Strouhal number, an adverse interaction between the vortices results in a vortex dipole that directs flow upstream. A comparison between a NACA 0012 and flat-plate profile provides further insight into the advantages and disadvantages of using a thinner profile in l...