Plunging Oscillations of Airfoils and Wings: Progress, Opportunities, and Challenges

The vortex dynamics of thrust and lift generation for periodically plunging airfoils and finite wings at low Reynolds numbers is reviewed. Effects of vortex lock-in phenomenon, leading-edge vortex, vortex merging and pairing, vortex-wing interactions, leading-edge geometry, wake deflection, and chordwise and spanwise flexibility on lift and thrust are discussed. Mean lift force of the oscillating airfoil at post-stall angles of attack becomes maximum at optimal frequencies due to the vortex lock-in at the fundamental frequency, subharmonic, and first harmonic of the natural shedding frequency. Bifurcations of wakes of plunging airfoils, which depend on the mean angle of attack and the initial conditions and can produce large mean lift coefficients, are not observed for finite wings. One or two arch-type vortical structures may develop from the leading-edge vortices shed on low-aspect-ratio wings. Significant spanwise flows can develop on finite wings due to the dynamics of tip vortices or leading-edge vortices. Chordwise and spanwise flexibility of plunging airfoils and wings are known to enhance the mean thrust and lift; however, the physical mechanism of the optimal conditions is not well understood. Various flow control opportunities to exploit the flow physics as well as remaining challenges and unresolved issues are summarized.