Vortex Mode Bifurcation and Lift Force of a Plunging Airfoil at Low Reynolds Numbers

Force and supporting particle image velocimetry measurements were performed on a plunging NACA 0012 airfoil at a Reynolds number of 10,000, at pre-stall, stall, and post-stall angles of attack. The lift coefficient for pre-stall and stall angles of attack at larger amplitudes showed abrupt bifurcations with the branch determined by initial conditions. With the frequency gradually increasing, very high positive lift coefficients were observed, this was termed mode A. At the same frequency with the airfoil impulsively started, negative lift coefficients were observed, this was termed mode B. The mode A flow field is associated with trailing-edge vortex pairing near the bottom of the plunging motion causing an upwards deflected jet, and a resultant strong upper surface leading-edge vortex. The mode B flow field is associated with trailing-edge vortex pairing near the top of the plunging motion causing a downwards deflected jet, and a resultant weak upper surface leading-edge vortex. The bifurcation was not observed for small amplitudes due to insufficient trailing-edge vortex strength, nor at larger angles of attack due to greater asymmetry in the strength of the trailing-edge vortices, which creates a natural preference for a downward deflected mode B wake.