Three-Dimensional Effects on Plunging Airfoils at Low Reynolds Numbers

We present two-dimensional and three-dimensional (3-D) direct numerical simulations of large-amplitude plunging maneuvers at Reynolds numbers of Re=1000 and 5000, with velocity ratios of G=0.5, 1, and 2. For all cases, the evolution of the force coefficients is qualitatively similar. The lift coefficient presents a pronounced peak toward the end of the acceleration phase of the maneuver, a local minimum in the deceleration phase, and a second peak at the end of the maneuver. The amplitude of the main peak increases linearly with G, with limited effect of the Reynolds number and a negligible effect of the three-dimensionality of the flow. On the other hand, both the Reynolds number and three-dimensionality have a stronger effect on the amplitude of the maximum value of the lift coefficient at the end of the maneuver, as well as on the subsequent transient decay toward the static values. The comparison of the evolution of the flow structures near the airfoil shows that these differences in the force coefficients are due to subtle interactions between the various vortices generated during the maneuver, as well as to the development of a 3-D boundary layer on the suction side of the airfoil triggered by the instability of the trailing-edge vortices.