Thrust Generation by Simultaneous Flapping Airfoil in Tandem Configuration

Abstract

Thrust generation by a simultaneously flapping airfoils in tandem configuration is parametrically studied with respect to frequency, amplitude and relative location. Navier- Stokes solver with overset grid topology is employed to compute the unsteady flowfields. The numerical results indicate that when the two airfoils stroke in-phase, that is flapping phase lag is zero, the propulsive efficiency and thrust generation are maximized for most frequency and amplitude range. At a flapping amplitude of 0.2 chord length and a reduced frequency of 0.75, the propulsive efficiency of aft airfoil was enhanced by about 37 % compared to that of the forward airfoil. However, if flapping frequency exceeds some critical value, a strong leading edge vortex of the aft airfoil fortified by the trailing edge vortex of forward airfoil results in a poor propulsive efficiency. It is also found that out-of-phase stroking has relatively low propulsive efficiency and thrust, since vorticial wake of the forward airfoil interacts with the leading edge vortex of aft airfoil in the apposite sense. The total thrust and propulsive efficiency are shown to decrease with the horizontal miss distance of aft airfoil. On the contrary, the vertical miss distance has little effect on the overall aerodynamic performance.