Oscillation Frequency and Amplitude Effects on Plunging Airfoil Propulsion and Flow Periodicity

The effect of varying the plunging amplitude (h) at different reduced frequencies (k) on the propulsive performance and the periodicity of the fluidflowover a plungingNACA0012 airfoil was studied usingNavier–Stokes numerical simulations at a Reynolds number of 20,000. For propulsive performance at a given kh (or Strouhal number), a plunging airfoil at moderate to higher frequencies generates more thrust at higher efficiency than a plunging airfoil operating at lower frequencies. By examining the vortex dynamics, this increase in the propulsive performance with k at a given kh is attributed to larger suction caused by smaller-sized leading-edge vortices formed at higher k and lowerh, and to the timing of the shedding of these vortices beingmore synchronizedwith themotion of the airfoil. For flowperiodicity, results show that, at a given k or a given kh, increasing the plunging amplitude causes a change in flow behavior from periodic to quasi periodic to chaotic. The increased size and random interaction of leading-edge vortices for larger plunging velocities (kh> 1:25) were found to be responsible for chaotic behavior. This phenomenon, initially found in two-dimensional simulations, was verified with three-dimensional simulations, showing that the chaotic flow was not an artifact of the two-dimensional flowfield assumption.