On wake mode transition of a foil with combined heaving and pitching motion

The drag to thrust transition and its correspondence with changes in wake topology is investigated numerically for an oscillating foil with combined heaving and pitching motion. Quantitative and qualitative evaluations at a range of reduced frequency (0.16<f^*<0.64), phase difference (0^o<Π<315^o) and Reynolds number (1000<Re<12000) reveal no direct correspondence between changes in the propulsive performance and wake modes. For f^*<0.32, several wake configurations ranging from 2P+2S, wBvK and 2P are observed at increasing Π, although the propulsive performance was still observed to be drag dominated. For f^*>0.24, different spatial configuration of 2P mode is observed for 90^o<Π<225^o, which are characterized by either vortex pairs aligned with the wake centerline (2P^H) or dipoles (2P^D). However, no drag-to-thrust transition coincides with these changes in wake topology. Further investigation into the mean flow corresponding to 0.24<f^*<0.4 revealed bifurcation of the thrust producing jet with increasing Π from 0^o to 90^o. Dynamic interaction of coherent structures in the wake provides a qualitative reasoning for this bifurcation. This serves as a possible route to drag production in biological swimming and hence confines the range of Π that yields an optimum propulsive efficiency.