Route to transition in propulsive performance of oscillating foil

Transition in the propulsive performance and vortex synchronization of an oscillating foil in a combined heaving and pitching motion is numerically investigated at a range of reduced frequencies (0.16 f * 0.64), phase offsets (0 degrees phi 315 degrees), and Reynolds number (1000 Re 16000). Focusing on the common case of Re = 1000, the drag to thrust transition is identified on a phi - f* phase map. Here, the range of 90 degrees phi 225 degrees depicted a drag-dominated regime for increasing reduced frequency. However, thrust-dominated regimes were observed for phi 90 degrees and phi 225 degrees, where increasing the reduced frequency led to an increased thrust production. The isoline-depicting drag-thrust boundary was further observed to coincide with transitions in the characteristic near-wake modes with increasing reduced frequency, which ranged from 2P+2S to 2P and reverse von Karman modes. However, evaluation of the wake with changing phase offsets at individual reduced frequencies only depicted effects on the spatial configuration of the vortex structures, while the number of vortices shed in one oscillation period was unchanged. The existence of similar wake modes with significantly different propulsive performance clearly suggests that transitions of the wake topology may not always be a reliable tool for understanding propulsive mechanisms of fish swimming or development of underwater propulsion systems. We further assessed a possible route to drag production via investigation into the mean velocity fields at increasing phase offset and at intermediate reduced frequencies ranging from 0.24 to 0.40. This revealed bifurcation of a velocity jet behind the foil on account of the wake topology and dynamics of shed vortex structures. The changes posed by increasing phi on wake structure interactions further hints at potential mechanisms that limit the achievement of optimum efficiency in underwater locomotion.

Automated summary

Numerical investigation was conducted on the propulsive performance and vortex synchronization of an oscillating foil in combined heaving and pitching motion at different reduced frequencies, phase offsets, and Reynolds numbers. The transition from drag to thrust dominance was observed at specific phase offsets and reduced frequencies, coinciding with transitions in near-wake modes. Changing phase offsets only affected the spatial configuration of vortex structures, while the number of vortices shed in one oscillation period remained constant. These findings suggest that wake topology transitions are not always reliable for understanding propulsive mechanisms in fish swimming or underwater propulsion systems.