Thrust enhancement of a flapping foil through interaction with a Kármán vortex street

Abstract

How to increase the thrust without efficiency deterioration is a significant issue for an oscillating-foil propeller. This paper presents a numerical study to the phenomenon of thrust enhancement of a flapping foil through interaction with a Kármán vortex street. A circular cylinder is placed upstream the foil to generate the Kármán street. Simulations are performed at different combinations of the gap between the cylinder and the foil and the reduced frequencies. It is found that the upstream vortices are helpful to increase both the thrust and propulsive efficiency simultaneously. Under specific oscillation parameters, the synchronization between the vortices shedding and the foil's oscillation will occur, which further results in the phenomenon of thrust enhancement, with a high thrust more than twice that of the same flapping foil in the uniform oncoming flow, as well as a higher efficiency. The occurrence of thrust enhancement is sensitive to the oscillation amplitudes of the foil and the optimal pitching amplitude is found in the range of [30°,40°]. Flow analysis shows that a large relative flow angle higher than 90° is induced around the leading edge of the foil when a vortex of the cylinder approaches the foil, which prompts the generation of the leading-edge vortex and further results in an expanding wake pattern as two triplets (2 T). Besides, the phase between the Kármán street and the oscillation of the foil can be effectively adjusted by forcing the cylinder to vibrate, which can further control the generation of the thrust enhancement.