Oscillating foils of high propulsive efficiency

TLDR

The study investigates thrust‑producing harmonically oscillating foils by measuring force, power, and visualizing flow to classify key wake characteristics. The authors used digital particle image velocimetry at Re 1100 and measured force and power at Re 40 000 to capture the flow and performance of the foils. Experimental results show good agreement with theory when the wake consists of alternating vortices, achieving up to 87 % propulsive efficiency, and high efficiency is linked to alternating leading‑edge vortices that interact with trailing‑edge vorticity to form a reverse Kármán street, with the phase angle between transverse oscillation and angular motion being the key parameter.

Abstract

Thrust-producing harmonically oscillating foils are studied through force and power measurements, as well as visualization data, to classify the principal characteristics of the flow around and in the wake of the foil. Visualization data are obtained using digital particle image velocimetry at Reynolds number 1100, and force and power data are measured at Reynolds number 40 000. The experimental results are compared with theoretical predictions of linear and nonlinear inviscid theory and it is found that agreement between theory and experiment is good over a certain parametric range, when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. High propulsive efficiency, as high as 87%, is measured experimentally under conditions of optimal wake formation. Visualization results elucidate the basic mechanisms involved and show that conditions of high efficiency are associated with the formation on alternating sides of the foil of a moderately strong leading-edge vortex per half-cycle, which is convected downstream and interacts with trailing-edge vorticity, resulting eventually in the formation of a reverse Kármán street. The phase angle between transverse oscillation and angular motion is the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion.