OSCILLATING FOILS OF HIGH PROPULSIVE EFFICIENCY

Thrust-producing harmonically oscillating foils are studied through fo rce and power measurements, as well as visualization data, to classify the principal characteristics of the flow around and in the wake of t he foil. Visualization data are obtained using digital particle image velocimetry at Reynolds number 1100, and force and power data are meas ured at Reynolds number 40 000. The experimental results are compared with theoretical predictions of linear and nonlinear inviscid theory a nd it is found that agreement between theory and experiment is good ov er a certain parametric range, when the wake consists of an array of a lternating vortices and either very weak or no leading-edge vortices f orm. High propulsive efficiency, as high as 87%, is measured experimen tally under conditions of optimal wake formation. Visualization result s 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 vorti city, resulting eventually in the formation of a reverse Karman 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.