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 trailingedge vorticity, as well as the efficiency of propulsion. IntroductionFish and cetaceans employ their oscillating tails to produce propulsive and maneuvering forces. The tails of some of the fastest swimming animals closely resemble high-aspect-ratio foils. Because of the presumed optimal propulsive performance of fish, oscillating foils have been studied extensively using theoretical and numerical techniques (Lighthill 1975;Wu 1961Wu , 1971Longvinovich 1971;Cheng & Murillo 1984;Karpouzian, Spedding & Cheng 1990;McCune & Tavares 1993), and experimentally (Scherer 1968;DeLaurier & Harris 1982;Lai, Bose & McGregor 1993).A foil in steady forward motion and a combination of steady-state harmonic heaving and pitching motion produces thrust through the formation of a flow downstream from the trailing edge, which when averaged over one period of oscillation has the form of a jet. This average jet flow is unstable, acting as a narrow-band amplifier of perturbations. The harmonic motion of the foil causes unsteady shedding of vorticity from the trailing edge, while there are conditions when leading-edge vortices form as well. The interaction between the unsteady vorticity shed by the foil and the inherent dynamics of the unstable wake result in the formation of patterns of

We present experimental force and power measurements demonstrating that the power required to propel an actively swimming, streamlined, fish-like body is significantly smaller than the power needed to tow the body straight and rigid at the same speed U. The data have been obtained through accurate force and motion measurements on a laboratory fish-like robotic mechanism, 1.2 m long, covered with a flexible skin and equipped with a tail fin, at Reynolds numbers up to 10 6 , with turbulence stimulation. The lateral motion of the body is in the form of a travelling wave with wavelength λ and varying amplitude along the length, smoothly increasing from the front to the tail end. A parametric investigation shows sensitivity of drag reduction to the non-dimensional frequency (Strouhal number), amplitude of body oscillation and wavelength λ, and angle of attack and phase angle of the tail fin. A necessary condition for drag reduction is that the phase speed of the body wave be greater than the forward speed U. Power estimates using an inviscid numerical scheme compare favourably with the experimental data. The method employs a boundary-integral method for arbitrary flexible body geometry and motions, while the wake shed from the fish-like form is modelled by an evolving desingularized dipole sheet.

It is shown experimentally that free shear flows can be substantially altered through Subsequently, the experiments are repeated in a different facility at larger scale, resulting in Reynolds number 20000, in order to obtain force and torque measurements. The purpose of the second set of experiments is to assess the impact of flow control on the efficiency of the oscillating foil, and hence investigate the possibility of energy extraction. It is found that the efficiency of the foil depends strongly on the phase difference between the oscillation of the foil and the arrival of cylinder vortices. Peaks in foil efficiency are associated with the formation of a street of weakened vortices and energy extraction by the foil from the vortices of the vortex street.

No abstract

No abstract