Oscillating foils of high propulsive efficiency
TLDR
In this article, 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.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 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.read more
Citations
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References
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TL;DR: The dynamique des : fluides Reference Record created on 2005-11-18 is updated on 2016-08-08 and shows improvements in the quality of the data over the past decade.
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An Introduction to Fluid Dynamics. By G. K. Batchelor. Pp. 615. 75s. (Cambridge.)
TL;DR: In this paper, the Navier-Stokes equation is derived for an inviscid fluid, and a finite difference method is proposed to solve the Euler's equations for a fluid flow in 3D space.
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Digital Particle Image Velocimetry
Christian Willert,Morteza Gharib +1 more
TL;DR: In this article, the directional ambiguity associated with PIV and LSV is resolved by implementing local spatial cross-correlations between two sequential single-exposed particle images, and the recovered velocity data are used to compute the spatial and temporal vorticity distribution and the circulation of the vortex ring.
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Optimal Thrust Development in Oscillating Foils with Application to Fish Propulsion
TL;DR: In this paper, it was shown that a stable coexistence of the jet profile and the large-scale patterns is ensured only at the frequency of maximum amplification, hence at this frequency optimal efficiency is obtained, i.e., maximum thrust per unit input energy.
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Vortical patterns in the wake of an oscillating airfoil
TL;DR: In this article, the vortical flow patterns in the wake of a NACA 0012 airfoil pitching at small amplitudes were studied in a low speed water channel, and it was shown that a great deal of control can be exercised on the structure of the wake by the control of the frequency, amplitude and also the shape of the oscillation waveform.