Peter Dewey

Propulsive performance of unsteady tandem hydrofoils in an in-line configuration

Experiments are reported on the behavior of two hydrofoils arranged in an in-line configuration as they undergo prescribed pitching motions over a wide range of phase lags and spacings between the foils. It is found that the thrust production and propulsive efficiency of the upstream foil differed from that of an isolated one only for relatively closely spaced foils, and the effects attenuated rapidly with increasing spacing. In contrast, the performance of the downstream foil depends strongly on the streamwise spacing and phase differential between the foils for all cases considered, and the thrust and propulsive efficiency could be as high as 1.5 times or as low as 0.5 times those of an isolated foil. Particle image velocimetry reveals how the wake interactions lead to these variations in propulsive performance, where a coherent mode corresponds to enhanced performance, and a branched mode corresponds to diminished performance.

Propulsive performance of unsteady tandem hydrofoils in a side-by-side configuration

Experimental and analytical results are presented on two identical bio-inspired hydrofoils oscillating in a side-by-side configuration. The time-averaged thrust production and power input to the fluid are found to depend on both the oscillation phase differential and the transverse spacing between the foils. For in-phase oscillations, the foils exhibit an enhanced propulsive efficiency at the cost of a reduction in thrust. For out-of-phase oscillations, the foils exhibit enhanced thrust with no observable change in the propulsive efficiency. For oscillations at intermediate phase differentials, one of the foils experiences a thrust and efficiency enhancement while the other experiences a reduction in thrust and efficiency. Flow visualizations reveal how the wake interactions lead to the variations in propulsive performance. Vortices shed into the wake from the tandem foils form vortex pairs rather than vortex streets. For in-phase oscillation, the vortex pairs induce a momentum jet that angles towards the centerplane between the foils, while out-of-phase oscillations produce vortex pairs that angle away from the centerplane between the foils.

Linear instability mechanisms leading to optimally efficient locomotion with flexible propulsors

We present the linear stability analysis of experimental measurements obtained from unsteady flexible pitching panels. The analysis establishes the connections among the wake dynamics, propulsor dynamics, and Froude efficiency in flexible unsteady propulsion systems. Efficiency is calculated from direct thrust and power measurements and wake flowfields are obtained using particle image velocimetry. It is found that for flexible propulsors every peak in efficiency occurs when the driving frequency of motion is tuned to a wake resonant frequency, not a structural resonant frequency. Also, there exists an optimal flexibility that globally maximizes the efficiency. The optimal flexibility is the one where a structural resonant frequency is tuned to a wake resonant frequency. The optimally tuned flexible panels demonstrate an efficiency enhancement of 122%–133% as compared to an equivalent rigid panel and there is a broad spectrum of wake resonant frequencies allowing high efficiency swimming over a wide range of operating conditions. At a wake resonant frequency we find that the entrainment of momentum into the time-averaged velocity jet is maximized.

The Swimming of Manta Rays

Manta rays propel themselves by combining oscillating and undulating motions of flexible surfaces. We describe two experiments to study the effects of excitation and flexibility on the wake flowfield: experiments on undulating and flapping three-dimensional fins of elliptical planform, and experiments on pitching two-dimensional panels of rectangular planform with varying flexibility. To interpret the results on thrust and efficiency, we propose scalings for aspect ratio and flexibility, and develop a stability analysis called wake resonance theory. Here we focus on the insights provided by wake resonance theory.