Maximizing the efficiency of a flexible propulsor using experimental optimization

Abstract

Experimental gradient-based optimization is used to maximize the propulsive efficiency of a heaving and pitching flexible panel. Optimum and near-optimum conditions are studied via direct force measurements and particle image velocimetry (PIV). The net thrust and power scale predictably with the frequency and amplitude of the leading edge, but the efficiency shows a complex multimodal response. Optimum pitch and heave motions are found to produce nearly twice the efficiencies of optimum heave-only motions. Efficiency is globally optimized when (i) the Strouhal number is within an optimal range that varies weakly with amplitude and boundary conditions; (ii) the panel is actuated at a resonant frequency of the fluid-panel system; (iii) heave amplitude is tuned such that trailing-edge amplitude is maximized while the flow along the body remains attached; and (iv) the maximum pitch angle and phase lag are chosen so that the effective angle of attack is minimized. The multi-dimensionality and multi-modality of the efficiency response demonstrate that experimental optimization is well-suited for the design of flexible underwater propulsors.

Effects of non-uniform stiffness on the swimming performance of a passively-flexing, fish-like foil model

“…We also predicted that the 0 o angle of attack pitching program would further enhance swimming performance, based on the results of Quinn et al [32], and would lead to more fish-like kinematics than seen in previous studies. Finally, we discuss the significance of these improvements in the context of a whole fish body and propose hypotheses about the mechanisms driving the observed performances.…”

“…Refer to Quinn et al [32] for a schematic diagram of the system, and photographs of similar versions of the experimental set-up were provided by Lauder et al [19] and Lauder et al [33].…”

“…Based on the results of Riggs et al [34], Shelton et al [21], and Cleaver et al [35], we would predict that biological non-uniform stiffness distributions would increase thrust. Furthermore, we would expect maximum swimming speeds and efficiencies when the foils are actuated in biologically-relevant motions including both heave and pitch [32,33]. Using a mechanical flapping foil controller, we compared the swimming performance of uniform and non-uniform models with biologically-relevant stiffnesses and actuation.…”

“…There are at least three probable factors. First, as a whole, the 0 o angle of attack program has been shown to reduce leading edge separation and delay vortex breakdown in the wake, reducing the amount of energy lost to the wake, thereby increasing swimming efficiencies [32].…”

A fast Chebyshev method for simulating flexible-wing propulsion

“…These studies have demonstrated that flexibility can drastically improve propulsive performance, especially when a wing or fin is driven near resonance [51,50,21,54,69]. Several studies even performed optimization to uncover wing properties and/or actuation strategies that deliver peak performance [94,2,57,73,69]. All of the studies mentioned above, though, considered only the simplest material distributions, such as uniform flexibility [33,2,4,51,50,21,57,69] or flexibility that is localized through a torsional joint [84,54].…”