Simplified geometries can be used to isolate the effects of flexibility in swimming fish. Here we used an oscillating flexible rectangular panel to explore how efficiency scales with panel stiffness and frequency. We found that as frequency increases, efficiency passes through local maxima, corresponding to resonant modes of the fluid-structure system. Our results suggest that fish and fish-inspired vehicles may benefit from operating at their resonant frequencies. (This work is from PI Quinn’s research prior to the SFS Lab and is archived here for reference.)
Authors: Daniel Quinn, George Lauder, Alexander Smits
Abstract: We present an experimental investigation of flexible panels actuated with heave oscillations at their leading edge. Results are presented from kinematic video analysis, particle image velocimetry, and direct force measurements. Both the trailing edge amplitude and the mode shapes of the panel are found to scale with dimensionless parameters originating from the Euler–Bernoulli beam equation. The time-averaged net thrust increases with heaving frequency, but experiences localized boosts near resonant frequencies where the trailing edge amplitude is maximized. These boosts correspond to local maxima in the propulsive efficiency. For a constant heave amplitude, the time- averaged net thrust coefficient is shown to be a function of Strouhal number over a wide range of conditions. It appears, therefore, that self-propelled swimming (zero net thrust) only occurs over a small range of Strouhal numbers. Under these near-constant Strouhal number conditions, the propulsive economy increases with higher flexibilities and slower swimming speeds.