Session F23: Biological Fluid Dynamics: Locomotion Swimming - Foils and Plates

Connections between resonance and nonlinearity in swimming performance of a flexible heaving plate

The use of flexibility in underwater vehicles is of interest as a means to augment propulsive performance. As an intermediate step to this aim, the effect of flexibility on performance is often considered for flow past a heaving flexible flat plate. In this setting, previous work has found flexibility to improve performance for certain parameters. While these benefits have been found for a range of flow regimes, the physical role of flexibility remains unclear. Often, resonance is cited as the source of performance peaks, though in certain settings heaving at non-resonant frequencies has been found to be optimal. Other studies have found relationships between flow structures and performance, though the connection of these observations to resonance is still unclear. We use high-fidelity nonlinear simulations and a global linear analysis of the fully-coupled fluid-structure interaction system to study two-dimensional viscous flow past a heaving geometrically nonlinear Euler-Bernoulli beam. The linear analysis is used to unambiguously define resonant behavior in the presence of a viscous fluid. Comparisons between these linear results and nonlinear, finite-amplitude-heaving simulations are made to determine the role of nonlinearity in optimal performance.   

Thrust enhancement of a flexible foil oscillating near an air-water interface

Fish swimming near water surfaces is widely observed in nature. In this study, two-dimensional (2D) simulation was performed to analyze the hydrodynamic effects of the air-water interface on the propulsive performance of a heaving flexible foil. A sharp immersed boundary method was used to solve the fluid-flexible-body interaction problem, and a coupled level-set and volume-of-fluid (CLSVOF) method was adopted to capture the air-water interface. The flexible foil was modeled as a thin shell based on the nonlinear Kirchhoff thin-shell theory, and a plunging motion was prescribed on its leading edge. The effects of the submergence depth and the free-stream velocity were examined. The thrust produced by the foil was reduced near the air-water interface when the free-stream velocity was relatively low. The foil moving in a high-velocity stream, on the other hand, produced more thrust near the air-water interface than that far from the interface. The thrust enhancement or reduction due to the free-surface effect was found to be associated with the propagation of the leading-edge-induced wave, the phase velocity of which was estimated using the nonlinear dispersion relation. It was found that the thrust enhancement can  be amplified for up to 49% due to the free-surface effect.   

Experimental study on the effects of trailing edge geometry and pitching amplitude on the wake structure of bio-inspired pitching panels

Some aquatic swimmers create thrust with a caudal fin or a fluke found at the rear of the animal. These surfaces display many geometries, including those with different trailing edges. In the current work, the effects of systematically varying the trailing edge shape are studied using phase and time-averaged velocity fields obtained from stereoscopic particle image velocimetry in a water tunnel experiment. Results focus on the three-dimensional wakes produced by panels with straight, forked, and pointed trailing edges pitched at multiple amplitudes for Strouhal numbers, St, between 0.22 and 0.53. Results show that geometry and St influence wake behavior and dynamics. Portions of the phase-averaged wakes are often comprised of linked vortex rings, formed by greater amounts of vorticity as St number is increased. The time-averaged wakes indicate that increasing St leads to an injection of more momentum into the flow. In general, pointed trailing edges appear to generate more vorticity and momentum than straight and forked trailing edges.