Meeting Abstract
Simple mechanical models like passively-flexing foils have been used recently to study specific aspects of fish locomotion without the confounding complexity of the entire fish body. Yet, unlike these uniform-stiffness foils, fish are notable for non-uniform material properties along the body length. In particular, passive flexural stiffness of fish decreases along the anterior-posterior body axis. Thus, to expand upon previous models, this study examined the role of non-uniform stiffness in swimming propulsion. We studied four foil configurations made by sandwiching layers of plastic to produce discrete regions of high and low stiffness. This resulted in two uniform control foils and two foils with high-stiffness anterior and low-stiffness posterior regions. With a mechanical flapping foil controller, we measured forces and torques in three directions and quantified swimming performance under both heaving and constant zero angle of attack motion programs. Foils swam at Reynolds numbers of 45,000-80,000 and Strouhal numbers of 0.20-0.30. Self-propelled speeds (SPS) of the foils increased with frequency, but did so in different manners across stiffness distributions and motion programs. Non-uniform foils often achieved SPS, thrust coefficients, and efficiencies similar to or greater than uniform foils, but often required more power to do so. We also evaluated cost of transport during the two motion programs. Future research will seek to understand the fluid mechanics driving these differences and will evaluate swimming performance of more advanced models with continuous stiffness gradients.
