Bio-inspired propulsion: Manta ray swimming

Evolution has provided us with countless “solutions” to the problem of swimming and flying. We seek to gain understanding to some of these “solutions” in an engineering context by learning about and applying the physical mechanisms observed in nature. One species of particular interest to our work is the manta ray, the largest of the ray species. The manta ray is a migratory species that can weigh up to 5,000 pounds and be 25 feet in finspan. Despite their large size, the manta displays quick bursts of acceleration and turn-on-a-dime capabilities, making them an ideal candidate for inspiration to next generation autonomous underwater vehices (AUV’s).
We work in collaboration with researchers from the University of Virginia, UCLA, and West Chester University to understand more about these animals and their potential for use in engineering applications. From an experimental fluids perspective, a quick glance at the manta ray yields a fascinating result: the cross-section of their fins is identically that of a symmetric airfoil. They flap these flexible fins producing both spanwise curvature and a chordwise traveling wave motion to propel themselves through the water. In short, we are dealing with an unsteady aerodynamics problem with flexible fins, also known as aerodynamics on steroids.
Digital particle image velocimetry (DPIV), dye-flow visualization, and thrust measurements are just a few of the techniques used to investigate the manta ray. We hope to gain insight into how the various kinematic motions that we see in ray species relate to performance characteristics (efficiency, turning, acceleration, etc.) and wake structures generated by the flapping motion. Thus far, our studies have focused on determining the mechanism which causes a drop in efficiency as the Strouhal number, a non-dimensional flapping frequency, is increased for a given peak-to-peak amplitude. We are also investigating the effects of a passively flexible trailing edge of the fin (shown below) to determine how passive flexibility alters thrust producing characteristics and wake structures.