Swimming without a Spine: Hydrodynamics and Swimming Performance of some Marine Invertebrates

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Johns Hopkins University
The primary focus of the present study is to employ computational modeling to investigate the hydrodynamics of free-swimming marine invertebrates. A high-fidelity computational tool based on a sharp interface immersed boundary method (ViCar3D), is developed, and this solver incorporates a non-inertial reference frame treatment in order to significantly reduce the computational cost of these simulations. The analysis includes the details of the wake characteristics and correlation with thrust generation mechanisms, as well as the swimming performance evaluated by coefficient variety of metrics. Simulations of free-swimming of three distinct marine invertebrates - the Spanish Dancer, Aplysia and the marine Flatworm, are performed. These animals are known to be active and effective swimmers and exhibit swimming gaits including body/mantle undulation and/or body bending, which are generally representative of a wide range of soft-bodied swimmers. Simulation show that despite a somewhat ungainly swimming motion, the swimming speed and propulsive efficiency of the Spanish Dancer are quite comparable to other more proficient swimmers. For the Spanish Dancer, a body planform with a wider caudal region has better swimming performance and this might explain the planform shape typically found for these animals. This importance of body-bending become apparent when examining free-swimming in marine Flatworms with two kinematic models- pure lateral flapping (LF) and combined lateral flapping and body pitching (CFP). While the body pitch magnitude is quite small (with maximum deflection angle of 15°), this small addition results in a significantly larger swimming speed (with increments ranging from 16% to 121% depending on the phase differences between lateral flaps and body pitch) and a higher Froude efficiency with increases ranging up to 43%. For the Aplysia, it is found that animals swimming with kinematics that match field videos have high propulsive efficiency and a relatively high swimming speed. By examining the Froude efficiency and power coefficient for various body planforms and kinematics, it is found that animals that employ the LF gait fall into one group, whereas other animals that employ the CFP gait fall into another, regardless their body shape. In general, the CFP gait is found to be more effective for swimming than the LF gait. Wake characteristics of the free-swimming of these animals are also analyzed. A bifurcated train of vortex rings is identified within the wake of the Spanish Dancer, and these vortex rings are found responsible for thrust generation. Other vortices in the wake are found to be drag producing. In the wake of the Aplysia, three distinct trains of vortex rings are identified in addition to other spanwise vortex structures resembling the Karman vortex street. For some models of Flatworms, the addition of a small body pitch was found to significantly change the wake topology, and a well-organized wake with distinct vortex rings is found to be associated with improved swimming performance. The current study provides a first-of-its-kind view of swimming in invertebrates and the comparative analysis performed here could provide insights into how these animals have adapted for life under water. The current research could also provide data and insights for the design of bioinspired soft swimming robots.
Biolocomotion, Invertebrates, Swimming performance, Wake topology