Hydrodynamics of flexible-body swimming motions

Abstract

The performance and agility of fish swimming motions have intrigued both biologists and fluid mechanicians for many years. Both have endeavored to understand the mechanics of fish swimming and to resolve the paradoxical observations surrounding the performance of fish, yet the unsteady hydrodynamics are not well-understood. In this thesis, the hydrodynamics of the fish-like swimming motions of a flexible body are examined through numerical simulation. Two- and three-dimensional boundary integral panel methods are developed which can model the steady straight-line swimming and unsteady maneuvering motions of a flexible-body of arbitrary thickness. Multiple, desingularized, infinitesimal wake sheet representation models the nonlinear dynamics of thin shear layer vorticity shed from an arbitrary number of predefined wake separation edges. The integrated performance quantities and the near-body unsteady flow features are corroborated through experimental comparisons. Employing this numeric scheme for a variety of fish forms, the unsteady flow dynamics are resolved in great detail and are found to be much more complex than that predicted by linear theory. In addition, fundamental mechanisms of near-body flow actuation, body-generated vorticity release, and wake vorticity control are found which allow the fish to generate thrust efficiently, to achieve outstanding performance, and to generate large, short-duration maneuvering forces. Specifically, the straight-line swimming motions of a flexible-body are studied through simulation of several fish geometries. Comparison to classical linear theory highlights the importance of the vortical dynamics in achieving performance and the complexity of the near-body flow patterns. The flow around the body is found to be highly longitudinal through systematic visualization of the body cross-sectional and waterline planes. A body-generated vortex ring structure, created through localized body undulations, actuates the smooth near-body longitudinal flow patterns around much of the fish body, resulting in strong vertical vorticity components bounding the wake thrust jet. Regions of high propulsive efficiency are identified for certain prescribed kinematics, and the performance is found to be strongly dependent on kinematic variation, recoil motions, and geometric modeling choices. Maneuvering hydrodynamics of fish swimming are studied through the simulation of a 60° "C" -turn of a Giant Dania. The formation and controlled release of body-generated vorticity through local contortions of the backbone is shown to affect the formation of a turning thrust jet for rapid maneuvering. The interaction body-generated free vorticity and regions of high fluid momentum with the sweeping motion of the tail fin is similarly shown to affect both the strength and direction of the turning jet. Through simulation of these straight-line swimming and unsteady maneuvering motions, fundamental mechanisms of vorticity control utilized by the fish are identified. Body-generated vorticity released by the body upstream is actuated by the motion of the oscillating tail fin, resulting in complex wake-wake-body interactions for varying kinematics. These interactions may enhance the performance by increasing thrust or increasing efficiency; similarly, large drag forces may be enhanced through constructive interaction of the wakes. Several novel vorticity control modes are elucidated for both straight-line steady swimming and unsteady maneuvering motions. Mechanisms of near-body flow actuation and vorticity control by the motions of swimming flexible-bodies and oscillating lifting surfaces may have tremendous potential for application to vehicle design and to unsteady maneuvering systems.