| Laboratory studies have shown that fishes swimming in uniform flow produce thrust by undulating their bodies in a stereotypical manner. However, in nature many fishes routinely encounter turbulent flows that arise from flow past stationary objects or from the propulsive movements of other fishes, as when holding station behind a rock in a stream or when schooling. This dissertation uses high-speed visible and infrared video motion analyses, quantitative flow visualization, electromyography and pharmacological lateral line treatment to expose a novel mechanism by which fishes can extract energy from environmental vortices to enhance their swimming performance, herein referred to as the Karman gait. When exposed to experimentally generated vortices shed behind a cylinder, many species of fish adopted the Karman gait by synchronizing their body kinematics to the shed vortices, the motions of which resemble a flag flapping slowly in a breeze. Digital Particle Image Velocimetry was used to visualize and quantify the size, strength, and shedding rate of the cylinder vortices. Synchronization of flow visualization images to the kinematic profile of Karman gaiting trout revealed that fish were slaloming in between vortices rather than intercepting each one. Electromyographic recordings indicate that trout swimming in uniform flow sequentially activate axial muscles along their entire body from head to tail. In contrast, during the Karman gait relatively little activity is needed: only the red axial muscles near the head are active, suggesting that muscle activity controls stability rather than provide propulsion. The trout lateral line system detects water flow and is used when interacting with cylinder vortices, since experimentally blocking this sensory modality changes Karman gait kinematics significantly. In the absence of visible light, trout both with and without an intact lateral line sense prefer not to swim in the vortex street if given other flow environments to choose from, often balancing near the suction region. These results document a new mode of locomotion for fishes, shed light into the patterns of fish distributions in schools and riverine environments, provide inspiration for autonomous underwater vehicles designed to negotiate turbulent flows, and promise to further the design of fish ladders used to guide fish around hydroelectric dams. |
