Kinematic Mode Related Hydrodynamic Interactions in Fish Schools Across Body-Propulsor Configurations

Aquatic animals have developed specialized morphological features and distinctive locomotion strategies over the course of evolution to interact with surrounding fluids to achieve efficient swimming under active changing flow conditions. As a result, fish are developed to possess specialized locomotion modes to achieve optimal interaction effectiveness with the surrounding fluids, which can be attributed to the flow induced by the nearby conspecies peers. The hydrodynamic interactions between the trailing fish and the upstream flow conditions produced by its peers are commonly observed in fish group swimming which can be beneficial for their propulsive performance.
This dissertation utilizes numerical approaches to examine the mode related fluid dynamic interactions in stagger schools and the underlying flow physics and the interaction mechanisms in aquatic animals across major forms of body-fin configurations and the corresponding locomotion modes. By defining the propulsive force production body portion, fishes can be categorized as median-paired fin (MPF) swimmers and body-caudal fin swimmers (BCF), and by the locomotion features, swimming motions can be defined as oscillatory and undulatory. High-fidelity computational models with swimming kinematics are produced with prescribed motions derived from high-speed videos of live fish locomotion and biological measurements. Numerical simulations are then performed using an in-house developed sharp interface immersed boundary method based direct numerical flow solver to obtain hydrodynamic performances and the detailed three-dimensional flow information for the analyses of performance enhancements achieve through schooling interactions and the relation to the kinematic modes used by the swimmers.
The interaction mechanisms involving multiple schooling members are examined across different body configurations and kinematic modes. For the schooling interaction in the MPF oscillatory swimming in manta ray, a maximum of 72% overall thrust enhancement is found on the follower due to the interactions with the upcoming flow condition induced by the leader. For the schooling interaction in the BCF oscillatory swimming in tuna, it is found that the presence of an upstream peer can provide a thrust enhancement by 16% and a drag reduction by 15%. At the optimal streamwise separation, the follower continues to benefit from the preferred hydrodynamic force productions regardless of the swimming phase change. A further follower’s drag reduction of 19% is found when it is at a phase change of 270°. For the schooling interaction in the MPF undulatory swimming in stingray, simulation results have shown a consistent thrust enhancement by the leader at 5% - 7% throughout the examined schooling configurations, while the thrust increase on the follower only appears in selected locations. At these locations, the follower benefits from a 9% overall thrust improvement and a 7% improved propulsive efficiency. For the schooling interaction in the BCF undulatory swimming lamprey, it is found that the hydrodynamic benefits on the trailing fish relate closely to the vortex spatial phase regardless of the wavelength of the fish in the swimming kinematics. While the pressure field interaction dominates drag production, the wake collision is found to be the mechanism responsible for the thrust enhancement on the trailing fish.
Major mechanisms responsible for the hydrodynamic performance enhancement can be attributed to the increase of the transverse flow across the leading edge or the trailing edge of the propulsor, and the vortex collision in the wake region in the examinations of the kinematic mode related fluid dynamic interactions in staggered schools presented in this dissertation work.
The overall findings of this dissertation advance the understanding of kinematic mode related schooling interactions in biological propulsion and provide novel physical insights into the deployment of aerial/underwater unmanned vehicle swarms from a fluid dynamics perspective.