Swimming Underwater: Effects of Body/Fin-Fin Interaction, Gait Selection and Propulsor Shape Optimization on High-Performance Bio-Inspired Propulsion

Aquatic animals have been considered skillful at performing fast, efficient and agile locomotion because of their deformable fins, body/fin-fin interactions, diverse swimming gaits, as well as energy extraction from their surroundings. Despite the compelling progress that has been made in the last two decades to understand aquatic locomotion, our knowledge of the underlying high-performance mechanisms is still incomplete. Furthermore, achieving similar levels of hydrodynamic performance as biological system in engineering design has proven to be challenging. This dissertation describes efforts toward understanding the effects of body/fin-fin interaction, swimming gait selection and propulsor shape optimization in high-performance aquatic locomotion. This work also aims to bridge some of the gaps between aquatic propulsion systems found in nature and bio-inspired flapping propulsors in engineering design.
The topics covered in the dissertation are as follows:
1. With a three-dimensional (3D) bluegill sunfish model, the impacts of body-fin and fin-fin interactions are studied. The results elucidate that, by adjusting their shapes and movements, the dorsal and anal fins influence the hydrodynamic performance of the caudal fin and body trunk. The fin-fin interaction effects are then further studied with a simplified canonical model, which consists of multiple foils pitching in line. The performance and corresponding wake patterns are analyzed in detail.
2. With the 3D model reconstructed from the high-speed photogrammetry of a trout performing intermittent swimming, the kinematics analysis is performed on the swimming gait and its effects on the hydrodynamic performance are investigated. Furthermore, a simplified canonical model is created with the kinematic data extracted from the rainbow trout. Forces and energetics are compared between the continuous and intermittent swimming gaits, and the effects of the time ratio between the burst phase and the intermittent swimming period (duty cycle) are investigated.
3. The fluke of cetaceans (dolphin, etc.) presents a significant morphological difference with fish, because they possess fluke composed of fibrous tissue instead of a membrane supported by fin rays. Performing an integrated study combining geometric parameterization methods, high-fidelity direct numerical simulations and gradient-based optimization, the effects of propulsor shape in aquatic locomotion are investigated.
The primary contributions of this dissertation are in the discovery and characterization of strategies aquatic animals used for high-performance locomotion, as listed below:
1. Larger and phase-leading dorsal and anal fins enhance the propulsive efficiency of the caudal fin and reduce the trunk drag significantly. This body trunk drag reduction is attributed to the dorsal and anal fins debilitating the interactions between the left- and right-stroke posterior body vortices.
2. The multiple foil propulsion system demonstrates that branched momentum jets in the downstream can indicate a better hydrodynamic performance than the coherent jet structure.
3. The rainbow trout is found to increase its tail tip flapping amplitude, flapping frequency, body wave traveling speed and body curvature to generate large thrust and accelerate its body weight during the burst phase.,
4. At the same swimming speed, the intermittent swimmer presents a lower energy cost than the continuous swimmer in the higher duty cycle range. For the intermittent swimming gait, both swimming speed and cost of transport reduce with the reduction of duty cycle.
5. The foil shape (cross section) optimization is first carried out on flapping foil and shows that, for the foil conducting large-amplitude pitching-heaving motions, a foil shape with thicker forebody and sharper trailing edge can improve the propulsive efficiency significantly.
By enumerating these body/fin-fin interaction, gait selection and propulsor shape optimization strategies across diverse species, our knowledge of aquatic locomotion is substantially advanced and the integrated applications of these strategies are expected to aid the designs of next-generation unmanned underwater vehicles.