| dc.description.abstract | Many swimming fishes in nature have been endowed with high hydrodynamic efficiency and performance, such as low resistance, high speed, and good maneuverability, both when swimming individually and in groups, i.e., using schooling arrangements. The main purpose of the present thesis is to study the fish hydrodynamics through both experimental and numerical investigations, to better understand the physical mechanisms behind high propulsive skills of fish-like bodies, and to build knowledge relevant for the design of greener and more efficient bio-inspired underwater vehicles, which could lead to positive consequences for the environment and the society.
Observational experiments were carried out in swim tunnels under different conditions and examined two fish species using different swimming modes, i.e., a labriform swimmer (shiner perch, Cymatogaster aggregate) belonging to median and/or paired fin (MPF) propulsion category, and a sub-carangiform swimmer (Atlantic salmon, Salmo salar) belonging to the body and/or caudal fin (BCF) propulsion category. In the experimental investigations performed on the shiner perch, the examined fish were randomly assigned to one of the three experimental scenarios: a solitary fish (Single), a schooling pair of fish (Pair), and a false pair where a single fish swam alongside a video of a conspecific fish (False pair). The swimming behaviours and metabolisms of the fish were analyzed and discussed among different arrangements. The comparisons suggested that schooling confers both hydrodynamic and behavioral advantages over swimming alone for a gregarious fish, but that the relative contribution of the two mechanisms depends on the speed of swimming. In the experiments carried out on Atlantic salmon, the observed results showed that the examined hatchery-reared fish had lower critical swimming speed than biological data of wild fish and fish swimming in larger flumes. The influence of fish body size and swim tunnel boundary walls on the salmon behaviors in the flume was discussed to identify important factors affecting the fish swimming capability. The analysis may provide current velocity threshold for farmed salmon during on-growing phase and give suggestions for experimental set-ups more suited for hydrodynamic and biological studies on fish.
In order to gain deep insights into the propulsive mechanics of swimming fish and to complement experimental studies, a series of hydrodynamic scenarios has been examined by performing two-dimensional CFD simulations within the OpenFOAM open-source platform. Three rigid flapping foils with different fish-like profiles, the tear-shaped semi-circle foil, NACA 0012 foil and NACA 0021 foil, have been studied through systematic numerical simulations to grasp the key propulsion characteristics. The predicted hydrodynamic forces and wake scenarios of the semi-circle foil have shown good consistency with available experimental data. The critical Strouhal number of the rigid pitching foils at drag-thrust transition is found to be a decreasing function of the Reynolds number, and a universal scaling law of force transition quantifying foil shape effect has been attempted. The analysis of the body-shape influence showed that the forepart of the flapping foil dominates the friction force component, while the trailing edge shape matters for the pressure force. Based on the results of the examined rigid pitching foils, a morphing foil strategy has been proposed to combine the advantages of two rigid foils associated, respectively, with the largest mean thrust and with the lowest mean input power requirement. A parametric analysis of the phase between pitching and morphing motions has been performed and an optimal morphing strategy has been identified. These results can pave the way for the use of morphing strategies to target optimized propulsive behavior and, in a broader context, to overcome limitations of rigid vehicles/devices in marine applications.
The swimming performance of 2D fish-like foils with carangiform propulsion mode has been numerically studied in two different scenarios: one is the fish-like foil, with prescribed undulatory deformations and restrained longitudinal motions, under an incoming uniform stream, and the other one is the fish self-propelling itself in the fluid. In the former case, the computed hydrodynamic loads of the forced swimming foil agreed well with reference data. The influence of body shape and Reynolds number on fish swimming behavior has been examined. The comparisons between two different fish-like foils confirmed that a fish with a blunter body experiences larger drag and has more limited capability to generate thrust compared to a slimmer fish. For the second scenario, a numerical method capable of simulating self-propelled bodies with periodic lateral flexible motions has been proposed. The implemented self-propulsion strategy in OpenFOAM, verified to be numerically accurate and efficient through studying an oscillating elliptic foil case, has been adopted to predict the swimming perfomance of a carangiform fish-like body under various conditions. The self-propelled swimming scenario allows to examine the effect of recoil. The influence of boundary wall conditions on fish locomotion has also been investigated systematically. The results showed that the forward swimming speed and the propulsive efficiency of the fish dropped significantly with the decrease of swim tunnel width. This analysis provided an indicative minimum swim tunnel width to limit the boundary wall influence on the swimming features. | en_US |
