Fluid-Structure Interactions and Active Control in High-Performance Thunniform Swimming

In terms of bulk swimming metrics such as speed, efficiency, maneuverability, and stealth, fishes vastly outperform human-made underwater vehicles. Motivated by this performance gap, this thesis uses both idealized models of propulsors and reduced-order three-dimensional robotic platforms to distill the essential physics and design features of high-performance swimming.
A unique wireless experimental system was developed at the beginning as the foundation of this thesis. This experimental system has the ability to perform automatic direct force measurements and dynamic position tracking, which creates a bridge between traditional tethered tests and autonomous platforms. The modularized design allows us to configure the experimental system with different hardware add-ons and software for various studies. Together with the newly developed Three-Dimensional Particle Image Velocimetry (3D PIV) system, this experimental system is able to couple hydrodynamic features with performance measurements, thereby offering a powerful tool for studies in this thesis.
When fishes/robots swim near the substrate or near each other, they introduce unsteady interactions with the boundary. Both dynamic position tracking and direct force measurements reveal that unsteady steady ground effect leads to asymmetric lift generation and a boost in thrust without efficiency loss. For the first time, we proved the existence of equilibrium altitudes. When close to the ground, the time-averaged lift is zero at certain altitudes and acts to return the foil to these equilibria. When the foil moves very close to the ground (d/c≤0.35), there exists another unstable equilibrium altitude that pushes the foil away from or towards the ground. In all cases, the stable equilibrium altitudes move higher with increasing Strouhal number or with decreasing reduced frequency, but the unstable equilibrium altitudes are less sensitive. Increasing aspect ratio leads to stronger unsteady effects, including a stronger thrust boost, a larger asymmetric force, and a higher stable equilibrium altitude. The inviscid nature of these phenomena indicates that similar effects might exist when swimming near another out-of-phase swimmer.
Besides high-efficiency steady swimming, fishes are highly maneuverable compared to man-made underwater vehicles. Maneuvers are inherently transient, so they are often studied via observations of fish and fish-like robots, where their dynamics cannot be recorded directly. In the case of the fish-inspired maneuver study, we present a set of experiments in which a semi-autonomous hydrofoil performs repeatable in-plane maneuvers in a water tunnel. We show that modulating the hydrofoil’s frequency, amplitude, pitch bias, and stroke speed ratio produce streamwise and lateral maneuvers with mixed effectiveness. Our findings provide a framework for considering in-plane maneuvers and streamwise/lateral trajectory corrections in fish and fish-inspired robots.
Most fishes are equipped with multiple fins that can be used for manipulating unsteady three-dimensional interactions. In the case of multi-fin interactions during fish swimming, our study revealed the importance of dorsal fin shape on swimming performance enhancement and its role in multi-fin interactions. In particular, we used a tuna-inspired fish model with variable fin sharpness to study the interaction between elongated dorsal/analfins and caudal fins. We found that the performance enhancement is stronger than previously thought (15%increase in swimming speed and50%increase in swimming economy)and is governed by a three-dimensional dorsal-fin-induced cross-flow that lowers the angle of attack on the caudal fin and promotes spanwise flow. Both simulations and multi-layer particle image velocimetry reveal that the cross-flow stabilizes the leading-edge vortex on the caudal fin, similar to how wing strakes prevent stall during fixed-wing aircraft maneuvers. Unlike other fin–fin interactions, this mechanism is phase-insensitive and offers a simple, passive solution for flow control over oscillating propulsors.
Since flexible elements are widely present in fishes, it is reasonable to expect that flexibility might be relevant to their high swimming performance over a wide range of speeds. In the case of tail flexibility, we reveal one of the secrets of high fish efficiencies: tunable flexibility. Motivated by fish, who use muscles to modulate their stiffness, we derived a model that explains how and why tuning stiffness affects performance. We show that to maximize efficiency, muscle tension should scale with swimming speed squared, offering a simple tuning strategy for fish and fish-like robots. Tuning stiffness can double swimming efficiency at tuna-like frequencies and speeds (0-6 Hz; 0-2 body lengths/sec). Energy savings increase with frequency, suggesting that high-frequency fish and robots have the most to gain from tuning stiffness.