Session J03: Locomotion: High Reynolds Numbers

Here Fishy Fishy: Resolvent Analysis of the Boundary Layer a Swimming Foil

This study uses resolvent analysis to explore the dynamics and coherent structures in the boundary layer of a foil that swims via a travelling wave undulation. A modified NACA foil shape with undulatory kinematics is used to represent fish-like bodies at realistic Reynolds numbers (Re = 10,000 and Re = 100,000) in both thrust and drag producing propulsion regimes. We introduce a novel coordinate transformation that enables us to use data-driven resolvent analysis to investigate the stability of the boundary layer of the swimming foil. This is the first study to use resolvent analysis on deforming bodies with non-zero thickness and at realistic swimming Reynolds numbers. The analysis shows that swimming kinematics drives the system's physics. In drag-producing regimes, it reveals breakdown mechanisms of the propulsive wave, while thrust-producing regimes show uniform wave amplification across the foil's back half. The key thrust and drag mechanisms scale with boundary-layer thickness, implying geometric self-similarity in this Re regime. Additionally, we identify a mechanism less strongly coupled to body motion. We compare this to a rough foil that reduces the amplification of this mechanism, demonstrating how roughness can control the amplification of key mechanisms in the flow. These results provide valuable insights into the dynamics of swimming bodies and suggest ways to develop opposition control strategies.   

Spin is all you need for fast and efficient swimming

The biomimetic engineering of fish-like robots has sought to recreate the specific characteristics of body or tail undulutaions with limited success in emulating the efficiency, speed or agility of the fish. In this talk an alternative design of a fish-like robot is presented, in which the spin of an unbalanced internal rotor is used generate parametric oscillations of a flexible tail and small amplitude oscillations of the main body. The oscillations of the tail generate thrust by the added mass effect according to the slender body theory and the reverse Karman wake it generates. The tail oscillations can switch from the first to the second mode depending on the spin frequency of the rotor. This novel locomotion generated by spin parametic oscillations is highly efficient at about a third of the efficiency of a tuna at speeds of 2.5 body lengths per second. Such efficiency in a free-swimming robot (untethered and unsupported by any cables in a water tunnel) is the highest amongst swimming robots. Moreover the the spin motion of the internal rotor confer gyroscopic roll stability to the robot and at the same time allow controlled roll and turning ability.   

Unsteady wake-surface interactions of rapid near-surface swimming

Many natural swimmers, such as dolphins and sailfish, can swim rapidly near the free surface. The underlying unsteady wake-surface interactions are thought to be governed by the unsteady number. However, the unsteady number can arise from either high flow speed with low motion frequency (low Strouhal number, e.g., hydrokinetic energy harvesters) or low flow speed with high motion frequency (high Strouhal number, e.g., accelerating dolphins), leading to distinct unsteady wake patterns. The impact of these different wake patterns, particularly the high Strouhal number related to biological swimming, remains underexplored. To identify the interplay between unsteady and Strouhal numbers, we conducted extensive parameter sweeps covering high unsteady and Strouhal number regimes, closing gaps to rapid swimming in nature. Additionally, 3D Particle Image Velocimetry (PIV) will be conducted to identify key wake features that influence changes in thrust and stability, providing deeper insights into rapid near-surface swimming.   

Hydrodynamic characteristics of the ocellate river stingray Potamotrygon motoro

The ocellate river stingray (Potamotrygon motoro) is a negatively buoyant and dorsoventrally flattened fish, resembling a hydrofoil, that swims extremely close to the substrate in South American river basins. Previous research, utilizing force transducers and particle image velocimetry (PIV), demonstrated that P. motoro exhibited positive lift-drag ratios when swimming near the ground. Conversely, negative lift-drag ratios were observed when the specimen was positioned further from the substrate, attributed to the unexpected negative lift inherently generated by the ray's form. This finding suggests that the form of P. motoro might intrinsically stabilize the ray's vertical position close to the ground. However, the hydrodynamic mechanism inducing negative lift away from the substrate remains unclear. This study employed a 3D-printed model, based on a scanned specimen of P. motoro, to confirm similar lift-drag patterns relative to the model's height from the substrate in a recirculating flume for different body size. Computational fluid dynamics is also used to test whether the high contoured slope between the lip and head of P. motoro induces this negative lift and to determine if a vertical stabilizing mechanism is indeed generated by the ray's form. Not only does this advance our understanding of locomotion in stingrays, but it also have implications for the design of bioinspired underwater vehicles moving along the ground.     

Two-stage Optimization of a Tapered Elastic Swimmer Using a Parameterized Genetic Algorithm

Fish are able to swim very efficiently at high speeds, even though most of their body is not being actuated. Part of what allows them to do this is the tapering in their bodies near their tail, which changes the size and phase of the oscillations at each point. By tailoring the shape of the tapering, we can increase the thrust and efficiency of an oscillating hydrodynamic plate. To search through the design space without resorting to full fluid-structure interaction (FSI) models, we use an evolutionary genetic algorithm to test and optimize the tapering shapes. As this model cannot directly calculate power and thrust, we use parameters that are correlated with swimming performance, including tip displacement, standing wave ratio, and displacement area. The best-performing shapes are validated using the FSI model, and gradient descent is done on these shapes to determine the optimal weight function.