Session T14: Biofluids: High Re Locomotion III

Hydrodynamic loads and vortex evolution from a flapping fin near a solid body

The pectoral fins of fish play an important role in the fine control of fish motion. The dynamics of a simplified fin-body configuration is examined in experiments to study the hydrodynamic loads and the vortex dynamics of fin-body interactions under selected actuation inputs. The loads show different mean values, amplitudes, and hysteresis depending on the mean fin angle and the flapping frequency. Particle image velocimetry results reveal the influence of the shear layer and the fin-tip vortex on the hydrodynamic loads. Preliminary analysis of the quantitative contributions from the vortical, viscous, and inviscid flow effects is performed using the force/moment partitioning method.   

Determining self-propelled swimming speeds of isolated fin models for a range of kinematic and planform geometry parameters

When moving through an aquatic environment, many swimming animals use an oscillating caudal fin or fluke to generate forward propulsion. Among the various species of swimming animals, propulsive appendages may display significant diversity in geometric factors like planform shape, leading edge shape, and trailing edge shape. The current work builds on prior experimental work that investigated the swimming speeds of bio-inspired pitching and heaving propulsors in a recirculating water tunnel. Rather than focusing on performance measurements captured for a fixed velocity freestream flow, the current work studies performance using experiments to determine self-propelled swimming speeds for combined pitching and heaving motions. Trailing edge and leading edge shapes, as well as kinematic parameters, are varied for a series of bio-inspired panels with a nominally trapezoidal planform. Thirteen unique panel geometries were actuated through multiple motion profiles until the resultant self-propelled swimming speed was determined. Panel geometries were systematically changed in a manner that allows for investigations into specific geometric factors and their influence on non-dimensional swimming speeds. Results are discussed in the context of changes to planform shape, area, aspect ratio, geometric centroid, and kinematics. The current work has implications on the design of bio-inspired vehicles during conditions of constant velocity swimming.   

Oscillatory and undulatory swimming of a stingray-inspired platform

Different stingray species have evolved different swimming modes depending on how closely they live to the sea floor, i.e., substrate. Oscillatory rays (fin wavenumber < 1) tend to live far from the substrate (pelagic), while undulatory rays (fin wavenumber > 1) tend to live near the substrate (benthic). The hydrodynamic differences between oscillatory and undulatory stingray locomotion have not previously been studied using a single platform. Here, we study the hydrodynamic performance of both swimming modes using a stingray-inspired robotic platform that prescribes wavenumber using a modular rotary cam-train system. Near-ground swimming tests reveal that both swimming types produce a frequency-dependent lateral suction force that is inversely related to the wavenumber. Therefore, the oscillatory mode requires a higher positive angle-of-attack to maintain level swimming comparing with the undulatory mode. At matched frequencies, oscillatory swimming outperforms undulatory swimming in both thrust generation and efficiency, regardless of substrate proximity. Finally, we measure the three-dimensional wake structures using multi-layer stereo PIV to explain the observed performance differences. This study advances our understanding of oscillatory and undulatory swimming, offering valuable insights into stingray locomotion and the design of bio-inspired robotic systems.   

On the stability of interacting flapping plates

The motion of several plates in an inviscid and incompressible fluid is studied numerically, the general motivation being to understand the hydrodynamic interactions in schooling and flocking behavior in animal collectives. We consider two to four plates initially placed in-line, one behind the other, separated by a specified distance d0, and move each in the vertical direction with a prescribed oscillatory motion. This yields horizontal plate motion due to their self-induced thrust and the fluid drag forces. The plates are observed to approach an equilibrium distance between each other that depends on d0, with the front plate moving practically the same as a single plate. In this talk we address the stability of these equilibria. We find that the equilibria lose stability as either the number of plates increases, or the oscillation amplitude decreases. A simple mechanism is implemented and shown to successfully stabilize the motion. The stabilization has a remarkable impact on the regularity of the vortex pattern in the wake.   

Hydrodynamic interactions between shape and substrate could provide a stabilizing effect in the vertical position of river stingrays

Potamotrygon motoro is a foil-shaped stingray that swims along the ground in South American river basins. In this study, we measured lift and drag forces, and the posterior flow field using particle image velocimetry (PIV), to characterize the hydrodynamic performance of P. motoro as a function of flow speed and distance from the ground. The experiments were conducted in a recirculating flume where a dead ray (14cm width, W) was attached to load cell at an angle of attack of 0°, to measure forces and flow at different heights from the substrate (0.001-0.85W) at a range of speeds (0-1.33W/s). The ray generated negative lift when positioned furthest away from the ground (>0.5W). However, lift changed to positive and increased in value as the ray was positioned closer to the ground. Furthermore, as expected, drag decreased as the ray was positioned closer to the ground. From the lift-drag ratios (L/D) we observed three distinctive regions: weak ground interaction with negative L/D due to negative lift (>0.5W), intermediate ground interaction with slightly positive and constant L/D (0.07-0.5W), and strong ground interaction with high L/D (<0.07W). Hence, P. motoro may benefit from hydrodynamic interactions with the substrate that inherently stabilizes the fish near the ground when swimming.   

Effect of Turbulence on the Hydrodynamics of Fish Schools

The collective behavior of fish, observed across various species, offers numerous advantages such as improved foraging, sensing, and interaction. While previous research has primarily focused on steady flow due to the complexities of measurements and high computational costs, the dynamic and often turbulent environment in which fish naturally exist motivates the study of fish schools in turbulent flow. In our previous work, we used a sharp-interface immersed boundary flow solver to simulate the hydrodynamics of a minimal school, demonstrating that enhancement of the leading-edge vortex (LEV) could improve the swimming performance of fish. However, the robustness of the LEV-based mechanisms under turbulent flow conditions remains unclear. In this direct numerical simulations-based study, we imposed a turbulent inflow to the fish school. We then examine the data to gain insights into the effect of turbulence intensity and scale on the swimming performance of fish in schools.    

Hydrodynamic Interactions in Schooling Fish: A Comparative Study of BCF and MPF Swimmers

Fish locomotion involves two primary propulsion strategies: Body-Caudal Fin (BCF) and Median-Paired Fin (MPF) modes, each with distinct body compositions. BCF swimmers have propulsors downstream of their body trunks, while MPF swimmers possess propulsors parallel to them. Despite these differences, both types have been observed swimming in groups. To investigate the hydrodynamic effects of different propulsion strategies in schooling, this study employs a tuna and a manta ray model, representing BCF and MPF swimmers, respectively. Three-dimensional (3D) numerical simulations were utilized to compare the flow features and hydrodynamic interactions of a tuna-like and ray-like swimmer pair. Detailed flow analysis revealed beneficial fin-fin and body-fin interactions in staggered configurations. The vortex-capturing mechanism played a crucial role in the hydrodynamic benefits of the staggered follower. These findings shed light on the performance differences between BCF and MPF swimmers in schooling and provide valuable insights into vortex interactions for bio-inspired robotics and underwater vehicle design.   

On Lighthill's elongated-body theory

In 1971, Lighthill published a paper entitled "Large-amplitude elongated-body theory of fish locomotion". In this paper, he derived a potential flow theory for the undulatory locomotion of an elongated swimmer. He showed, in particular, that the average thrust force due to the undulatory motion only depends on the kinematics of the tail. To this day, it has been the only theory for undulatory locomotion derived from first principles. 

During this talk, we will revisit Lighthill's elongated-body theory. We will show in particular how it can be used to derive the time-dependent force along the backbone at any instant. We will also show the potential flow associated with this theory and compare it with numerical simulations.