Session Q13: Biological Fluid Dynamics: Locomotion, High Reynolds, Number Swimming III

Hydrodynamic Interactions in Schooling Fish: Disentangling Mechanisms via Highly Resolved Direct Numerical Simulations and Force Partitioning

A fish swimming in the proximity of others may obtain hydrodynamic benefits such as increased thrust generation and reduced power consumption due to the interaction with the flow induced by neighbors. Several different mechanisms (drafting, "channeling," constructive wake-interference, vortex phase matching, LEV enhancement, etc.) have been proposed but results have often been contradictory and difficult to reconcile. Ultimately, the swimming performance is determined by the time-varying pressure forces on the fish, but pressure is simultaneously affected by vortices, viscous diffusion, and added-mass effects, thereby making it difficult to determine causality. In the present study, the hydrodynamic interactions between a pair of swimming fish are examined by performing a series of high-resolution, three dimensional simulations. The simulation results show that the thrust and power of the trailing fish interacting with the wake of the leading fish changes about +/- 10% and +/- 5%, respectively, depending on the streamwise distance and the tail beat phase difference. The mechanisms underlying these interactional effects are analyzed by applying a force partitioning method, which enables quantification of the effect of vortices around the fish on the force generation.   

Enhancing bio-locomotion of elastic propulsors by combining internal and external actuation

We investigate using three-dimensional computer simulations the hydrodynamics of a bio-inspired elastic propulsor that undergoes plunging motion in a Newtonian fluid. To enhance the hydrodynamic performance of the propulsor, in addition to the heaving motion at the base, the propulsor is actuated by a periodic internal bending moment imposed along the propulsor length. We vary the phase difference between the two actuation modes to probe their synergetic effect on the propulsor hydrodynamics. The simulations reveal that by tuning the phase difference, the propulsor thrust and free-swimming velocity can be varied in a wide range while maintaining high hydrodynamic efficiency. Furthermore, the propulsor with the hybrid actuation significantly outperforms the propulsors with either of the single actuation modes. We relate the improved performance to reduced center of mass displacement with increased trailing edge displacement of the propulsor achieved at specific values of the phase difference. The results of our study are useful for designing highly efficient robotic swimmers utilizing propulsors made of smart materials such as piezoelectric macro-fiber composites.   

Helical swimming by jet-propelled salp colonies in the ocean

Helical swimming is common among microorganisms and has inspired the development of swimming microrobots owing to their high performance at low Re.  However, there are few reported cases of larger swimmers that rely on a helical swimming pattern. Salps are cm- to meters-long colonial organisms that coordinate multiple jet-propelled swimming units to achieve efficient swimming.  The salp species, Weelia cylindrica, swims in a helical pattern but it is unknown what drives this swimming pattern and whether helical swimming confers any propulsive advantages over non-helical swimming salp species.  We recorded the three-dimensional swimming paths of W. cylindrica via a SCUBA diver-operated stereo camera system to address the following questions: 1) are helical swimming trajectories driven by morphology, kinematics or, both? and, 2) do helically swimming salps achieve higher swimming speeds than their non-helical-swimming counterparts?  The details of W. cylindrica swimming mechanics offer insights into performance of helical swimming driven by the coordination of multiple jets.   

Active Control of Caudal Fin-Ray Stiffness and Propulsion

 Fish fin ray features a unique bilaminar structure that comprises a pair of concave-shaped hemitriches, each of which is pulled independently by muscles to control the shape and stiffness of the fin ray. Fish precisely control the shape and stiffness of each ray to generate desired hydrodynamics forces on the fins. In the current study, the active control of the fin ray stiffness in propulsion through the hemitrich structure is studied using numerical simulations. A realistic fin-ray structure is constructed by including the two hemitriches and intra-ray layer. The fin ray control is implemented by specifying a dynamic hemitrich offset at the root. The effect of the dynamic parameters of the offset on the flexural stiffness and the resulted propulsion performance of the ray is investigated through flow-structure interaction simulations. The study is expected to improve the understanding of fish fin active stiffening in propulsion.   

A stingray is affected by the ground differently depending on its aspect ratio

Animals and bio-inspired robots can swim/fly faster near solid surfaces, with little to no loss in efficiency. How these benefits change with propulsor aspect ratio is unknown. Here we show that lowering aspect ratio weakens unsteady ground effect: thrust enhancements become less noticeable, stable equilibrium altitudes shift lower and become weaker, and wake asymmetries become less pronounced. Water channel experiments and potential flow simulations reveal that these effects are consistent with known unsteady aerodynamic scalings. We also discovered a second equilibrium altitude even closer to the wall (<0.35chord lengths). This second equilibrium is unstable, particularly for high-aspect-ratio foils. Active control may therefore be required for high-aspect-ratio swimmers hoping to get the full benefit of near-ground swimming. The fact that aspect ratio alters near-ground propulsion suggests that it may be a key design parameter for animals and robots that swim/fly near a seafloor or surface of a lake.   

Tunable stiffness enables fast and efficient swimming in fish-like robots

Fish maintain high swimming efficiencies over a wide range of speeds. A key to this achievement is their flexibility, yet even flexible robotic fish trail real fish in terms of performance. Here, we explore how fish leverage tunable flexibility by using their muscles to modulate the stiffness of their tails to achieve efficient swimming. 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-like robots. Tuning stiffness can double swimming efficiency at tuna-like frequencies and speeds (0 to 6 hertz; 0 to 2 body lengths per second). Energy savings increase with frequency, suggesting that high-frequency fish-like robots have the most to gain from tuning stiffness.   

Experimental investigation of the relationship between the propulsive performance and vortex rings produced by bio-inspired pitching panels

Many animals propel themselves through the water using either a caudal fin or a fluke. These propulsive appendages display wide diversity in planform, including those with different trailing edge shapes. In the current work, trailing edge shape and pitching amplitude are varied for bio-inspired pitching panels with a nominally trapezoidal planform. In total, five unique panel geometries, each with a different trailing edge shape, were pitched in a constant free stream flow at mulitple amplitudes. Experimental results are discussed in the context of changes to Strouhal number, which ranged between 0.09 and 0.66. Results show that the time-averaged performance of low aspect ratio panels with pointed trailing edges exceeds that of high aspect ratio panels with forked trailing edges. However, lower aspect ratio panels exhibit enhanced performance despite the presence of significant three-dimensional effects that manifest through the shedding of streamwise vorticity into the wake, which is typically associated with degraded performance. Through the actions of deforming vortex rings in the wake, streamwise vortices can be linked to the development of vortex-induced velocity fields that have a significant streamwise component. The development of accelerated streamwise flows in the wake, which are associated with the actions of streamwise vortices, is ultimately beneficial for increased time-averaged thrust and enhanced propulsive performance. The current work also focuses on the implications of these findings on the design of bio-inspired vehicles and the effects that propulsor geometry and kinematics may have on swimming animals.   

Efficient flapping foil flow at high Re 2.5 x 105 for large underwater vehicle

The recent development of Autonomous Underwater Vehicles (AUVs) has spurred many studies to understand the efficient swimming performance of animals. Despite these rigorous studies, the propulsive performance of those AUVs is far less efficient than animals. This study explores the possibility of propulsion parameters from animals to be applied at autonomous vehicles using high-fidelity Boundary Data Immersion Method (BDIM) simulations. We find that despite the large leading-edge vortex breakdown on the foil, the flow characteristics are still overpowered by the oscillating kinematics for the optimal Strouhal number range St=[0.2,0.4]. This is due to a laminarization process produced by the flapping motion which was previously reported at lower Reynolds number by Zurman-Nasution et al. [JFM, 2020]. The near-wall flow behavior is presented along with the hydrodynamic forces, providing a detailed explanation of this laminarization behavior. This result allows the dimension-scaling of flapping foil for large underwater vehicles.   

Measuring the 3D hydrodynamics of swimming snakes

Snakes are anguilliform swimmers. Although the hydrodynamics of anguilliform swimming can be predicted through idealized numerical models, they are yet to be observed in three dimensions. 

We used volumetric 3-component velocimetry (V3V) to capture the instantaneous 3D wake structure of two female and one male freely swimming dice snakes (Natrix tessellata).

We observed that the wake topology after a tailbeat cycle is a hairpin-like, curved-shaped head vortex street. We compare quantitatively different sequences of the same individual and of different individuals using slices of the velocity field in the wake of the snake. 

These results are a first step towards the understanding of the efficiency of anguilliform swimming and will be compared to results for other species of aquatic snakes that have independently evolved an aquatic life-style. This may help future developments of  bio-inspired aquatic systems.   

Numerical simulation of the flow around larvae from the Drusinae subfamily

Drusinae is a caddisfly subfamily whose larvae are found on the rock bed of small mountain creeks, being exposed to a turbulent flow. It is hypothesized that their head morphologies are related to the specific hydrodynamic properties of the different habitats of the members of the subfamily. For that reason, the flow past different larvae species of the subfamily is computed by LES to investigate the stresses acting on different larvae. The three-dimensional body shapes are constructed from micro-tomography data of the selected species. The flow parameters (Reynolds number) are selected based on spatio-temporally filtered velocity data measured during field excursions. The divergence-free synthetic eddy method (DFSEM) is employed to provide turbulent inlet conditions into a channel within which the larva sits on the wall. The simulations with the tested range of Reynolds numbers show differences in the total forces acting on each morphotype, supporting the hypothesis of preferred hydraulics between the different species. Furthermore, higher shear stresses were found on the head location for the larva that feeds by filtering the water. The higher velocity fluctuations may enhance the encounter rates of particles on their filtering devices.