Session X07: Biofluids: General Locomotion III

Swimming Fast Versus Swimming Efficiently

Fish-like swimming gaits that generate maximum thrust differ from those that achieve maximum efficiency. The focus of this study was to examine and compare the optimal gaits for fish-like swimming to attain maximum thrust and maximum efficiency. To identify these optimal swimming gaits, we developed and utilized an efficient adjoint-based optimization algorithm. This algorithm enabled us to identify periodic swimming gaits that maximize the time-averaged thrust or propulsive efficiency. Our investigation utilized a 2D oscillating plate as the propulsor in the Direct Numerical Simulation (DNS). Our analysis will compare the optimal swimming gaits for maximum thrust and maximum efficiency, elucidating their associated flow characteristics. Furthermore, we will incorporate the sensitivity information provided by the adjoint method.   

Maximizing performance of ≥3 in-line oscillating fins

Fin-fin interaction of underwater swimmers is beneficial for swimming performance of schooling animal swimmers and is becoming increasingly utilized in bioinspired propulsion technology. However, there is a lack of research focused on the benefits and possible plateau in performance of in-line fins in underwater propulsion for systems with greater than two fins. Using coupled experiments and immersed boundary simulations, we find the idealized kinematics and spacing of a three fin system, followed by exploring the asymptotic behavior of larger systems. We study fins oscillating in both pitch and heave with variable spacing, phase, amplitude, and frequency. We find the ideal swimming configuration through parameter space exploration and optimization, focused separately on maximizing thrust and efficiency. We find that there are further benefits to be gained by adding more than two fins.   

Modeling multiple pacemaker control in jellyfish locomotion

Recent studies [1] have found that there are importantneuromechanical constraints that arise from the timescales associated with neuromuscular activation and the elastic response of flexible appendages or bodies. In jellyfish, the neuromuscular response is governed by the interaction of pacemakers with the underlying motor nerve net that communicates with the musculature. This set of equally-spaced pacemakers, located at the bell rim, alter their firing frequency in response to environmental cues, allowing for different swimming modes to be activated when sets of pacemakers fire in concert. In this work, we explore the control of neuromuscular activation with a 3D computational FSI model of a jellyfish bell immersed in a viscous fluid and use numerical simulations to describe the interplay of multiple pacemakers. We will look at the role a single pacemaker can play, as well as when a pair of pacemakers fire in unision and the resulting fluid dynamics that can result from their interaction. We will then comment on the pacemaker firing frequency implications on control and stability.

[1] Hoover, A. P., Xu, N. W., Gemmell, B. J., Colin, S. P., Costello, J. H., Dabiri, J. O., & Miller, L. A. (2021). Neuromechanical wave resonance in jellyfish swimming. Proceedings of the National Academy of Sciences, 118(11), e2020025118.   

Large Eddy Simulations of a Self-Propelled Tuna

This study numerically investigates the swimming characteristics of a self-propelled virtual Bluefin Tuna. The self-propelled Tuna is simulated using Curvilinear Immersed Boundary (CURVIB) method, and the turbulence is modeled using a dynamic Smagorinsky Model for Large Eddy Simulations (LES) at various Reynolds numbers. The body kinematics are prescribed based on experimental observations provided by the Hopkins Marine Station at Stanford University and the swimming velocity is computed based on the forces on the fish body. An over-set mesh is used to increase the grid resolution near the fish body for High Reynolds number flows to reduce the computational costs. The simulations demonstrate that as the Reynolds number of the Tuna decreases, Strouhal number increases, swimming efficiency decreases, and wake spreading increases. At the initial time, the swimming velocity is zero, and as time progresses, the fish gets accelerated until it reaches the steady swimming speed. The computational resources are provided by the High-Performance Research Computing (HPRC) at Texas A&M University.   

Exploring the efficacy of metachronal swimming and pumping at intermediate Reynolds numbers with magnetically actuated artificial cilia

Flexibility is a key feature of biological structures involved in swimming and flying, enhancing efficiency and performance. Advances in soft robotics enable engineering solutions that can more closely mimic such structures compared to rigid mechanical designs, helping us explore the role of flexibility across a range of biological and hydrodynamic contexts. Here we explore the performance of a coordinated, magnetically actuated array of soft robotic paddles at intermediate Reynolds numbers, modeled after the propulsors of ctenophores (comb jellies). Ctenophores’ paddle-like appendages are arranged in rows and beat sequentially, with a wave travelling opposite to the beating direction (antiplectic metachronal coordination). Our bioinspired propulsors consist of magnetic elastomers, actuated with an external magnetic field. Different frequencies, wave speeds, and phase lags can be encoded in the soft robotic array, allowing us to use particle image velocimetry (PIV) to explore the hydrodynamics of a biologically relevant performance space. We find distinct shifts in momentum flux with phase lag and frequency, showing the versatility of the soft robotic platform to explore the role of flexibility and metachronal coordination across a range of intermediate Reynolds numbers.   

Studying vortex interactions in water walking insects using physical and computational fluid dynamics

Amongst the genera of water walking insects, Microvelia and Mesovelia are some of the insects which locomote on both water and land effortlessly. Unlike Gerridae, these water walkers employ an alternating tripod gait in their locomotion. During power strokes, these insects generate pairs of counter-rotating vortices on either side from their middle and hind legs. In the case of microvelia, these vortices are sometimes re-energized during the strokes of hind legs, whereas such vortex interactions were absent in the locomotion of Mesovelia. We systematically studied a physical model to unveil the role of vortex interactions in these water walkers. Furthermore, we use computational fluid dynamics to gain an in-depth understanding of the fluid dynamics involved in such events involving vortices. In addition to the advancing the knowledge of fluid dynamics of water walking insects, we hope that these findings may lead to better designs of next generation amphibious micro-robots.   

Flapping dynamics of a multi-segmented structure bending uni-directionally

Considering flapping motion for drag-based swimming gait, the effective area of a propulsion unit is directly linked to the generation of thrust. In this study, the change in effective area is achieved through the deformation of an elastic structure used as the propulsion unit. Alike the articulated limbs of animals, an artificial multi-segmented structure with uni-directional elastic hinges connecting the segments is proposed. The uni-directionality of the hinges causes greater asymmetry of the deformation profiles between the power and recovery strokes and thereby enhances cyclic thrust. Experimental and theoretical analyses are conducted to investigate the deflection and drag characteristics of the structure undergoing flapping motions. To explore the effects of distributed flexibility and find the optimal configuration of the multi-segmented structure, kinematic parameters such as flapping amplitude and frequency and design parameters such as the number, location, and bending stiffness of the elastic hinges are varied. Through appropriate definitions of the bending stiffness and tip deflection of the whole structure, namely effective stiffness and weighted tip deflection respectively, the deflection of the multi-segmented structure could be well characterized.   

The role of hydrofoil shape in rigid/flexible oscillating propulsors

The field of aerodynamics has countless libraries of foil shapes that are optimized for steady flight performance. However, it is unclear if these same foil characteristics are ideal for foils used as unsteady propulsors, as found naturally in biological swimmers and increasingly used in underwater vehicles. We detail the role that hydrofoil shape characteristics like maximum thickness and thickness distribution have in swimming speed and efficiency. Through optimization, we arrive at the ideal foil shape for a rigid foil and a foil that passively pitches through a leading edge spring. Results are primarily obtained through a boundary data immersion method flow solver and compared to experiments. Through shape optimization, we are able to dramatically increase the propulsive performance of underwater swimmers.   

Optimal stiffness distributions in propulsive plates with time-periodic stiffness

We study the optimal spatial stiffness distributions in flexible flat plates swimming in an inviscid and incompressible fluid including time-periodic stiffness variation. It has been shown previously that time-periodic stiffness can lead to dramatic improvements on the thrust performance of an oscillating propulsor. Our goal is to explore the spatio-temporal stiffness parameter space to find optimal distributions for propulsive thrust and efficiency. The optimal stiffness distribution for spatially only varying stiffness approaches a rigid plate with a torsional spring at its leading edge. We seek to compare the optimal spatio-temporal stiffness distributions with the optimal spatial distributions found in the literature. We also compare results found with a full two-dimensional fluid structure interaction solver.   

The trade-off between locomotion speed and hydromechanical efficiency determines the optimal orifice ratio of a salp-inspired swimmer

Different from most marine invertebrates that swim by rear single-jet propulsion (such as jellyfish and squid), salps take water in at the front and expel water out at the rear during locomotion. Inspired by this unique dual-jet propulsion mode, we develop a two-dimensional numerical model to investigate the hydrodynamic performance of a salp-inspired swimmer. Our results show that higher orifice ratio can improve the hydrodynamic efficiency of the salp-inspired swimmer, but at the cost of its locomotion speed. Moreover, while smaller duty cycle is benefit for both locomotion speed and hydrodynamic efficiency, the lower bound of duty cycle is restricted by energy consumption coefficient. In particular, the salp-inspired swimmer reaches a trade-off between the locomotion speed and the hydromechanical efficiency when the orifice ratio is around 0.4, a value in agreement with the case of real salps in nature. We also found that its locomotion speed depends on the jet-based Reynolds number with a simple 4/5 scaling law, which can be explained by the force balance during locomotion.   

Free surface-induced ground effect for flapping swimmers

Numerous flying and swimming creatures use the ground effect to boost their propulsive performance, with the ``ground" referring to either a solid boundary or a free surface. While our knowledge of how a solid boundary affects biolocomotion is relatively comprehensive, little is understood about the effect of a free surface. To address this limitation, we conduct a numerical investigation on the propulsion performance of a flapping plate under a free surface, subject to a range of control parameters. When the Froude number is very low (i.e., little surface deformation), the effects of a free surface are similar to those of a solid boundary, with enhanced thrust and input power but little change in efficiency. However, as the Froude number increases (i.e., more surface deformation), our results reveal an optimal Froude number of approximately 0.6, where the free surface induces a more streamlined flow around the flapping plate, effectively reducing the added mass. This results in a significant decrease in input power and greatly enhanced efficiency. Our simulation results are supported by measurement data of fish Froude numbers from the literature.   

Robotically Controlled Jellyfish: Modifying Swimming Dynamics with Mechanical Forebodies

Ocean monitoring tools yield extensive data for understanding climate change but energy consumption is a limiting factor for mission duration. In contrast, Aurelia aurita jellyfish have a cost of transport 97% lower than some underwater vehicles and are adaptable to a wide range of ocean environments. Here we explore mechanically modifying jellyfish bells with added forebodies to reduce drag on swimming animals. By equipping jellyfish with microelectronic swim controllers, we create an ocean monitoring tool capitalizing on jellyfish regenerative capabilities paired with inexpensive electronics. Previous work has demonstrated stimulated jellyfish vertical swimming speeds of 2.8 times baseline speeds without swim controllers. We model the unsteady swimming dynamics of jellyfish with attached forebodies to study the impact of changes in drag and added mass on the animal dynamics. To inform this model, we conduct drop tests of various hemiellipsoid forebodies to empirically determine drag at terminal velocity. We validate the model by attaching forebodies onto the jellyfish exumbrellar surface to create a more streamlined shape. We utilize a 6m tall water tank to test the animal’s swimming speed and endurance experimentally with attached 3D printed caps.