Self-propelled Swimming Research Articles

Numerical simulation on the interaction of median fins for enhancing vortex dynamics and propulsion performance in fish self-propelled swimming

Studies have shown that fish can enhance propulsion performance by utilizing the interaction between median fins (dorsal, anal, and caudal fins), compared to fish with only caudal fin. However, most of the current studies are based on the fish oscillating in-place, and the analysis of median fins interaction to improve swimming propulsion performance is still insufficient, and the mechanism needs further study. This study applied three-dimensional numerical simulation methods to solve the process of grass carp accelerating from a stationary state to cruising state under different body and median fins combination, as well as different motion parameter models. A comparative quantitative analysis of different models was conducted to assess the impact of median fins interactions on enhancing swimming performance, with a detailed analysis of the hydrodynamic mechanisms and their relationship with vortex dynamics. The results indicated that interactions between median fins could generate significant hydrodynamic benefits, with the fish's average swimming speed increasing up to 4.6 times, thrust up to 33.47%, and swimming efficiency up to 25.48%. This study found that the enhancement of propulsion performance was due to the formation of high-intensity and persistent posterior body vortices by the movements of the dorsal and anal fins, which were captured by the leading-edge of the caudal fin, greatly enhancing the strength of the leading-edge vortex. This study elucidates the hydrodynamic mechanisms of the interaction between median fins and could provide new insights into the efficient swimming mechanism of fish in nature.

Machine learning assisted sorting of active microswimmers.

Active matter systems, being in a non-equilibrium state, exhibit complex behaviors, such as self-organization, giving rise to emergent phenomena. There are many examples of active particles with biological origins, including bacteria and spermatozoa, or with artificial origins, such as self-propelled swimmers and Janus particles. The ability to manipulate active particles is vital for their effective application, e.g., separating motile spermatozoa from nonmotile and dead ones, to increase fertilization chance. In this study, we proposed a mechanism-an apparatus-to sort and demix active particles based on their motility values (Péclet number). Initially, using Brownian simulations, we demonstrated the feasibility of sorting self-propelled particles. Following this, we employed machine learning methods, supplemented with data from comprehensive simulations that we conducted for this study, to model the complex behavior of active particles. This enabled us to sort them based on their Péclet number. Finally, we evaluated the performance of the developed models and showed their effectiveness in demixing and sorting the active particles. Our findings can find applications in various fields, including physics, biology, and biomedical science, where the sorting and manipulation of active particles play a pivotal role.

Hydrodynamic interactions between two self-propelled flapping plates swimming towards each other

Hydrodynamic interactions between two self-propelled flapping plates swimming towards each other

Real-time position and pose prediction for a self-propelled undulatory swimmer in 3D space with artificial lateral line system

This study aims to investigate the feasibility of using an artificial lateral line (ALL) system for predicting the real-time position and pose of an undulating swimmer with Carangiform swimming patterns. We established a 3D computational fluid dynamics simulation to replicate the swimming dynamics of a freely swimming mackerel under various motion parameters, calculating the corresponding pressure fields. Using the simulated lateral line data, we trained an artificial neural network to predict the centroid coordinates and orientation of the swimmer. A comprehensive analysis was further conducted to explore the impact of sensor quantity, distribution, noise amplitude and sampling intervals of the ALL array on predicting performance. Additionally, to quantitatively assess the reliability of the localization network, we trained another neural network to evaluate error magnitudes for different input signals. These findings provide valuable insights for guiding future research on mutual sensing and schooling in underwater robotic fish.

A miniature swimmer actuated by a PZT ring ultrasonic underwater propulsion system

This study investigates a scheme utilizing a ring transducer for an acoustic underwater propulsion system. Acoustic underwater propulsion systems are well suited for the inspection and repair of underwater robots due to their small size, high power density, and simple structure. Previous research has focused on self-propelled swimmers utilizing disc transducers. However, the radial vibration component of disc transducers makes it difficult to provide propulsion for an acoustic underwater propulsion system driven by an acoustic driving force. Pure longitudinal vibration requires a greater thickness to achieve the same vibration area, resulting in higher impedance and reduced driving efficiency. In this paper, simulation, and measurements of vibration distribution demonstrate that a ring transducer exhibits a vibration distribution closely resembling pure longitudinal vibration. A prototype swimmer using a ring transducer was fabricated for experimental evaluation through measurements of admittance characteristics, zero-speed propulsion, and no-load speed in water.

Hydrodynamic analysis of the upright swimming of seahorse

The seahorse is the only creature in the ocean that can maintain an upright posture while swimming. This paper mainly discusses the hydrodynamic characteristics and the flow field structure of the seahorse when it swims upright. Using a three-dimensional seahorse model, numerical simulations of self-propelled swimming are conducted by establishing the kinematic equations of its dorsal fin. The focus is on elucidating the effects of the undulation frequency and the inclination angle on swimming performance. The results indicate that a higher undulation frequency of the dorsal fin leads to better acceleration performance, or in other words, greater hydrodynamic forces. The inclination angle of the seahorse's body also directly affects its hydrodynamics and the flow field structure. Unlike other fish that swim horizontally, the seahorse generates forward and upward thrust as the flow field simultaneously spreads backward and downward. Since the upright posture makes the forward thrust much smaller than the upward one, the seahorse has low efficiency in forward propulsion when swimming upright. As the inclination angle decreases, the forward thrust gradually increases and exceeds the upward force, which allows for a rapid improvement in the swimming velocity. The simulation findings of this study are consistent with previous experimental observations.

A novel miniature swimmer propelled by 36° Y-cut lithium niobate acoustic propulsion system

Self-propelled swimmers using ultrasonic transducers have been proposed and studied. The screw and bionic propulsion systems have dominated underwater robots. However, the complex structures make miniaturization difficult. The swimmers with ultrasonic transducers have no moving parts such as fins or propellers and can be driven by the nanomechanical-vibration in the surface of the transducer. Simple structure and high driving frequency make it easy to miniaturize. A 36° Y-cut lithium niobate (LN) thickness-vibration-mode transducer is used to investigate an underwater acoustic propulsion system. LN transducers are manufactured to 10 × 10 mm, 7 × 7 mm, and 4 × 4 mm square plates. The zero-speed propulsion and no-load speed are investigated to evaluate the swimmer performance. A mm-order swimmer is studied with a 4 × 4 mm LN transducer. At last, the multi-degrees-of-freedom swimmer driven by three 7 × 7 mm LN transducers is demonstrated with the advance and sway locomotion. In the future, a small-scale and agile self-propelled swimmer with an acoustic propulsion system can be expected for narrow and complex environment tasks, such as seagrass-prone areas and narrow pipelines.

Vortex phase matching of a self-propelled model of fish with autonomous fin motion

It has been a long-standing problem how schooling fish optimize their motion by exploiting the vortices shed by the others. A recent experimental study showed that a pair of fish reduce energy consumption by matching the phases of their tailbeat according to their distance. In order to elucidate the dynamical mechanism by which fish control the motion of caudal fins via vortex-mediated hydrodynamic interactions, we introduce a new model of a self-propelled swimmer with an active flapping plate. The model incorporates the role of the central pattern generator network that generates rhythmic but noisy activity of the caudal muscle, in addition to hydrodynamic and elastic torques on the fin. For a solitary fish, the model reproduces a linear relation between the swimming speed and tailbeat frequency, as well as the distributions of the speed, tailbeat amplitude, and frequency. For a pair of fish, both the distribution function and energy dissipation rate exhibit periodic patterns as functions of the front-back distance and phase difference of the flapping motion. We show that a pair of fish spontaneously adjust their distance and phase difference via hydrodynamic interaction to reduce energy consumption.

A systematic investigation into the effect of roughness on self-propelled swimming plates

This study examines the effects of surface topography on the flow and performance of a self-propelled swimming (SPS) body. We consider a thin flat plate with an egg-carton roughness texture undergoing prescribed undulatory swimming kinematics at Strouhal number $0.3$ and tail amplitude to length ratio $0.1$ ; we use plate Reynolds numbers $Re=6$ , 12 and $24\times 10^3$ , and focus on $12\,000$ . As the roughness wavelength is decreased, we find that the undulation wave speed must be increased to overcome the additional drag from the roughness and maintain SPS. Correspondingly, the extra wave speed raises the power required to maintain SPS, making the swimmer less efficient. To decouple the roughness and the kinematics, we compare the rough plates to equivalent smooth cases by matching the kinematic conditions. We find that all but the longest roughness wavelengths reduce the required swimming power and the unsteady amplitude of the forces when compared to a smooth plate undergoing identical kinematics. Additionally, roughness can enhance flow enstrophy by up to 116 % compared to the smooth cases without a corresponding spike in forces; this suggests that the increased mixing is not due to increased vorticity production at the wall. Instead, the enstrophy is found to peak strongly when the roughness wavelength is approximately twice the boundary layer thickness over the $Re$ range, indicating the roughness induces large-scale secondary flow structures that extend to the edge of the boundary layer. This study reveals the nonlinear interaction between roughness and kinematics beyond a simple increase or decrease in drag, illustrating that roughness studies on static shapes do not transfer directly to unsteady swimmers.

Effect of trailing-edge shape on the swimming performance of a fish-like swimmer under self-propulsion

Tuna, dolphin and whale as typical thunniform mode fish have obvious differences in body size and caudal fin shape. As the main propeller, the difference in the shape of caudal fin will have a significant impact on the swimming performance of fish. In this work, numerical simulations are adopted to study the hydrodynamics of self-propelled thunniform swimming. By changing the trailing-edge profile parametrically, three different tails varying from forked tuna-like tail to the square-shaped tail are constructed. The results revealed that the tuna-like tail is most beneficial to fish long distance cruise, while the square-shaped tail is the most unfavorable. It is interesting to note that for tuna-like tail, decreasing the undulating amplitude can improve fish's long-distance cruising ability more than decreasing the pitching amplitude, but the opposite is true for the square-shaped tail. Further study on the vortex dynamics and the surface pressure distribution indicate that the backward expansion of the trailing-edge profile will delay the shedding of the leading-edge vortex and increase the existence time of low-pressure region on the caudal fin. Since the pitching angle is reduced synchronously, the delayed disappearance of the low-pressure region will generate more lateral force. This will lead to more energy loss.

Multi-Body Hydrodynamic Interactions in Fish-Like Swimming

Abstract Many animals in nature travel in groups either for protection, survival, or endurance. Among these, certain species do so under the burden of aero/hydrodynamic loads, which incites questions as to the significance of the multibody fluid-mediated interactions that are inherent to collective flying/swimming. Prime examples of such creatures are fish, which are commonly seen traveling in highly organized groups of large numbers. Indeed, over the years, there have been numerous attempts to examine hydrodynamic interactions among self-propelled fish-like swimmers. Though many have studied this phenomenon, their motivations have varied from understanding animal behavior to extracting universal fluid dynamical principles and transplanting them into engineering applications. The approaches utilized to carry out these investigations include theoretical and computational analyses, field observations, and experiments using various abstractions of biological fish. Here, we compile representative investigations focused on the collective hydrodynamics of fish-like swimmers. The selected body of works are reviewed in the context of their methodologies and findings, so as to draw parallels, contrast differences, and highlight open questions. Overall, the results of the surveyed studies provide foundational insights into the conditions (such as the relative positioning and synchronization between the members, as well as their swimming kinematics and speed) under which hydrodynamic interactions can lead to efficiency gains and/or group cohesion in two- and three-dimensional scenarios. They also shed some light on the mechanisms responsible for such energetic and stability enhancements in the context of wake-body, wake-wake, and body-body interactions.

Effects of body stiffness on propulsion during fish self-propelled swimming

Many fish propel themselves using wave-like lateral flexion of their body and tail in the water. The undulatory body is driven by the distributed muscles, and locomotion is achieved by internal muscular stimulation and the external action of the fluid. As one of the material properties, the stiffness of the body being propelled plays an important role in the deformation process, especially for the muscle power input and phase lag. In this paper, a three-dimensional self-propelled elongated body model is employed to numerically investigate the effects of stiffness on the propulsion performance, including the forward speed, energy consumption, and energy-utilization ratio. According to various deformation characteristics and energy-utilization ratios, three deformation modes corresponding to high, medium, and low stiffness are identified. Our results indicate that a deforming body with medium stiffness has the highest efficiency, and its corresponding deformation is closest to that of fish in nature. When the stiffness of the fish body is higher than the normal level, more muscle energy is needed to sustain the tail beating of the same amplitude. A lower level of stiffness produces a more obvious phase lag in the fish body, which might lead to slow control responses. We also show that the stiffness of the fish body affects the scaling relationship between the swimming speed and the tail beating velocity. The upper and lower limits of the scaling exponent correspond to high and low levels of stiffness, respectively, and are also affected by the wavelength of the muscle contraction.

Hydrodynamic interactions coordinate the swimming of two self-propelled fish-like swimmers

A muscle-contraction model of pre-strains is utilized to conduct a numerical investigation on the collective swimming of two self-propelled fish-like swimmers. To simulate the conscious swimming control exhibited by live fish, a PID control algorithm is integrated with the numerical simulation model. Specifically, in a scenario involving straight swimming, we conduct a comprehensive simulation study on the hydrodynamic interactions between the leading and following swimmers. The research focuses on the effects of phase difference and initial spacing between the two fish on their collective behavior. Notably, our simulation model considers both the alteration of muscular activities and the energy-saving effect during collective fish swimming. The results demonstrate that a simple PID controller can effectively imitate the conscious swimming control of live fish, ensuring the formation of collective mode of two freely swimming fish. By varying the initial spacing and phase difference, the two fish can attain the same swimming speed, and a similar steady flow field structure can be observed within a certain range of parameters. The passive hydrodynamic interactions, acting as both facilitators and impediments, aid in coordinating the swimming of fish individuals. Furthermore, for efficient swimming of the following fish, a ‘lazy’ strategy proves to be essential.

Self-Propelled Micro-/Nanomotors of ZnO Nanoshuttles Induced by Surface Defects

To accomplish various complex tasks in the microworld, micro-/nanomotors (MNMs) with diverse swimming characteristics have attracted more and more attention. In this work, ZnO nanoshuttles were synthesized by a simple hydrothermal method. In addition, MNMs based on ZnO nanoshuttles have been designed, exhibiting self-propelled swimming characteristics with three motion modes of shake, rotation, and translation in water. It is revealed that the asymmetrically distributed defects on the surface of ZnO nanoshuttles are responsible for the swimming modes. The unique shuttle-like structure helps to reduce the drag in a low Reynolds number environment and endows the self-propelled ZnO nanoshuttle motor with a relatively high swimming velocity (5.16 μm/s). In addition, to demonstrate the versatility and openness of ZnO nanoshuttle MNMs, ZnO/Co magnetic MNMs and ZnO/TiO2 Janus MNMs were prepared by simple Co doping and TiO2 coating, respectively. This study reveals the possible role of surface defects in regulating the motion behavior of ZnO-based MNMs and also provides a feasible low-cost method for the large-scale fabrication of self-asymmetric fuel-free micro-/nanomotors.

The Role of Double-Tentacled Cooperative Kinematics on the Hydrodynamics of a Self-propelled Swimmer

In this study, we proposed an underwater robotic swimmer integrating dual-actuated composite tentacles. We employed overlapping grid technology to manipulate virtual swimmers and performed simulations of incompressible viscous flow. To facilitate the distinction between three driving modes (the reverse, homologous, and interlace modes), the rear flexible module of the swimmer was divided into three components: thigh links, calf links, and caudal fins. The cooperative motion mechanism behind the double-tentacled module exhibited special hydrodynamic properties. Under the same kinematic parameters, the reverse mode exhibited the best energy-saving and propulsion effect, whereas the homologous mode was affected by lateral energy loss, thus resulting in the worst propulsion effect. However, the joint system exhibited anti-interference and spanwise flexibility. The interlace mode produced a certain error in the lateral displacement, and the propulsion efficiency was between the former two modes. Compared with traditional fish-like robots, the diverse actuation morphologies of the swimmer reported in this study exhibit extremely powerful self-propelled functionality, and its key features, including the geometry of an aquatic squid and the kinematics of the stretched body-caudal fin pattern, offer insights into the analysis of self-propelled hydrodynamics.

Pre-asymptotic dispersion of active particles through a vertical pipe: the origin of hydrodynamic focusing

When motile algal cells are exposed to gyrotactic torques, their swimming directions are guided to form radial accumulation, well known as hydrodynamic focusing. The origin of hydrodynamic focusing from the effects of active swimming, ambient flow and particle anisotropy is elucidated in the present study on the pre-asymptotic dispersion of active particles through a vertical pipe. With an extension of the Galerkin method to pipe flows, time-dependent solutions directly from the Smoluchowski equation in the position and orientation space are derived by series expansions of spherical harmonics and Bessel functions. Ballistic and diffusive scaling laws are examined with the predominance of self-propelled swimming, and computation is validated against an explicit benchmark solution and Lagrangian particle simulation. In the limit of extreme shear, the competitive roles of shear dispersion and Brownian rotation are reflected concretely in the pre-asymptotic phase of hydrodynamic focusing. For flows with various shear strengths, a concentration peak in near-wall regions with a smooth transition to hydrodynamic focusing is illustrated with richer phenomena in upwelling and downwelling flows. A newly observed regime through a vertical pipe, named transient effective trapping, is revealed as a transitional mode towards hydrodynamic focusing. The pre-asymptotic approach to hydrodynamic focusing is elaborated intensively through extensive solutions of concentration moments and macroscopic transport coefficients characterised by swimming and flow Péclet numbers. The unique findings for the origin of hydrodynamic focusing could provide insight into related micro-algae reactor technology and contribute to flow control and biomass transfer in confined environments.

Chemotaxis of two chiral squirmers

External gradients can strongly influence the collective behavior of microswimmers. In this paper, under an external linear chemical gradient, we study the behavior of two hydrodynamically interacting self-propelled chiral swimmers in the low-Reynolds number regime. We use the generalized squirmer model called the chiral squirmer, a spherically shaped body with an asymmetric surface slip velocity, to represent the swimmer. We find that the external gradient favors the attraction between the swimmers and, in some situations, leads to a bounded state in which the swimmers move in a highly synchronous manner. Furthermore, due to this cooperative motion, these swimmers reach the chemical target faster than individual swimmers. This study may help in understanding the collective behavior of chiral swimmers and in designing synthetic microswimmers for targeted drug delivery.

Dynamics of a Wavy Axisymmetric Swimmer

A novel underwater locomotion method is proposed. In this method, the propulsion system is designed as an empty tube with two open ends. The sidewall of this tube is deformable so that the diameter of the tube varies both spatially and temporally, following a wavy pattern that travels from head to tail. By using a fluid dynamics model based on the axisymmetric Navier–Stokes equations, we numerically demonstrate that this design is able to generate thrust for self-propelled swimming. The sources of the thrust have been identified. Specifically, the physical mechanisms that assist with the swimming performance are 1) the momentum flux through the front and back openings; and 2) the normal stress at the front and back openings, most notably the suction force at the front opening due to negative pressure there. Moreover, a parametric study has been conducted to investigate the effect of geometric and kinematic parameters upon the performance of the system.

Computational framework for efficient high-fidelity optimization of bio-inspired propulsion and its application to accelerating swimmers

A new computational framework for high-fidelity optimization of kinematic gaits during self-propelled undulatory swimming is developed. A computational framework utilizes a spectral-element method on moving body-fitted grids for a simulation of self-propelled swimming, and a surrogate-based optimization (SBO) procedure. A new volume-conservation method for reconstruction of a swimmer's geometry during the undulatory motion is proposed to ensure numerical stability of the fluid-structure interaction solver in an incompressible flow framework. A surrogate-based optimization algorithm that utilizes a Kriging response surface method is adopted and further developed in this work to manage the optimization process in the presence of physiological constraints on the fish body motion. A grid convergence of the optimization results is established, and the influence of the polynomial refinement on the results of optimization procedure is assessed. The increase in polynomial order does not change the optimum gaits of locomotion or relative efficiency rankings between the modes, but it results in slightly lower predicted efficiency for all the modes. The optimum solution is characterized by a kinematic gait that generates the reverse Karman vortex street associated with high propulsive efficiency. Efficiency of sub-optimum modes is found to increase with both the tail amplitude and the effective flapping length of the swimmer, and a new scaling law is proposed to capture these trends. Lastly, the SBO algorithm converged to an optimized gate with significantly less function evaluations than typically observed for evolutionary algorithms. This suggests that the SBO framework is a well suited alternative for high-fidelity optimization of fluid and structure problems.

Self-Propelled Swimming of a Flexible Propulsor Actuated by a Distributed Active Moment

The self-propelled swimming of a flexible propulsor is numerically investigated by using fluid-structure interaction simulations. A distributed active moment mimicking the muscle actuation in fish is used to drive the self-propulsion. The active moment imposed on the body of the swimmer takes the form of a traveling wave. The influences of some key parameters, such as the wavenumber, the amplitude of moment density and the Reynolds number, on the performance of straight-line swimming are explored. The influence of the ground effect on speed and efficiency is investigated through the simulation of near-wall swimming. The turning maneuver is also successfully performed by adopting a simple evolution law for the leading-edge deflection angle. The results of the present study are expected to be helpful to the design of bio-inspired autonomous underwater vehicles.