Study on the mud swimming motion of Paramisgurnus dabryanus

In the context of the rapid development of bionic technology, inspired by the swimming behavior of fish, a variety of robotic fish have been designed and applied to different underwater works and even military applications. However, in some operations, such as detection and salvage, vehicles need to travel under mud, a medium that is different from fluids. This complicating factor put higher requirements on robotic fish design. In this study, Paramisgurnus dabryanus, a fish species adept at swimming into the mud, was taken as a research object to investigate its profile and mud swimming behavior. First, a three-dimensional (3D) image scanner is used for profile scanning to acquire the point cloud data of the profile features of the loach. After modification, data coordinate points are extracted and used to fit the profile curve of loach and build geometric and mathematical models by means of Fourier function fitting. The next step includes the analysis of the motion of loach, determination of main parameters of the wave equation, and establishment of the fish body wave curve of a loach in the swimming using MATLAB software. Saturated mud having a water content of 37% is adopted as an environmental medium to numerically simulate the swimming behavior in mud, identifying the distribution of vortex path, and velocity field of loach’s motion. The rationality of simulation results is verified by the loach mud swimming test, and the simulating results agree well with the experimental data. This study lays a preliminary foundation for the outer contour design of the robotic fish operating under mud and aims to carry out the drag reduction and accelerating design of the robotic fish. The robotic loach may be applied in fishery breeding, shipwreck salvage operations, and so on.

Undulatory Swimming Performance Explored With a Biorobotic Fish and Measured by Soft Sensors and Particle Image Velocimetry

Due to the difficulty of manipulating muscle activation in live, freely swimming fish, a thorough examination of the body kinematics, propulsive performance, and muscle activity patterns in fish during undulatory swimming motion has not been conducted. We propose to use soft robotic model animals as experimental platforms to address biomechanics questions and acquire understanding into subcarangiform fish swimming behavior. We extend previous research on a bio-inspired soft robotic fish equipped with two pneumatic actuators and soft strain sensors to investigate swimming performance in undulation frequencies between 0.3 and 0.7 Hz and flow rates ranging from 0 to 20 cms in a recirculating flow tank. We demonstrate the potential of eutectic gallium–indium (eGaIn) sensors to measure the lateral deflection of a robotic fish in real time, a controller that is able to keep a constant undulatory amplitude in varying flow conditions, as well as using Particle Image Velocimetry (PIV) to characterizing swimming performance across a range of flow speeds and give a qualitative measurement of thrust force exerted by the physical platform without the need of externally attached force sensors. A detailed wake structure was then analyzed with Dynamic Mode Decomposition (DMD) to highlight different wave modes present in the robot’s swimming motion and provide insights into the efficiency of the robotic swimmer. In the future, we anticipate 3D-PIV with DMD serving as a global framework for comparing the performance of diverse bio-inspired swimming robots against a variety of swimming animals.

Creating underwater vision through wavy whiskers: a review of the flow-sensing mechanisms and biomimetic potential of seal whiskers

Seals are known to use their highly sensitive whiskers to precisely follow the hydrodynamic trail left behind by prey. Studies estimate that a seal can track a herring that is swimming as far as 180 m away, indicating an incredible detection apparatus on a par with the echolocation system of dolphins and porpoises. This remarkable sensing capability is enabled by the unique undulating structural morphology of the whisker that suppresses vortex-induced vibrations (VIVs) and thus increases the signal-to-noise ratio of the flow-sensing whiskers. In other words, the whiskers vibrate minimally owing to the seal's swimming motion, eliminating most of the self-induced noise and making them ultrasensitive to the vortices in the wake of escaping prey. Because of this impressive ability, the seal whisker has attracted much attention in the scientific community, encompassing multiple fields of sensory biology, fluid mechanics, biomimetic flow sensing and soft robotics. This article presents a comprehensive review of the seal whisker literature, covering the behavioural experiments on real seals, VIV suppression capabilities enabled by the undulating geometry, wake vortex-sensing mechanisms, morphology and material properties and finally engineering applications inspired by the shape and functionality of seal whiskers. Promising directions for future research are proposed.

Intersegmental coordination of the central pattern generator via interleaved electrical and chemical synapses in zebrafish spinal cord

A significant component of the repetitive dynamics during locomotion in vertebrates is generated within the spinal cord. The legged locomotion of mammals is most likely controled by a hierarchical, multi-layer spinal network structure, while the axial circuitry generating the undulatory swimming motion of animals like lamprey is thought to have only a single layer in each segment. Recent experiments have suggested a hybrid network structure in zebrafish larvae in which two types of excitatory interneurons (V2a-I and V2a-II) both make first-order connections to the brain and last-order connections to the motor pool. These neurons are connected by electrical and chemical synapses across segments. Through computational modeling and an asymptotic perturbation approach we show that this interleaved interaction between the two neuron populations allows the spinal network to quickly establish the correct activation sequence of the segments when starting from random initial conditions and to reduce the dependence of the intersegmental phase difference (ISPD) of the oscillations on the swimming frequency. The latter reduces the frequency dependence of the waveform of the swimming motion. In the model the reduced frequency dependence is largely due to the different impact of chemical and electrical synapses on the ISPD and to the significant spike-frequency adaptation that has been observed experimentally in V2a-II neurons, but not in V2a-I neurons. Our model makes experimentally testable predictions and points to a benefit of the hybrid structure for undulatory locomotion that may not be relevant for legged locomotion.

Design of Fin-Curvature-Based Feedback Controller for Efficient Swimming

Over the past few decades, biologists and engineers have attempted to elucidate the swimming mechanism of fish and developed a fish-like robot to perform fast swimming in water. Such a robot will have wide applicability in investigations and exploration in the sea. There have been many studies on fish-type robots; however, the propulsion efficiency of the introduced robots is far from that of the actual fish. The main reason is that the robot controller for generating motions is conventionally designed by trial and error, and little attention has been placed on designing a motion controller that matches the body structure of a real fish. In this paper, we present an approach that uses fin-curvature-based feedback to design a motion controller. A swimming robot composed of a body with two actuated joints and a flexible tail fin is developed. After examining the relationship between the swimming speed and tail fin curvature in feedforward (open-loop) system experiments, we propose to reflect the tail fin curvature to the actuation inputs (phase difference between the two cyclic oscillations), which will perform the efficient swimming motion. Further, the results show that implementing the proposed feedback controller in a fish-type robot makes it swim similar to a real fish. In addition, the proposed controller functions to find inappropriate actuation according to the body structure.

Swimming of the Trophont Zooid of Vorticella Convallaria

Vorticella convallaria is a ciliated protozoan found in freshwater habitats. In the sessile or stalked trophont form, V. convallaria is shaped somewhat like a balloon as it has a body or zooid (the head of the balloon) that is about 40 μm large with cilia around its oral part, and a stalk (the string of a balloon) anchoring the zooid to a solid surface. When a trophont zooid of V. convallaria detached from the stalk, the zooid swims around in water by creating water flow using its oral cilia. In contrast to the stalk contraction of V. convallaria that has been well studied, the swimming motility of V. convallaria is little known. In this study, we measured the swimming trajectories of the stalkless trophont zooid of V. convallaria using video microscopy and Hele-Shaw cells with a gap height of 25 μm, traced the swimming zooid using image processing, and analyzed the swimming motion in terms of swimming velocity and mean square displacement. The stalkless trophont zooid of V . convallaria was found to swim in circular patterns with intermittent ballistic motions in the confinement, and the average swimming speed ranged from 20 μm/s to 110 μm/s. Since the swimming pattern of V. convallaria appeared to be affected by the level of confinement, we will continue characterizing the ciliate’s swimming in the Hele-Shaw cell with different gap heights. Our study is expected to reveal the swimming motility of V. convallaria and to advance general understanding of swimming of microorganisms.

A Numerical Model for the Analysis of the Locomotion of a Cownose Ray

Among all aquatic species, mantas and rays swim by flapping their pectoral fins; this motion is similar to other fishes in terms of efficiency, but it gives better maneuverability and agility in turning. The fin motion is featured by a traveling wave going opposite to the forward motion, producing a force thanks to momentum conservation. This article aims at understanding the swimming dynamics of rays, focusing on energy efficiency. A CFD model of the swimming motion of a cownose ray has been implemented in OpenFOAM, simulating the acceleration of the fish from still to the steady-state velocity using an overset mesh. In this analysis, the 1-DOF dynamics of forward swimming is solved together with the fluid velocity and pressure. The effect of frequency and wavelength of fin motion on thrust, power, and velocity has been investigated and an analysis of the vortices in the wake showed has been performed. The energy efficiency of a self-propelled body has been defined in a novel way and it has been calculated for different motion conditions. The results showed that batoid fishes swim with high energy efficiency and that they are a promising source of inspiration for biomimetic autonomous underwater vehicles.

Bio-Inspired Design, Modeling, and Control of Robotic Fish Propelled by a Double-Slider-Crank Mechanism Driven Tail

This paper presents the design, modeling, and control of a three-joint robotic fish propelled by a Double-Slider-Crank (DSC) driven caudal fin. DSC is a mechanism that can use one DC motor to achieve oscillating foil propulsion. Its design is guided by a traveling wave equation that mimics a fish’s undulatory locomotion. After multiple tests, the robotic fish displayed good performance in mimicking a real fish’s swimming motion. DSC mechanism is proven to be an effective propulsion technique for a robotic fish. With the help of another servo motor at the first joint of the fish’s tail, the robotic fish can have a two-dimensional free-swimming capability. In experiments, the robotic fish can achieve a swimming speed of 0.35 m/s at 3 Hz, equivalent to 0.98 body length (BL) per second. Its steering rate is proportional to a bias angle. The DSC benefits the control of the robotic fish by independently adjusting its steering and swimming speed. This characteristic is studied in a hydrodynamic model that derives the thrust within a DSC frame. Besides the dynamic model, a semi-physics-based and data-driven linear model is established to connect bias angle to robotic fish’s heading angle. The linear model is used for designing a state feedback control, and the controller has been examined in simulation and experiments.

Homeostatic swimming of zooplankton upon crowding: the case of the copepod Centropages typicus

Crowding has a major impact on the dynamics of many material and biological systems, inducing effects as diverse as glassy dynamics and swarming. While this issue has been deeply investigated for a variety of living organisms, more research remains to be done on the effect of crowding on the behaviour of copepods, the most abundant metazoans on Earth. To this aim, we experimentally investigate the swimming behaviour, used as a dynamic proxy of animal adaptations, of males and females of the calanoid copepod Centropages typicus at different densities of individuals (10, 50 and 100 ind. l −1 ) by performing three-dimensional single-organism tracking. We find that the C. typicus motion is surprisingly unaffected by crowding over the investigated density range. Indeed, the mean square displacements as a function of time always show a crossover from ballistic to Fickian regime, with poor variations of the diffusion constant on increasing the density. Close to the crossover, the displacement distributions display exponential tails with a nearly density-independent decay length. The trajectory fractal dimension, D 3D ≅ 1.5, and the recently proposed ‘ecological temperature’ also remain stable on increasing the individual density. This suggests that, at least over the range of animal densities used, crowding does not impact on the characteristics of C. typicus swimming motion, and that a homeostatic mechanism preserves the stability of its swimming performance.