Undulating Fin Propulsion Research Articles

Hydrodynamic performance analysis of undulating fin propulsion

A desire to further comprehend the hydrodynamic characteristics of three-dimensional undulating fin propulsion is what motivates the work. First, a high-resolution numerical technique based on the constraint immersed boundary method is utilized to simulate the fluid-fin system. The simulation results reveal fundamental variation laws between the hydrodynamic performance of the undulating fin and kinematic parameters. According to the simulation data, an in-depth analysis of the scaling law is conducted. A key contribution of this work is to build the force scaling formula and extend the law to complicated cases involving different incoming flow velocities. The important application of the force scaling law is that it can be used to estimate the self-propelled speed and wave efficiency of the undulating fin in different kinematic conditions. The results show that the wave efficiency exhibits a monotonically bounded increasing trend as the amplitude grows, is basically independent of the frequency, and decreases monotonically with the increasing wavelength. Finally, the work discusses the evolution of vortex structures in undulating fin propulsion. The analysis indicates that the streamwise central jet formed throughout the fin body is the primary reason for thrust generation in undulating fin propulsion. Furthermore, the basic dynamic mechanisms of two types of vortex rings, related to the formation of the central jet, are investigated in the work. The results further reveal the macro-interaction mechanism between the undulating fin and fluid flow. The findings could make a contribution to explaining some biological phenomena and developing bionic engineering.

Underwater collision avoidance using undulating elongated fin propulsion

Underwater navigation to survey littoral regions or complex environments such as offshore structures, coral reefs or sensitive coastal zones requires a highly maneuverable propulsion mechanism. Conventional propulsors have limited maneuverability, often incapable of providing adequate torques and forces to maneuver in such environments. Elongated ribbon fin propulsion, a bio-inspired propulsion technique, can overcome these shortcomings. In the present work, we used a robotic vessel with elongated fin propulsion to test and implement three collision avoidance strategies based on potential field methods. We found that having a circular sensing envelope in the robotic vessel helps to smoothly transition to the mission path after collision is avoided. However, under this strategy, the distance to return to the desired path tends to be longer. The selection of adequate collision avoidance strategies will depend on multiple factors, including others sensing capabilities, desired reaction time, rate of turning and objects to avoid. Although this work focused on relatively simple scenarios for obstacle avoidance, we envision that utilizing more complex maneuver techniques, including rapid turns and hovering, collision avoidance strategies for underwater vessels with undulating fin propulsion can take advantage of the full fin kinematics manipulation to maneuvers as elegant as seen on the fish counterpart.

Inspired by the Black Ghost Knifefish: Bionic Design of Undulatory Fin with 2-DOF Rays and Its Propulsion Performance

The demand for high-performance underwater thrusters in marine engineering is increasing. The concealed, mobile, and efficient underwater ability of fish provides many directions for research. The black ghost knifefish uses only wavy ventral fins to swim and can hover and roll in the water. Based on the physiological and morphological characteristics of the black ghost knifefish, we explored the structure and movement mode of the ventral fin, so as to establish a two-degree of freedom (2-DOF) structural model and kinematic model. We reveal the motion mechanism of the undulating fin propulsion through the constructed model and computational fluid dynamics. It is found that when the fin surface fluctuates, a pair of vortices with opposite directions will be formed on the concave side of the fin surface. These vortices will produce a central jet on the fin surface, provide a reverse impulse for the ventral fin, and make the fin obtain power. In addition, we found that the propulsive force of the ribbon fin along the body direction is positively correlated with the swing amplitude and frequency of the fin movement, and the propulsive torque of the ribbon fin to realize the maneuvering movement increases first and then decreases with the increase of the torsion angle. The research on the structure and motion mechanism of the ribbon fin of the black ghost knifefish provides a basis for the development of a bionic prototype of multi-DOF motion and the control strategy of high-mobility motion.

Cost of Transport of Undulating Fin Propulsion.

Autonomous robots are used to inspect, repair and maintain underwater assets. These tasks require energy-efficient robots, including efficient movement to extend available operational time. To examine the suitability of a propulsion system based on undulating fins, we built two robots with one and two fins, respectively, and conducted a parametric study for combinations of frequency, amplitude, wavenumber and fin shapes in free-swimming experiments, measuring steady-state swimming speed, power consumption and cost of transport. The following trends emerged for both robots. Swimming speed was more strongly affected by frequency than amplitude across the examined wavenumbers and fin heights. Power consumption was sensitive to frequency at low wavenumbers, and increasingly sensitive to amplitude at high wavenumbers. This increasing sensitivity of amplitude was more pronounced in tall rather than short fins. Cost of transport showed a complex relation with fin size and kinematics and changed drastically across the mapped parameter space. At equal fin kinematics as the single-finned robot, the double-finned robot swam slightly faster (>10%) with slightly lower power consumption (<20%) and cost of transport (<40%). Overall, the robots perform similarly to finned biological swimmers and other bio-inspired robots, but do not outperform robots with conventional propulsion systems.

A comparison for hydrodynamic performance of undulating fin propulsion on numerical self-propulsion and tethered models

Undulating fin propulsion holds considerable potential in the design of novel underwater vehicles. However, there exist gaps in the understanding of undulating fin propulsion mechanisms, particularly within the context of different numerical strategies. This study aims to evaluate hydrodynamic performance of undulating fin propulsion base on two strategies, namely, the tethered model (TM) and the self-propulsion model (SPM). Specifically, effects of kinematic factors on hydrodynamic performance and wake structures at different conditions are investigated. Importantly, the work presents pros and cons of the two numerical models on evaluating hydrodynamic performance by comparing the simulation results. A result is that there exist different change rules of hydrodynamic parameters even under the same kinematic conditions for the two models. The wake structure in the TM could present three different structures, depending on the traveling wave speed and the inflow speed. However, the wake always presents the 2S-type and thrust-type vortex street in the SPM. The study shows the TM focus on presenting a periodic, fully developed and steady result, but the SPM is more suitable to explore instantaneous hydrodynamic interactions between the flexible body and the developing flow field. The findings offer some insights into understanding the differences between the two models.

The hydrodynamic effects of undulating patterns on propulsion and braking performances of long-based fin

Bio-inspired long-based undulating fin propulsion is commonly employed in biological autonomous underwater vehicles (BAUVs), while the hydrodynamic characteristics of various undulating patterns are different. To investigate what kind of undulating pattern has outstanding propulsion or braking performance for BAUVs in directional maneuvers, undulations with four basic undulating patterns are numerically examined under the Open-source Field Operation And Manipulation environment at the Reynolds number of 5 × 102, 5 × 103, and 5 × 104, corresponding to viscous, transitional, and inertial flow regimes, respectively. The study is conducted at various non-dimensional phase speeds c (0.5–2.0, normalized by incoming flow speed) at a constant maximum amplitude of 0.08 and a wavelength of 0.5 (both are normalized by the fin cord length) to imitate the long-based fin. The numerical results indicate that the undulating fin motion with the amplitude envelope gradually increasing from the anterior part to the posterior (conical sinusoidal wave) part may be preferable for thrust generation; undulating with the amplitude envelope increasing from the anterior part to the mid part and decreasing toward the posterior (fusiform sinusoidal wave) presents the superior braking performance when the phase speed is low enough. Moreover, the influence of undulating patterns on the wake structure is analyzed. Through further comparative analysis for propulsion and braking performances, the results obtained here may have instructional significance to the propulsion mechanism in bionic design.

Investigations on vortex structures for undulating fin propulsion using phase-locked digital particle image velocimetry

Investigations on vortex structures for undulating fin propulsion using phase-locked digital particle image velocimetry

Reinforcement learning-based optimization of locomotion controller using multiple coupled CPG oscillators for elongated undulating fin propulsion.

This article proposes a locomotion controller inspired by black Knifefish for undulating elongated fin robot. The proposed controller is built by a modified CPG network using sixteen coupled Hopf oscillators with the feedback of the angle of each fin-ray. The convergence rate of the modified CPG network is optimized by a reinforcement learning algorithm. By employing the proposed controller, the undulating elongated fin robot can realize swimming pattern transformations naturally. Additionally, the proposed controller enables the configuration of the swimming pattern parameters known as the amplitude envelope, the oscillatory frequency to perform various swimming patterns. The implementation processing of the reinforcement learning-based optimization is discussed. The simulation and experimental results show the capability and effectiveness of the proposed controller through the performance of several swimming patterns in the varying oscillatory frequency and the amplitude envelope of each fin-ray.

Design and Preliminary Evaluation of A Biomimetic Underwater Robot with Undulating Fin Propulsion

Biomimetic undulating fin propulsion systems have the advantages for enhancing the maneuverability and efficiency of underwater robot when compared with conventional driving forms. However, there are still some defects existing in this approach, such as low speed and difficulty in depth control. This paper presents the mechanical design, control system design and experimental evaluation of a biomimetic underwater robot with undulating fin propulsion. First, a 3D model of the robot was established in CATIA. According to the kinematic model of the fins, the design of undulating fin propulsion system have been optimized and the phase control method has been proposed to fit with the design. Next, a control system with hybrid structure switching between autonomous mode and remote mode has been implemented to address the problem that the robot is out of control in underwater environment due to the attenuation of radio signals. Finally, the underwater locomotion performance of the robot has been validated in a swimming pool. The result indicates that the robot can achieve multiple degree of freedoms (DOFs) motions by controlling the kinematic parameters of the fin and was able to swim as fast as 31.6 cm/s (nearly 0.8 body length per second).

Swimming performance of a bio-inspired robotic vessel with undulating fin propulsion

Undulatory fin propulsion exhibits a high degree of maneuver control—an ideal feature for underwater vessels exploring complex environments. In this work, we developed and tested a self-contained, free-swimming robot with a single undulating fin running along the length of the robot, which controls both forward motion and directional maneuvers. We successfully replicated several maneuvers including forward swimming, reversed motion, diving, station-keeping and vertical swimming. For each maneuver, a series of experiments was performed as a function of fin frequency, wavelength and traveling wave direction to measure swimming velocities, orientation angles and mean power consumption. In addition, 3D flow fields were measured during forward swimming and station-keeping using volumetric particle image velocimetry (PIV). The efficiency for forward swimming was compared using three metrics: cost of transport, wave efficiency and Strouhal number (St). The results indicate that the cost of transport exhibits a V-shape trend with the minimum value at low swimming velocity. The robot reaches optimal wave efficiency and locomotor performance at a range of 0.2–0.4 St. Volumetric PIV data reveal the shed of vortex tubes generated by the fin during forward swimming and station keeping. For forward swimming, a series of vortex tubes are shed off the fin edge with a lateral and downward direction with respect to the longitudinal axis of the fin. For station keeping, flow measurements suggest that the vortex tubes are shed at the mid-section of the fin while the posterior and anterior segment of the vortex stay attached to the fin. These results agree with the previous vortex structures based on simulations and 2D PIV. The development of this vessel with high maneuverability and station keeping performance has applications for oceanography, coastal exploration, defense, the oil industry and other marine industries where operations are unsafe or impractical for divers or human-piloted vessels.

Evaluating the Fin-Ray Trajectory Tracking of Bio-Inspired Robotic Undulating Fins via an Experimental-Numerical Approach

In the past decade, biomimetic undulating fin propulsion has been one of the main topics considered by scientists and researchers in the field of robotic fish. This technology is inspired by the biological wave-like propulsion of ribbon-finned fish. The swimming modes have aquatic application potentials with greater manoeuvrability, less detectable noise or wake and better efficiency at low speeds. The present work concentrates on the evaluation of fin-ray trajectory tracking of biorobotic undulating fins at the levels of kinematics and hydrodynamics by using an experimental-numerical approach. Firstly, fin-ray tracking inconsistence between the desired and actual undulating trajectories is embodied with experimental data of the fin prototype. Next, the dynamics' nonlinearity is numerically and analytically unveiled by using the computational fluid dynamics (CFD) method, from the viewpoint of vortex shedding and the hydro-effect. The evaluation of fin-ray tracking performance creates a good basis for control design to improve the fin-ray undulation of prototypes.

Braking Performance of a Biomimetic Squid-Like Underwater Robot

In this study, the braking performance of the undulating fin propulsion system of a biomimetic squid-like underwater robot was investigated through free run experiment and simulation of the quasi-steady mathematical model. The quasi-steady equations of motion were solved using the measured and calculated hydrodynamic forces and compared with free-run test results. Various braking strategies were tested and discussed in terms of stopping ability and the forces acting on the stopping stage. The stopping performance of the undulating fin propulsion system turned out to be excellent considering the short stopping time and short stopping distance. This is because of the large negative thrust produced by progressive wave in opposite direction. It was confirmed that the undulating fin propulsion system can effectively perform braking even in complex underwater explorations.

Experimental and analytical study on factors influencing biomimetic undulating fin propulsion performance based on orthogonal experimental design

This paper presents an experimental study on the structural and motion parameters of a biomimetic mechanical fin, as actuators for biomimetic underwater vehicles, and their influences on its propulsion performance. Orthogonal experimental design method is employed to optimize the test scheme due to its capability of reducing experiment time and rapid determination of influencing factors. A L64 orthogonal array is adopted in our seven-factor mixed factorial experiment with three structure parameters and four motion parameters. The experimental results of range analysis and variance analysis are discussed. The statistic results show that the fin-ray configuration, sway frequency, wave number, and rigidity of membrane are the primary factors affecting the integrated propelling performance of the biomimetic mechanical fin. Optimizing these parameters needs to compromise the propelling speed, efficiency, and maximum output power of the actuator. The analytic results are also in agreement with theoretical analysis and simulation study on inclined angle of fish fin-ray. Consequently, proper modification of the inclined angle of the fin-ray might improve not only the propelling speed and acceleration but also the propelling efficiency.

Computational Study on a Squid-Like Underwater Robot with Two Undulating Side Fins

Abstract The undulating fin propulsion system is an instance of the bio-inspired propulsion systems. In the current study, the swimming motion of a squid-like robot with two undulating side fins, mimicking those of a Stingray or a Cuttlefish, was investigated through flow computation around the body. We used the finite analytic method for space discretization and Euler implicit scheme for time discretization along with the PISO algorithm for velocity pressure coupling. A body-fitted moving grid was generated using the Poisson equation at each time step. Based on the computed results, we discussed the features of the flow field and hydrodynamic forces acting on the body and fin. A simple relationship among the fin's principal dimensions was established. Numerical computation was done for various aspect ratios, fin angles and frequencies in order to validate the proposed relationship among principal dimensions. Subsequently, the relationship was examined base on the distribution of pressure difference between upper and lower surfaces and the distribution of the thrust force. In efficiency calculations, the undulating fins showed promising results. Finally, for the fin, the open characteristics from computed data showed satisfactory conformity with the experimental results.

Modelling and parametric study of modular undulating fin rays for fish robots

Abstract Biomimetic robots borrow their senses and structure from animals, such as insects, fish and birds. Development of underwater vehicles is one of the areas where biomimetic robots can potentially perform better than conventional robots. In this paper, the biomimetic design and the workspace study of undulating fin propulsion mechanisms are considered and discussed. We are interested in fish with long and/or wide undulating body/fin – especially those of anguilliform, amiiform, rajiform, and gymnotiform. Two major mechanism layouts developed to mimetic fin undulations of real fish are compared and discussed. Various kinematics expressions of fin waves are presented and the model’s limitation is also discussed. For a parametric study, the geometry of a single fin segment of the assembled fin mechanisms and the fin wave generated are first developed. Next, the fin workspace of the single fin segment is derived based on a defined area ratio. By virtue of the obtained fin dimensions, a gymnotiform robot, Nanyang knifefish (NKF-II), has been designed and constructed. With the fin-ray linkages with sliders connecting in series, the fish robot is able to generate arbitrary undulating waveforms. The robot’s maneuvering and its depth control have also been achieved by the integration of a buoyancy tank with the undulating fin mechanisms. Initial pool testing has been conducted to demonstrate the basic performance of the fish robot underwater.