Study on thrust increase characteristics of separated undulating fins

Underwater collision avoidance using undulating elongated fin propulsion

A two-dimensional unsteady computational fluid dynamics (CFD) method using an unstructured, grid-based and unsteady Navier-Stokes solver with automatic adaptive re-meshing to compute the unsteady flow was adopted to study the hydrodynamic interaction between a periodic oscillating plate and a rigid undulating fin in tandem arrangement. The user-defined function (UDF) program was compiled to define the undulating and oscillating motion. First, the influence of the distance between the anterior oscillating plate and the posterior undulating fin on the non-dimensional drag coefficient of the fin was investigated. Ten different distances, D=0.2L, 0.4L, 0.6L, 0.8L, 1.0L, 1.2L, 1.4L, 1.6L, 1.8L and 2.0L, were considered. The performance of the fin for different distances (D) is different. Second, the plate oscillating angle (5.7°, 10°, 20°, 30°, 40°, 45°, 50°) and frequency (0.5 Hz, 1.0 Hz, 1.5 Hz, 2.0 Hz, 2.5 Hz, 3.0 Hz, 3.5 Hz, 4.0 Hz) effects on the non-dimensional drag coefficient of the fin were also implemented. The pressure distribution on the fin was computed and integrated to provide fin forces, which were decomposed into lift and thrust. Meanwhile, the flow field was demonstrated and analysed. Based on the flow structures, the reasons for different undulating performances were discussed. It shows that the results largely depend on the distance between the two objects. The plate oscillating angle and frequency also make a certain contribution to the performance of the posterior undulating fin. The results are similar to the interaction between two undulating objects in tandem arrangement and they may provide a physical insight into the understanding of fin interaction in fishes or bio-robotic underwater propulsors that are propelled by multi fins.

Influence of hydrofoil motion patterns on the hydrodynamic performance of undulating fin for biomimetic underwater robots

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.

The kinematics of steady forward swimming of Gymnarchus niloticus are described. The geometric features of the body and locomotive characteristic and parameters of the flexible dorsal fin are discussed. A simplified kinematic model on locomotion of the undulatory long dorsal fin propulsion is based on the the observation and experimental data. The hy-dromechanical performances of the undulatory long dorsal fin propeller of Gymnarchus niloticus are estimated applying the large-amplitude elongated-body theory. The hydromechanical efficiency of the undulatory long dorsal fin system ranged from 82.284% to 87.455% over a speed rang of 0.728-0.985 lengthes· s-1. It is suggested that the undulatory long dorsal fin propulsion is an adaptation to swimming with high hydromechanical efficiency.

We present experimental velocity, propulsion force and power measurements finding out that the significance level of shape and kinematics parameters of robotic fin to its propulsion performance. The data have been obtained through synchronous velocity, propulsive force and the power measurements on an undulatory robotic fin mechanism, at the Reynolds numbers 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> . The lateral motion of the robotic fin is in the form of a travelling wave, the kinematics parameters we investigated are frequency, amplitude, wavelength and envelope line. According to the data of velocity, propulsive force and the power measurements, the propulsion efficiency can be conducted. The experimental result indicates that the velocity, propulsive force and propulsion efficiency is optimum with the fin shape is isosceles trapezoid, the frequency and amplitude of fluctuation is 1.67Hz and 100 mm respectively. The optimization result is meaningful for the mechanical design of undulatory fin propulsion.

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

Considering the advantages of biomimetic undulating fin propulsion, a modular underwater robot was developed, and the linear underwater propulsion performance of its individual module was studied. The CFD method was used to analyze the hydrodynamic characteristics of the robot's underwater self-propulsion, verifying the feasibility of the modular robot's self-propulsion and the overall design's rationality. The variation in speed and thrust of the robot over time during its underwater self-propulsion process was clarified. The transient evolution of the undulating fin, robot and surrounding flow field during the steady-state propulsion cycle was analyzed. A parametric study was conducted to examine the effect of different undulating parameters of the fin on the robot's underwater propulsion performance. The effect of frequency on propulsion performance under different amplitude conditions showed a certain similarity. The effect of amplitude on propulsion performance under different frequency conditions followed the same trend, and when the frequency exceeded 2 Hz, the curve of propulsion performance with respect to amplitude became more similar. The research results provide a theoretical fluid dynamics foundation for undulating fin robot underwater propulsion and also serve as the basis for future research on underwater propulsion in modular reconfigured forms.

An autonomous underwater vehicle with a biomechanical propulsion system is a possible answer to the demand for small, silent sensor platforms in many fields. The design of Galatea, a bio-mimetic AUV, involves four aspects: hydrodynamic shape, the propulsion, the motion control systems and payload. The shape of the hull is based on a modified Wortmann FX 71-L-150/20 airfoil. Wind tunnel tests have been conducted to determine the hydrodynamic force coefficients. The propulsion system is based on bio-mimetic undulating fin propulsion. A test set-up is build to get more insight in the fundamentals of this mechanism. The swimming behaviour is currently manually controlled and will be developed into an fully autonomous system. In the future, more research on the undulating fin propulsion system will be carried out and a second, modular prototype robot will be developed.

Several physico-mechanical designs evolved in fish are currently inspiring robotic devices for propulsion and maneuvering purposes in underwater vehicles. Considering the potential benefits involved, this paper presents an overview of the swimming mechanisms employed by fish. The motivation is to provide a relevant and useful introduction to the existing literature for engineers with an interest in the emerging area of aquatic biomechanisms. The fish swimming types are presented, following the well-established classification scheme and nomenclature originally proposed by Breder. Fish swim either by body and/or caudal fin (BCF) movements or using median and/or paired fin (MPF) propulsion. The latter is generally employed at slow speeds, offering greater maneuverability and better propulsive efficiency, while BCF movements can achieve greater thrust and accelerations. For both BCF and MPF locomotion, specific swimming modes are identified, based on the propulsor and the type of movements (oscillatory or undulatory) employed for thrust generation. Along with general descriptions and kinematic data, the analytical approaches developed to study each swimming mode are also introduced. Particular reference is made to lunate tail propulsion, undulating fins, and labriform (oscillatory pectoral fin) swimming mechanisms, identified as having the greatest potential for exploitation in artificial systems.

A long uncompressed fin driven by fin rays is designed in this paper, and its kinematic model is established. The SST K-ω turbulence model is used to solve the N-S equation of three-dimensional unsteady incompressible fluid, and the hydrodynamic characteristics of the uncompressed fin are studied. The results show that when the undulating fin is started, the velocity of the surrounding fluid gradually increases with the movement of the fin surface, and the fluid moves from the leading edge of the undulating fin to the back edge along the fin surface. A relatively obvious anti-Karman vortex street is formed on both sides of the fin, and a jet is formed at the tail of the fin to generate forward thrust.

Undulatory fin propulsion, inspired by the swimming of knifefish and rays, holds considerable potential for enhancing the maneuvering and station-keeping capabilities of underwater vehicles. Most robotic fins are comprised by a series of individually actuated “fin rays”, interconnected via a flexible “membrane” sheet. Propulsive forces are generated by the propagation of a traveling wave along the fin, obtained via appropriately timed ray oscillations. In prior work, we have presented a dynamic model for an elementary two-ray fin system, analyzed as a standard robot mechanism with additional contributions arising from the elastic deformation of the flexible membrane and from the hydrodynamic forces. This paper extends the model to also consider the dynamics of the fin ray actuators, and introduces a combined feedforward-feedback scheme for tracking of the rays' reference trajectories, in order to accurately implement a desired undulation pattern. Experiments conducted with a two-ray fin prototype confirm the efficacy of the proposed controller, and demonstrate its improved performance over non model-based control schemes.

Low-thrust electric propulsion system has drawn increasing attention from researchers because of its high propellant efficiency. The reachable domain of low-thrust spacecraft can provide a geometric insight for space mission planning. In this paper, analytical solutions of the envelope of reachable domain are obtained by employing the Pontryagin’s maximum principle in the well-known linearized model of relative motion, the Tschauner–Hempel equations. Specifically, this study focuses on two contributions. First, an analytical solution of the envelope of reachable domain is obtained, and the associated optimal control profile is derived. Second, an ellipsoid approximation of the reachable domain is proposed to represent the envelope directly based on the analytical solution. Numerical simulations are conducted to demonstrate the accuracy of the proposed solutions. The results show that the reachable domain obtained analytically coincides well with that solved by the numerical indirect method based on the two-body model with low thrust.

Simulation of Fluid Flow in a Channel Induced by Three Types of Fin-Like Motion

Multi-fin kinematics and hydrodynamics in pufferfish steady swimming