Effect of head swing motion on hydrodynamic performance of fishlike robot propulsion

This paper studies the effect of the head swing motion on the fishlike robot swimming performance numerically.Two critical parameters are employed in describing the kinematics of the head swing: the leading edge amplitude of the head and the trailing edge amplitude of the head.Three-dimensional Navier-Stokes equations are used to compute the viscous flow over the robot.The user-defined functions and the dynamic mesh technology are used to simulate the fishlike swimming with the head swing motion.The results reveal that it is of great benefit for the fish to improve the thrust and also the propulsive efficiency by increasing the two amplitudes properly.Superior hydrodynamic performance can be achieved at the leading edge amplitudes of 0.05L( L is the fish length) and the trailing edge amplitudes of 0.08 L.The unsteady flow fields clearly indicate the evolution process of the flow structures along the swimming fish.Thrust-indicative flow structures with two pairs of pressure cores in a uniform mode are generated in the superior performance case with an appropriate head swing,rather than with one pair of pressure cores in the case of no head swing.The findings suggest that the swimming biological device design may improve its hydrodynamic performance through the head swing motion.     

DYNAMICS OF FREE STRAIGHT SWIMMING OF ANGULLA ANGULLA INCLUDING FORWARD, BRAKING AND BACKWARD LOCOMOTION

Eels can swim backward by reversing the direction of the traveling wave along the body. The propulsive mechanism of an eel, angulla angulla, during its self-propelled straight swimming, including forward swimming, braking and switching direction to backward swimming was numerically studied. The problem was reasonably simplified to a loose-coupling problem of fish swimming dynamics and hydrodynamics only in the swimming direction. The approach involved the simulation of the flow by solving the two-dimensional unsteady incompressible N-S equations and the fish motion dynamic problem with Newton’s second law. Visualizations of flow fields and vortex structures were performed. The propulsive mechanism and dynamics during each process were investigated and the effects of controllable factors on forward free swimming were discussed.     

两种手姿态在不同攻角下划水的数值模拟

该文旨在用计算流体力学（CFD）的方法研究游泳推进问题,建立了手掌和前臂的联接模型,并生成3D非结构网格，计算了两种不同手姿态：并拢和张开在不同来流速度和攻角下的推进阻力和推进升力，并结合计算结果进行了流场分析，研究了两种手姿态对推进效率的影响。结果表明，在各个攻角状态下，手并拢划水均能获得更高的推进效率。     

Using spanwise flexibility of caudal fin to improve swimming performance for small fishlike robots

This paper examines the beneficial effects of the spanwise flexibility of the caudal fin for the improvement of the swimming performance for small fishlike robots. A virtual swimmer is adopted for controlled numerical experiments by varying the spanwise flexible trajectories and the spanwise flexible size of the caudal fin while keeping the body kinematics fixed. 3-D Navier-Stokes equations are used to compute the viscous flow over the robot. Elliptical, parabolic and hyperbola trajectories are chosen to describe the spanwise flexible profile of the caudal fin. According to the sign(positive or negative) of the phase difference of the swinging motion, the spanwise flexibility can be divided into the fin surface of "bow" and the fin surface of "scoop". It is observed that for both the fin surface of "bow" and the fin surface of "scoop", the spanwise elliptical trajectory has the optimal swimming velocity, thrust, lateral force, and efficiency. With comparisons, using the flexible caudal fin with the fin surface of "bow", the lateral force and the power consumption can be reduced effectively and the swimming stability can be increased while reducing little the swimming velocity and thrust. Meanwhile, using the flexible caudal fin with the fin surface of "scoop" can greatly improve the swimming velocity, thrust, and efficiency while increasing part of the lateral force and the power consumption. Three-dimensional flow structures clearly indicate the evolution process around the swimming robot. It is suggested that the fish, the dolphin, and other aquatic animals may benefit their hydrodynamic performance by the spanwise flexibility of the caudal fin.     

The mechanism of flapping propulsion of an underwater glider

To develop a bionic maneuverable propulsion system to be applied in a small underwater vehicle, a new conceptual design of the bionic propulsion is applied to the traditional underwater glider. The numerical simulation focuses on the autonomous underwater glider(AUG)'s flapping propulsion at Re= 200 by solving the incompressible viscous Navier-Stokes equations coupled with the immersed boundary method. The systematic analysis of the effect of different motion parameters on the propulsive efficiency of the AUG is carried out, including the hydrofoil's heaving amplitude, the pitching amplitude, the phase lag between heaving and pitching and the flapping frequency. The results obtained in this study can provide some physical insights into the propulsive mechanisms in the flapping-based locomotion.     

Numerical and experimental studies of hydrodynamics of flapping foils

The flapping foil based on bionics is a sort of simplified models which imitate the motion of wings or fins of fish or birds. In this paper, a universal kinematic model with three degrees of freedom is adopted and the motion parallel to the flow direction is considered. The force coefficients, the torque coefficient, and the flow field characteristics are extracted and analyzed. Then the propulsive efficiency is calculated. The influence of the motion parameters on the hydrodynamic performance of the bionic foil is studied. The results show that the motion parameters play important roles in the hydrodynamic performance of the flapping foil. To validate the reliability of the numerical method used in this paper, an experiment platform is designed and verification experiments are carried out. Through the comparison, it is found that the numerical results compare well with the experimental results, to show that the adopted numerical method is reliable. The results of this paper provide a theoretical reference for the design of underwater vehicles based on the flapping propulsion.     

Hydrodynamic performance of distributed pump-jet propulsion system for underwater vehicle

A type of distributed pump-jet propulsion system （DPJP） is developed with two or four specially designed pump-jet pods located around the axisymmetric underwater vehicle body symmetrically. The flow field is numerically simulated by solving the RANS equations with the finite volume method. The computational method is validated by comparing the calculated hull resistances of the SUBOFF AFF-3 model and the open water performance of a ducted propeller with experimental data. The hydrodynamic performances of the DPJP with different axial or radial positions and numbers of pump-jet pods are obtained to analyze the interactions between the hull and the pump-jet pods. It is shown in the calculated results that the decrease of the distance between the pods and the hull leads to an increase both in the efficiency of the pods and the thrust deduction factor due to the effect of the stern wake. And, a negative thrust deduction factor can be obtained by locating the DPJP at the parallel middle body near the aftbody of the vehicle to improve the hydrodynamic performance of the DPJP. Besides, the increase of the number of pods will cause a remarkable decrease of the total propulsive efficiency of the DPJP with the pods mounted on the stern planes, while a small decline of the total propulsive efficiency of the DPJP is observed with the pods mounted on the parallel middle body.     

蝌蚪模型并列游动的数值研究

该文通过数值求解雷诺平均N-S方程,对高雷诺数下蝌蚪模型同相位和反相位并排游动进行了数值研究,揭示了并列蝌蚪群游的流动机制。研究表明,非流线型的蝌蚪并列群游与流线型的鱼体不同。蝌蚪同相位并列游动时,总推力比单独游动时低,尽管能量消耗有所增加但推进效率更高;蝌蚪反相位并列游动时,总推力比单独游动时低,但比同相位并列游动时高,间距很近时消耗的能量会显著增加,而推进效率依然提高。蝌蚪的钝体头部产生的涡在群游时有利于增加推力。     

Propulsive performance of a passively flapping plate in a uniform flow

Propulsive performance of a passively flapping plate in a uniform viscous flow has been studied numerically by means of a multiblock lattice Boltzmann method. The passively flapping plate is modeled by a rigid plate with a torsion spring acting about the pivot at the leading-edge of the plate, which is called a lumped-torsional-flexibility model. When the leading-edge is forced to take a vertical oscillation, the plate pitches passively due to the fluid-plate interaction. Based on our numerical simulations, various fundamental mechanisms dictating the propulsive performance, including the forces on the plate, power consumption, propulsive efficiency and vortical structures, have been studied. It is found that the torsional flexibility of the passively pitching plate can improve the propulsive performance. The results obtained in this study provide some physical insights into the understanding of the propulsive behaviors of swimming and flying animals.     

山区流域洪涝预报水文与水动力耦合模型研究进展

受人类活动及气候变化的影响,我国山区流域洪涝灾害频发,研制洪涝模拟及预报模型对于流域洪涝灾害防治及水环境综合整治具有重要的理论意义和应用价值。论文评述了洪涝模拟与预报使用的水文及水动力模型的优缺点,指出单一的水文或水动力模型的局限性。基于此,分析并比较了国内外已有的水文模型与水动力模型的不同耦合方式,包括水文模型与水动力模型的串联耦合、水文模型与一维水动力模型的动态单向耦合、水文模型与二维水动力模型的动态单向耦合。总结了各类耦合模型的优势和适用性,评价了已有耦合模型在计算精度、数值格式稳定性和计算效率等方面所取得的创新。论文探讨了流域洪涝发生和发展的水文过程与水动力过程的互馈机制,分析了水文模型与水动力模型新的耦合方法及可行性,介绍了水文与二维及一维水动力的动态双向耦合模型。动态双向耦合模型预期能更加真实地反映流域洪涝发生及发展物理机制,具有提高洪涝模拟预报精度和计算效率的潜力。各类耦合模型各有其自身优势,可以根据流域及城市防洪实际,选取合适的水文与水动力耦合模型,为山区流域洪涝模拟和预报提供技术支持。     

The energy-saving advantages of burst-and-glide mode for thunniform swimming

This paper explores the energy-saving advantages of the burst-and-glide swimming and compares it with the normal self-swimming for a thunniform swimmer. The virtual swimmer allows us to perform controlled numerical experiments by varying the swinging tail number and the duty cycle while keeping the other parameters fixed. 3-D Navier-Stokes equations are used to compute the viscous flow over the swimmer. The user-defined functions and the dynamic mesh technology are used to simulate the burst-and-glide swimming. The results show that with the increase of the swinging tail number or the duty cycle, the swimming velocity, the power and the efficiency all increase, but the velocity-power ratio decreases somewhat. Therefore, choosing smaller swinging tail number and duty cycle is beneficial in reducing the power and increasing the velocity-power ratio, and thus to obtain the same velocity, less power is consumed. And to swim the same distance, the energy can significantly be saved. The power consumption, the efficiency and the velocity-power ratio in the burst-and-glide case are 43.9%, 40.6% and 1.15 times of those in the normal swimming case, respectively. The flow structures clearly show the evolution process around the fish in the burst-and-glide swimming. The findings can be used to reasonably plan the swimming action and to take the advantage of the external flow field energy for the fishlike robot, to be more efficient and energy-saving.     

NUMERICAL SIMULATION OF FISH SWIMMING WITH RIGID PECTORAL FINS

The numerical simulation of the self-propelled motion of a fish with a pair of rigid pectoral fins is presented.A Navier-Stokes equation solver incorporating with the multi-block and overset grid method is developed to deal with the multi-body and moving body problems.The lift-based swimming mode is selected for the fin motion.In the lift-based swimming mode,the fin can generate great thrust and at the same time have no generation of lift force.It can be found when a pair of rigid pectoral fins generates the hydrodynamic moment,it may also generate a lateral force opposite to the centripetal direction,which has adverse effect on the turn motion of the fish.Furthermore,the periodic vortex structure generation and shedding,and their effects on the generation of hydrodynamic force are also demonstrated in this article.     

静水中长鳍扭波推进的水动力数值研究

长鳍扭波推进是一种典型的鱼类游动方式,具有高效、低噪、机动灵活等一系列优点。该文以"尼罗河魔鬼"鱼为仿生研究对象,采用弹性光顺法和局部网格重划法的动网格技术对大摆幅的长鳍扭波推进运动流场进行了数值计算研究,鳍条最大摆幅高达85°,对长鳍扭波推进在系泊状态下的水动力进行了研究,并与试验结果进行了比较验证。数值分析了鳍面压力分布及其随相位的变化,以及与推力产生的关联,并分析了长鳍扭波推力随扭波频率的变化,研究表明,系泊状态下,推力系数不随频率而改变。     

考虑坝体柔性的重力坝坝面地震动水压力计算

为了准确模拟地震作用下重力坝坝面动水压力,采用比例边界有限元与有限元的无缝耦合理论,提出了一种考虑坝体和库底柔性的坝面动水压力计算方法。该方法采用比例边界有限元的理论,可仅离散坝水交界面实现半无限库水的高效高精度模拟,且能方便考虑库水的可压缩性以及库底和岸坡的吸收作用;采用有限元离散坝体结构,通过作用在大坝迎水面上的动水压力实现对大坝-库水系统的求解。算例计算结果表明,该方法计算得到的重力坝坝面动水压力与已有文献计算结果吻合较好;坝体柔性削弱了坝面动水压力;坝面动水压力随库底反射系数的减小而减小。     

重力坝坝体-库水-气幕相互作用的振动台试验

结合某碾压混凝土重力坝工程,模拟了坝体库水气幕的动力相互作用,振动台模型试验中测试了大坝自振频率、坝面动水压力、坝面加速度和上、下游坝面动应力响应、坝顶位移等,研究了气幕对坝面动水压力、加速度、位移等的影响,并将库水有限元模型的计算结果与模型试验成果进行比较。结果表明,气幕对坝面动水压力及其动力特性均有影响,坝前气幕的良好压缩性能对地震效应有很大的缓冲作用,能降低动水压力和坝体的动应力,改善和提高坝体的抗震性能。     

NUMERICAL STUDY ON PROPULSIVE PERFORMANCE OF FISH-LIKE SWIMMING FOILS

1. INTRODUCTION The study of the movement of fishes can be very informative in exploring mechanisms of unsteady flow control because movements in fish is a result of many millions of years of evolutionary optimization. Previous studies have shed light on …     

二维摆动水翼仿生推进水动力性能研究

以鱼类尾鳍摆动式推进模式为背景，对二维刚性和柔性水翼摆动运动的非定常水动力性能开展了数值计算，将刚性水翼的计算结果与实验值进行了对比，吻合较好。运动边界采用动网格技术，湍流模型选用SST模型，柔性变形采用Bose模式，探讨了水翼瞬时攻角、及柔度对摆动水翼推进性能的影响。     

NUMERICAL AND EXPERIMENTAL STUDIES OF INFLUENCE OF THE CAUDAL FIN SHAPE ON THE PROPULSION PERFORMANCE OF A FLAPPING CAUDAL FIN

This article presents a comprehensive study of the effects of the caudal fin shape on the propulsion performance of a candal fin in harmonic heaving and pitching.A numerical simulation based on an unsteady panel method was carried out to analyze the hydrodynamic performance of flapping caudal fins of three shapes(the whale caudal fin with the largest projected area,the dolphin caudal fin with the median projected area,and the tuna caudal fin with the smallest projected area).Then,a series of hydrodynamic ex...     

多关节鱼形机器人的动态特性的建模与仿真研究

本文以James LightHill的鱼类的细长体理论为基础,利用能量守恒、动量定理和波动理论来分析计算仿生机器鱼在运动中的受力。建立了转角、相位差、转动频率等重要参数的动态模型,在对模型进行仿真的基础上,对仿生机器鱼的效率进行了数值模拟和估算,分析了影响效率的一些因素,揭示了鱼类运动的高效的机理,为进一步展开对仿生机器鱼的研究提供了一个思路。     

Swimming behavior of crucian carp in an open channel with sudden expansion

The sudden‐expansion effect created by natural and man‐made structures such as unsubmerged boulders, spur dikes, and culvert structures is common in open channels. Migratory fishes may use the heterogeneous flow generated by sudden expansion to their benefit, to select habitats for balancing energy expenditure or for maximizing predation opportunities. This study explores the swimming behavior of three size classes of crucian carp in response to hydrodynamic characteristics in an experimental open channel with sudden expansion. The flow field was characterized using particle image velocimetry (PIV) measurements and featured a recirculation zone in the vicinity of the expansion. Based on the classification of the swimming trajectory, four specific swimming behaviours (SSBs) were utilized by crucian carp during migration. With increasing flow velocity, fish spent more time in the recirculation zone. Residence time in the recirculation zone was inversely correlated with fish body length. Due to the advantage in providing shelter from challenging heterogeneous hydraulic conditions, the recirculation zone was preferred by crucian carp, especially the smaller ones, indicating the importance of energy conservation in habitat occupation. These findings confirm that the recirculation zone generated by sudden expansion may be beneficial to the upstream passage of fish and in habitat restoration.