This paper addresses the physical mechanism of intermittent swimming by considering the burst-and-coast regime of fish swimming at different speeds. The burst-and-coast regime consists of a cycle with two successive phases, i.e., a phase of active undulation powered by the fish muscles followed by a passive gliding phase. Observations of real fish whose swimming gait is forced in a water flume from low to high speed regimes are performed, using a full description of the fish kinematics and mechanics. We first show that fish modulate a unique intrinsic cycle to sustain the demanded speed by modifying the bursting to coasting ratio while maintaining the duration of the cycle nearly constant. Secondly, we show using numerical simulations that the chosen kinematics by correspond to optimized gaits for swimming speeds larger than 1 body length per second.

粒子图像测速(PIV)技术是一种瞬态流动平面二维速度场测试技术，在细部流场实测领域得到重视，但是成熟的PIV产品价格高昂。鉴于此，介绍了一种简易的PIV装置，主要由高速摄像机、激光发射器、柱面透镜和示踪粒子构成，以较低成本即可基本实现商业用PIV产品的功能。为了验证简易PIV性能，设计了PIV简易装置，采用Fluent软件模拟，并结合PIV技术对比分析了简易PIV装置的优缺点，同时对影响结果的粒径大小和粒子跟随性进行了优化。结果表明，简易PIV装置适宜选用玉米粉作为示踪粒子，并需要根据示踪粒子跟随性所能达到的最大进口水流速度选择高速摄像机的帧率，最终能够较好地实现流场实测，进而达到对PIV技术进行开发和优化的目的，其分析结果将为后续研究者提供参考。

As a measurement technique of instantaneous 2-D velocity flow field, particle image velocimetry(PIV) has received much attention in detailed flow field measurement, but mature product of PIV is still expensive. In view of this, we introduce a simple device which consists of high-speed digital camera, laser transmitter, cylindrical lens and tracer particle. The fuctions of business PIV product can be achieved with low cost. To verify the simple PIV performance in this paper, we designed a simple device and analyzed the advantages and disadvantages of this simple device by software Fluent. Furthermore, we optimized the particle size and the tracking performance of particle. Results show that corn flour particle is suitable as the tracer particle of this simple device. Moreover, we should choose the frame rate of high-speed digital camera according to the maximum inlet water velocity which the tracking performance of tracer particle can reach. During this process, we can get a better flow field measurement, and develope and optimize the PIV technique. The analysis result offers important reference for further study.

We describe and characterize a method for estimating the pressure field corresponding to velocity field measurements such as those obtained by using particle image velocimetry. The pressure gradient is estimated from a time series of velocity fields for unsteady calculations or from a single velocity field for quasi-steady calculations. The corresponding pressure field is determined based on median polling of several integration paths through the pressure gradient field in order to reduce the effect of measurement errors that accumulate along individual integration paths. Integration paths are restricted to the nodes of the measured velocity field, thereby eliminating the need for measurement interpolation during this step and significantly reducing the computational cost of the algorithm relative to previous approaches. The method is validated by using numerically simulated flow past a stationary, two-dimensional bluff body and a computational model of a three-dimensional, self-propelled anguilliform swimmer to study the effects of spatial and temporal resolution, domain size, signal-to-noise ratio and out-of-plane effects. Particle image velocimetry measurements of a freely swimming jellyfish medusa and a freely swimming lamprey are analyzed using the method to demonstrate the efficacy of the approach when applied to empirical data.

Swimming animals commonly bend their bodies to generate thrust. For undulating animals such as eels and lampreys, their bodies bend in the form of waves that travel from head to tail. These kinematics accelerate the flow of adjacent fluids, which alters the pressure field in a manner that generates thrust. We used a comparative approach to evaluate the cause-and-effect relationships in this process by quantifying the hydrodynamic effects of body kinematics at the body-fluid interface of the lamprey, Petromyzon marinus, during steady-state swimming. We compared the kinematics and hydrodynamics of healthy control lampreys to lampreys whose spinal cord had been transected mid-body, resulting in passive kinematics along the posterior half of their body. Using high-speed particle image velocimetry (PIV) and a method for quantifying pressure fields, we detail how the active bending kinematics of the control lampreys were crucial for setting up strong negative pressure fields (relative to ambient fields) that generated high-thrust regions at the bends as they traveled all along the body. The passive kinematics of the transected lamprey were only able to generate significant thrust at the tail, relying on positive pressure fields. These different pressure and thrust scenarios are due to differences in how active versus passive body waves generated and controlled vorticity. This demonstrates why it is more effective for undulating lampreys to pull, rather than push, themselves through the fluid.© 2016. Published by The Company of Biologists Ltd.

To understand the mechanics of fish swimming, we need to know the forces exerted by the fluid and how these forces affect the motion of the fish. To this end, we developed a 3-D computational approach that integrates hydrodynamics and body dynamics. This study quantifies the flow around a swimming zebrafish (Danio rerio) larva. We used morphological and kinematics data from actual fish larvae aged 3 and 5 days post fertilization as input for a computational model that predicted free-swimming dynamics from prescribed changes in body shape. We simulated cyclic swimming and a spontaneous C-start. A rigorous comparison with 2-D particle image velocimetry and kinematics data revealed that the computational model accurately predicted the motion of the fish's centre of mass as well as the spatial and temporal characteristics of the flow. The distribution of pressure and shear forces along the body showed that thrust is mainly produced in the posterior half of the body. We also explored the effect of the body wave amplitude on swimming performance by considering wave amplitudes that were up to 40% larger or smaller than the experimentally observed value. Increasing the body wave amplitude increased forward swimming speed from 7 to 21 body lengths per second, which is consistent with experimental observations. The model also predicted a non-linear increase in propulsive efficiency from 0.22 to 0.32 while the required mechanical power quadrupled. The efficiency increase was only minor for wave amplitudes above the experimental reference value, whereas the cost of transport rose significantly.

Many fish species transform in body shape during growth, but it remains unclear how this influences the mechanics of locomotion. Therefore, the present study focused on understanding how drag generation during coasting is affected by ontogenetic changes in the morphology of zebrafish (Danio rerio). The shapes of the body and fins were measured from photographs of fish ranging in size from small larvae to mature adults and these morphometrics were compared to drag coefficients calculated from high-speed video recordings of routine swimming. We found that the viscous drag coefficient of larval and juvenile fish increased by more than an order of magnitude during growth and the inertial drag coefficient decreased at a comparable rate in adults. These hydrodynamic changes occurred as zebrafish disproportionately increased the span of their fins and their body changed shape from elongated to streamlined, as reflected by the logistic growth of a newly defined streamlining index, SL. These results suggest that morphological changes incur a performance cost by generating greater drag when larvae and juveniles operate in the viscous regime, but later provide a performance benefit by reducing pressure drag in the inertial regime of the adult stage.(c) 2006 Wiley-Liss, Inc.

Swimming of fish and other animals results from interactions of rhythmic body movements with the surrounding fluid. This paper develops a model for the body-fluid interaction in undulatory swimming of leeches, where the body is represented by a chain of rigid links and the hydrodynamic force model is based on resistive and reactive force theories. The drag and added-mass coefficients for the fluid force model were determined from experimental data of kinematic variables during intact swimming, measured through video recording and image processing. Parameter optimizations to minimize errors in simulated model behaviors revealed that the resistive force is dominant, and a simple static function of relative velocity captures the essence of hydrodynamic forces acting on the body. The model thus developed, together with the experimental kinematic data, allows us to investigate temporal and spatial (along the body) distributions of muscle actuation, body curvature, hydrodynamic thrust and drag, muscle power supply and energy dissipation into the fluid. We have found that: (1) thrust is generated continuously along the body with increasing magnitude toward the tail, (2) drag is nearly constant along the body, (3) muscle actuation waves travel two or three times faster than the body curvature waves and (4) energy for swimming is supplied primarily by the mid-body muscles, transmitted through the body in the form of elastic energy, and dissipated into the water near the tail.