斑马鱼S型起动运动学研究

快速起动是鱼类重要的一种机动能力,有助于鱼类的逃逸和捕食。快速起动通常分为C形起动和S形起动两种模式。本文利用数字式高速摄像机定量地研究了斑马鱼从低速巡游状态做出捕食反应的运动过程。通过分析斑马鱼的运动图像,发现S形起动过程大体可以分解为三个阶段:第一阶段,斑马鱼的头部和尾部都迅速向相同的一个体侧方向转动,整个身体近似弯曲反写的S型。第二阶段,斑马鱼的头部转向捕食方向,尾部摆动1~2个周期,整个身体近似成S型。第三阶段整个鱼体逐渐恢复平直形态,其前端2/3的部分不再摆动,维持一种类似在水中滑翔的游动姿态。本文定量分析了各个阶段鱼体躯干曲线的形状,给出了鱼体的速度分布,并定性地解释了S形起动的力学机理。     

斑马鱼C型起动中动力学特性的活体实验研究

提出了一种从鱼类自主游动的运动学实验数据出发推算其动力学特性的实验研究方法。该方法基于变形体动力学方程,不仅可以计算出变形鱼体的整体转动角速度以完善其运动学数据,还可以计算出作用在自主游动的鱼体上的流体合力和流体合力矩,进而分析鱼体的力能学特征。本文运用此方法研究了斑马鱼的两种典型C型起动,对比分析了它们的运动学及力能学特征。结果表明,在相仿的C型弯曲变形下,逃逸型C型起动与非逃逸型C型起动相比,前者推力较大,导致其在前进方向上的质心速率较大;前者侧向力较大,导致其转弯半径较小;前者转矩较大,但因在起动中后期的S型摆动产生了反向转矩,最终导致转动角度小于后者。通过对两种典型C型起动的实验研究发现,斑马鱼会因不同的运动需求而表现出不同的机动性能。     

鲤鱼尾鳍实时三维姿态测量及运动特性分析

为了实现鱼鳍三维运动特性的精确测量, 研究尾鳍的动态瞬时特性, 定量分析其连续变化过程, 建立了基于栅线投影和高速摄像的测量系统和与之对应的分析方法. 以自主游动的鲤鱼为实验对象, 将一组正弦光栅投射在其尾鳍表面, 产生包含三维信息的光栅条纹并由高速摄像机实时采集尾鳍摆动过程的序列图像; 对其中的每一帧图像作二维傅里叶变换、频谱滤波、逆傅里叶变换及三维位相展开等处理后, 重建尾鳍的瞬时三维形态, 再现尾鳍的连续运动过程; 据此测量结果分析研究尾鳍的运动特征及运动学参数. 结果表明, 鲤鱼游速为0.5L/s时, 尾鳍的摆动频率为1.42Hz; 鲤鱼巡游过程中尾鳍的主要运动为侧向摆动, 尾鳍的形状周期性地循环外展和内收, 且尾鳍上叶叶尖的平均摆幅比尾鳍下叶叶尖的平均摆幅约大15.6%.      

两关节压力驱动柔性仿生机器鱼的设计与仿真

为研究设计一种柔软度高、环境适应性强的新型仿生机器鱼,模仿鲨鱼外形及鲔科鱼类的游动姿态,设计了一种采用液压柔性驱动结构的仿生机器鱼.针对单关节液压驱动柔性机器鱼存在其C型摆动姿态不符合鲔科鱼类摆动规律的问题,采用两关节液压柔性驱动模拟鱼类S型摆动,并根据液压柔性驱动器原理设计仿生鱼的内部结构.依据理论波动方程确定机器鱼的摆动幅值,借助数值模拟计算施加在柔性驱动器内部的压强载荷大小,并分析计算液压柔性驱动器的驱动效率.应用有限元分析软件模拟仿生鱼在流体中的自主游动过程,并将两关节机器鱼与单关节机器鱼的自主巡游过程进行对比仿真,获得两种机器鱼在流体中自主巡游时的运动姿态、游动速度及流场情况.结果表明,在相同的频率与尾鳍摆幅下,两关节柔性机器鱼的巡游平均速度为0.29 BL/s(BL为鱼体体长),高于单关节机器鱼巡游平均速度0.15 BL/s,且由速度矢量图可得出两关节仿生鱼的S型摆动姿态更接近真实鱼类摆动规律,并在运动过程中会产生一系列离散的反向卡门涡街,推进效率高.     

两关节压力驱动柔性仿生机器鱼的设计与仿真

为研究设计一种柔软度高、环境适应性强的新型仿生机器鱼, 模仿鲨鱼外形及鲔科鱼类的游动姿态, 设计了一种采用液压柔性驱动结构的仿生机器鱼. 针对单关节液压驱动柔性机器鱼存在其C型摆动姿态不符合鲔科鱼类摆动规律的问题, 采用两关节液压柔性驱动模拟鱼类S型摆动, 并根据液压柔性驱动器原理设计仿生鱼的内部结构. 依据理论波动方程确定机器鱼的摆动幅值, 借助数值模拟计算施加在柔性驱动器内部的压强载荷大小, 并分析计算液压柔性驱动器的驱动效率. 应用有限元分析软件模拟仿生鱼在流体中的自主游动过程, 并将两关节机器鱼与单关节机器鱼的自主巡游过程进行对比仿真, 获得两种机器鱼在流体中自主巡游时的运动姿态、游动速度及流场情况. 结果表明, 在相同的频率与尾鳍摆幅下, 两关节柔性机器鱼的巡游平均速度为0.29 BL/s (BL为鱼体体长), 高于单关节机器鱼巡游平均速度0.15 BL/s, 且由速度矢量图可得出两关节仿生鱼的S型摆动姿态更接近真实鱼类摆动规律, 并在运动过程中会产生一系列离散的反向卡门涡街, 推进效率高.      

鲫鱼尾鳍的黏弹性力学性质测量

鱼类在游动过程中表现出高效率和机动灵活的特点。鱼类通过拍动鱼鳍获得动力,并对游动过程进行控制,因此鱼鳍的力学性质影响着鱼类在游动中的表现。在以往的研究中,研究者多将鱼鳍假设为弹性材料。本文通过单轴拉伸后的松弛实验测量了鲫鱼尾鳍的黏弹性力学性质。在松弛实验中,拉力随着时间的增加而逐渐减小。在实验的前100s时间内,拉力衰减至最大拉力的75%。本文采用五参数的线性黏弹性模型对松弛实验的数据进行了拟合。基于拟合得到的模型,发现在快速起动及巡游过程中,鲫鱼尾鳍的黏弹性性质能够增加鲫鱼尾鳍的表观刚度,同时在巡游过程中,由于黏性引起的能量耗散非常小。     

基于PIV的拉萨裸裂尻摆尾压力场特征分析

为研究鱼体摆尾时压力场分布特征及其游泳动力的形成过程,本文以拉萨裸裂尻幼鱼为研究对象,利用粒子图像测速技术(PIV)获得幼鱼在自由游泳摆尾的压力分布规律。结果表明:幼鱼需要借助尾鳍的摆动来形成射流推动鱼体前进,沿着尾部轮廓的凹陷处流体压力为负值,正压则沿着尾部轮廓的凸起处分布;拉萨裸裂尻幼鱼的整个摆尾过程可分为"S"形(T=0～50ms)、"C"形(T=50～150ms)和"C"形回摆(T=150～400ms)三个阶段。"S"形阶段,正压区合力F随时间T先减小后增大,负压区合力F随时间T先增大后减小,正压区和负压区的合力F分布范围为0.88～1.03mN/BL、-0.86～-0.38mN/BL;"C"形阶段,正压区和负压区的合力F分别逐渐增大至3.45mN/BL、-1.62mN/BL;"C"形回摆阶段,正压区和负压区的合力F分别逐渐减小至-0.43mN/BL、1.63mN/BL。     

鱼类波状摆动推进的流体力学研究

本文对大雷诺数下鱼类波状摆动推进的流体力学研究成果从物理上作一简要回顾和评述,其中包括作者最近的系列工作.本文列举了六个题目:(1)鳗鲡模式和鲹科模式的推进性能;(2)月牙尾的推进性能;(3)最佳运动分析和尾鳍形状的讨论;(4)大幅摆动和尾涡层演化的非线性理论;(5)变速运动的推进性能;(6)能耗和阻抗。最后,提出了今后研究方向.     

鱼形机器人快速转向的运动特性研究

基于C形转向的仿生学研究和简化物理模型,建立了鱼形机器人快速转向的运动学方程,分析体干中线运动步态和鱼体质心的运动规律.计算了转动惯量、驱动力矩和阻力矩,建立C形转向的动力学模型.通过前摆转向的多步态仿真游动实验,解析鱼体柔性段弧长和相对弯曲度对质心轨迹、转动惯量、力矩、转动角位移和最大转动角速度的影响,提出了有效提高鱼形机器人快速转向性能的方法.     

肌电测量技术和鲫鱼C形起动过程中肌电信号测量初探

鱼在水中游动时,骨骼肌在中枢神经系统的控制下进行主动收缩,将力和位移传递给鳍和皮肤等执行器官,再通过执行器官与水进行力的相互作用和能量的交换后推动鱼体自身运动.为研究鱼在不同运动过程中肌肉活动的方式与强度,本文引进肌电图测量术(EMG).将表面绝缘的不锈钢探针植入鲫鱼的红肌内,肌电信号通过自主设计制作的肌电放大器放大,由计算机记录,用以表征鲫鱼在C形起动过程中转弯一侧的红肌活动的持续时间及强度.本文着重阐述了肌电测量技术,肌电实验的手术操作过程,并通过对肌电信号的测量结果进行定量分析,得到了鲫鱼C形起动过程中转弯侧红肌基本同时开始活动,其活动时间大致相同,以及躯体前端红肌活动最为剧烈的结论.     

The coupling influence of airflow and temperature on the wall-wetted fuel film distribution

The coupling influence of airflow and temperature on the two-dimensional distribution of the film resulted from fuel spray impinging on a horizontal flat wall was studied with experiments. The horizontal airflow direction was perpendicular to the vertical axis of the injection spray. The results show that, as air velocity increases, the film shape turns from a circle to an oblong. As wall temperature increases, the film area shrinks. Film thickness decreases as wall temperature or air velocity increases. The boiling point of the fuel is an important temperature to affect the film area and the film thickness. Film center moves more far away in the downstream direction as air velocity increases. For a certain air velocity, film center moves less far away as wall temperature increases.     

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自然界中鱼类尾鳍形状各种各样,但前体大部分都是扁平的椭圆形状. 该文研究了不同形状 的前体对尾鳍推进的影响. 尾鳍统一采用月牙形状; 前体采用了两种模型： A-鱼类扁平椭圆形状和B-潜艇水滴回转体形状. 通过以Strouhal数为控制参数的FLUENT数 值模拟,发现前体形状对于尾鳍的静止状态和运动状态有不同的作用,采用模型A的前体形 状更有利于长时间巡游,故鱼类采用模型A这种前体形状是自然界优胜劣汰的进化必然选择.     

Similarity solutions for granular avalanches of finite mass with variable bed friction

This paper continues a series of studies on the plane flow of a pile of cohesionless granular material down a rough inclined plane. Internal and basal friction laws are assumed to obey the Mohr-Coulomb yield criterion but in contrast to previous investigations the angle of friction at the bottom of the pile is considered to depend on the position or on the velocity or on both. Similarity, i.e. shape preserving solutions are constructed. The depth of the pile and the profile of the total minus the centre of mass velocity are determined analytically, but the total length and the position of the centre of mass are calculated numerically. If the basal friction angle is constant, the centre of mass moves with constant acceleration and the length of the pile extends monotonically. These motions change, when the angle of friction varies along the pile — the length of the pile may extend, contract or remain stationary and the centre of mass motion may decelerate or even reach steady state. Eight special cases are exhibited which demonstrate the influence of the friction law on the speed and spread of the pile.     

Co-current flow effects on a rising Taylor bubble

The effects of co-current flows on a rising Taylor bubble are systematically investigated by a front tracking method coupled with a finite difference scheme based on a projection approach. Both the upward (the co-current flows the same direction as the buoyancy force) and the downward (the co-current moves in the opposite direction of the buoyancy force) co-currents are examined. It is found that the upward co-current tends to elongate the bubble, while the downward co-current makes the bubble fatter and shorter. For large N_f (the inverse viscosity number), the upward co-current also elongates the skirted tail and makes the tail oscillate, while the downward co-current shortens the tail and even changes a dimpled bottom to a round shape. The upward co-current promotes the separation at the tail, while the downward co-current suppresses the separation. The terminal velocity of the Taylor bubble rising in a moving flow is a linear combination of the mean velocity (U_C) of the co-current and the terminal velocity (U₀) of the bubble rising in the stagnant liquid, and the constant is around 2 which agrees with the literature. The wake length is linearly proportional to the velocity ratio (U_C/U₀). The co-currents affect the distribution of the wall shear stresses near the bubble, but not the maximum.     

Stereo-PIV study of flow around a maneuvering fish

Stereoscopic PIV was used to measure the three-component velocity distribution around a live fish. At the same time, shadow images of the fish were captured by additional cameras in order to reconstruct the three-dimensional shape of the fish. The side jet shed against the swept tail during turning was evident in the results. The height of the jet was approximately equal to the height of the tail fin, and a vortex ring was observed around the side jet. Based on the circulation and diameter of the vortex ring, the impulse and time-averaged force in terms of the side jet were estimated.     

Heat and Mass Transfer in MHD Micropolar Flow Over a Vertical Moving Porous Plate in a Porous Medium

An analysis is presented for the problem of free convection with mass transfer flow for a micropolar fluid via a porous medium bounded by a semi-infinite vertical porous plate in the presence of a transverse magnetic field. The plate moves with constant velocity in the longitudinal direction, and the free stream velocity follows an exponentially small perturbation law. A uniform magnetic field acts perpendicularly to the porous surface in which absorbs the micropolar fluid with a suction velocity varying with time. Numerical results of velocity distribution of micropolar fluids are compared with the corresponding flow problems for a Newtonian fluid. Also, the results of the skin-friction coefficient, the couple stress coefficient, the rate of the heat and mass transfers at the wall are prepared with various values of fluid properties and flow conditions.     

The supersonic motion of a rigid body in an elastic medium with friction

The plane problem of supersonic steady motion of a body in an elastic medium is solved. Two possible cases of body motion are considered depending on its velocity. In the first case, the body moves at a velocity greater than the velocity of transverse waves but smaller than the velocity of longitudinal waves. In the second case, the body moves at a velocity greater than the velocity of longitudinal waves. An analytic solution of the problem under study is obtained and analyzed. It is shown that friction substantially influences the penetration process.     

冰激结构频率锁定振动的发生机理及简单分析方法

冰激结构频率锁定振动是冰区海洋工程结构的危险工况.对频率锁定振动过程的传统机理解释没有体现这一过程的全部物理特征,导致现有的分析方法无法准确分析这一问题.本文基于对现场测量结果的分析,提出了一种海冰韧性损伤-破碎过程与结构振动耦合导致频率锁定振动的机理.该机理认为,海冰在直立结构频率锁定振动过程中发生韧性损伤-破碎行为,海冰的韧性损伤-破碎与结构运动相位耦合,导致了频率锁定振动. 海冰对结构作用产生的载荷为锯齿形,作用过程可以分为加载和卸载两个阶段,其中加载阶段时间长度约为卸载阶段3倍以上.在加载阶段,结构从平衡位置先与海冰运动方向相反振动到反向最大振幅位置后回摆,然后与海冰同向运动到正向最大振幅位置,这段过程中海冰与结构接触部位内部产生裂纹并扩展但未发生主要的破碎,在此阶段海冰发生韧性损伤;在卸载阶段,结构从最大振幅位置向平衡位置回摆,结构与海冰运动方向相反,这一过程中应变速率的突然增大导致裂纹加速扩展并失稳开裂,此时带有韧性损伤的海冰发生破碎.基于这一新的机理解释,本文提出了一种冰激结构频率锁定振动幅值的简单分析方法.该方法认为海冰破碎长度是频率锁定振动的关键参数. 理想状态下,海冰破碎长度约为结构水线处振动幅值的2.2倍.当冰速接近海冰破碎长度与结构自振周期的比值时,结构会发生频率锁定振动.该方法对评估海洋工程结构频率锁定振动的发生概率及疲劳损伤具有指导意义.