基于虚功率原理的鲹科鱼类波状推进分析

自然界中的大多数鱼类通过波状摆动的方式实现推进,这是动态变形的鱼体和周围流体相互作用的结果,研究推进中流体对鱼体变形的响应不仅可以增强对波状推进的认识,还可以为流动控制提供依据.以鲹科模式推进的仿生二维模型为研究对象,通过数值计算获得鱼体波动产生的流场以及鱼体受到的流体力数据.基于虚功率原理,将鱼体受力分解为4部分,分别是鱼体边界变速运动的瞬时贡献、流场中流体旋转和应变速率相对大小的贡献、壁面剪切应力的类摩阻分量和壁面摩阻分量.结果表明,当鱼体波状摆动产生推力时,鱼体边界变速运动是主要的正推力来源,并且该项80%的推力贡献来源于20%的鱼尾部分的边界变速运动.鱼尾两侧边界层中的流体旋转和应变速率的相对大小和壁面摩阻对推力都是负贡献.对于低雷诺数的情况,流体旋转和应变速率的相对大小的负贡献低于壁面摩阻的负贡献,而在高雷诺数的情况下,流体旋转和应变速率的相对大小的负贡献强于壁面摩阻的负贡献.壁面类摩阻分量相对于其他3项总是较小的.结合标度律分析,在摆动推进的标度关系中,与雷诺数无关的推力部分是由边界的变速运动、流场中流体旋转和应变速率共同提供,且流体旋转和应变速率也贡献了摆动推力中与雷诺数...     

鱼类巡游中的活体肌肉力学性能预测

在综合游动力学的框架下采用整体模化分析方法,对比研究了鳗鲡鱼类和鲹科鱼类巡游中红肌（驱动鱼类巡游的骨骼肌）的力学性能异同. 该方法将观测总结的鱼体动态变形规律作为已知条件,首先确定鱼游中体外作用（包括流体动力响应和惯性力）,然后间接预测活体的体内作用（包括肌肉主动力和生物组织被动应力）. 研究结果显示,鳗鲡鱼类尾部的肌肉强度明显低于其躯干部分,而鲹科鱼类尾部和躯干部分肌肉强度相当,这与各自的体内作用主导机制相适应. 且体外作用的区别也导致了鲹科鱼类的能效更高. 同时发现,两种模式游动下都存在鱼体躯干内部从头至尾的能量传递,其肌肉输出净功沿鱼体轴向都呈“钟形” 分布. 总体上沿轴向各点的肌肉都输出正的净功.      

PREDICTION OF MECHANICAL PROPERTIES OF FISH MUSCLE IN VIVO DURING STEADY SWIMMING

在综合游动力学的框架下采用整体模化分析方法,对比研究了鳗鲡鱼类和鲹科鱼类巡游中红肌（驱动鱼类巡游的骨骼肌）的力学性能异同. 该方法将观测总结的鱼体动态变形规律作为已知条件,首先确定鱼游中体外作用（包括流体动力响应和惯性力）,然后间接预测活体的体内作用（包括肌肉主动力和生物组织被动应力）. 研究结果显示,鳗鲡鱼类尾部的肌肉强度明显低于其躯干部分,而鲹科鱼类尾部和躯干部分肌肉强度相当,这与各自的体内作用主导机制相适应. 且体外作用的区别也导致了鲹科鱼类的能效更高. 同时发现,两种模式游动下都存在鱼体躯干内部从头至尾的能量传递,其肌肉输出净功沿鱼体轴向都呈“钟形” 分布. 总体上沿轴向各点的肌肉都输出正的净功.     

基于浸入边界法的鱼体自主游动的数值模拟

对游动或飞行生物自主运动特性的深入研究,可促进仿生学的进一步发展。本文以"C"型游动鱼作为研究对象,建立了自主游动的柔性鱼模型。此模型较为真实地反映了鱼自主游动时鱼体内力(由鱼体肌肉收缩提供)、鱼体运动和外界流体之间的耦合作用。基于传统的反馈力方法和混合有限元浸入边界法对鱼的自主游动进行了数值模拟。分析了鱼自主游动启动阶段和巡游阶段流场特性及鱼体运动特征。模拟结果表明,受到鱼体自身组织结构和外界流场作用,鱼游动时通过呈"C"型和类"S"型的不断转换,以获取能量,实现鱼体自主游动。     

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.      

鱼类自主游动水动力学特性的数值模拟

通过对推力和阻力进行重新定义, 从根本上解决了鱼游研究中推力和阻力无法区分的难题.在此基础上, 利用自适应网格下的ghost-cell浸没边界方法, 模拟了鱼类以鲹科模式在黏性流体(309 \le Re \le 14\,581)和无黏流体 (相当于雷诺数无穷大情形)中的二维自主游动.结果表明: (1) Strouhal数随雷诺数增大而减小,当雷诺数趋向于无穷时, Strouhal数趋向于0.25; (2)在所有雷诺数情况下, 推力主要来源于压力分量; 当Re<3000时, 阻力的压力分量小于黏性力分量, 而当Re>3000, 二者的关系就会反过来; (3)推进效率随着雷诺数的增大而增大,当雷诺数趋向于无穷大时, 推进效率最高可以达到70%, 说明鲹科模式适用于较高雷诺数下的游动.      

关于湍流标度律的争鸣

研究湍流结构函数的标度律。实验或数值模拟得到的湍流结构函数的标度指数是奇异的。很多学者认为:这一实验事实否定Kolmogorov1941年(K41)提出的正常标度律,各向同性湍流惯性区的标度律是奇异的。近年来作者发表一系列文章,提出不同的观点:由于有限雷诺数效应,有限雷诺数湍流的标度指数不等于真正的惯性区标度指数,湍流结构函数的标度指数的实验数据并不否定K41正常标度律,各向同性湍流惯性区的标度律可能是正常的。惯性区奇异标度律和正常标度律对应的湍流物理本质是完全不同的,因而研究解决这个争论具有重要的意义。      

壁湍流相干结构尺度及边界层内的SL标度律

利用高分辨率、高帧率PIV系统对湍流边界层中相干结构的多种空间尺度和边界层内SL 标度律在不同尺度下的具体表达形式进行了实验研究. 实验在两个动量损失厚度雷诺数 (Re_{\theta}=628.5和Re_{\theta}=1032.9)下测量平板湍流边界层中缓冲 层、对数区和外区的二维瞬时速度场. 应用 小波分析以及传统的统计学方法，在垂直于平板和平行于平板的平面内考察平板湍流边界层 中存在的相干结构的流向和展向尺度，并与已知的相干结构尺度实验结果进行了对比分析. 利用在动量损失厚度雷诺数628.5下测得的数据，对多种脉动结构(脉动速度结构等) 的空间关系及其标度律进行了研究. 第2项工作直接利用湍流边界层空间速度分布，对多种 流场尺度结构内部的She-Leveque(简称SL)标度律及自相似律进行了验证. 结果表明，各 单一流场尺度结构内部，流向脉动速度{\pmb u}'、法向脉动速度{\pmb v}'及 脉动涡分量\d {\pmb v}'/\d {\pmb x}的统计结构量均存在明显的标度律，标度 指数的形式与自相似律和SL标度律均非常吻合，只是常数随流场尺度的不同而不同， 且呈现一定的规律性. 但对于结构量的五阶矩随距离l的研究表明，自相似律和SL 标度律成立的范围并不完全一致，同时标度律成立的范围大小与流场尺度有明显关系.     

可解尺度各向同性湍流的标度律

针对大涡模拟, 首先利用 EDQNM 能谱和传输谱理论, 在3种不同的过滤器下分别计算可解尺度各向同性湍流的二阶、三阶结构函数标度律, 并通过速度差扭率验证了可解尺度湍流的 ESS 理论. 研究了该可解尺度湍流标度律及速度差扭率与多个因素的关系, 这些因素包括: 两点距离与过滤尺度的比、大涡模拟雷诺数和过滤器类型. 结果显示, 当两点距离位于过滤尺度量级时或大涡模拟雷诺数较小时, 可解尺度标度律与未过滤流场的标度律相差很大进而必须加以修正, 而且可解尺度流场也不再总是满足 ESS 理论进而对应的速度差扭率也需要修正. 然后通过3个例子介绍了这些结果在大涡模拟亚格子模型中的应用.      

????????????β?????????????о?

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.     

Hydrodynamics of a tandem fish school with asynchronous undulation of individuals

We perform numerical simulations using immersed boundary method for flow over a single and two fish in tandem performing traveling wave like motion for a range of Strouhal numbers. We investigate the hydrodynamic performance of single- and tandem-fish configurations using unsteady profiles of lateral side-force and drag coefficients, their time-averaged values, and wake behind these bodies. We present the spectra of hydrodynamic forces and find that the nature of these forces for a single fish resembles to those of stationary/oscillating bluff bodies and oscillating airfoils. For tandem cases, we vary the phase speed of undulatory motion of the rear fish while keeping the free-stream velocity constant. We show that hydrodynamic forces of the upstream and rear fish contain harmonics which are produced by nonlinear interaction of the oscillation frequencies of both fish. We find that the wake and time-averaged drag of the upstream fish remain almost independent of the undulating frequency of the rear fish at a certain Strouhal number. We also relate this observation with the absence of oscillation frequency of the rear fish in the Fourier spectra of hydrodynamic forces of the upstream fish. For the complete range of parameters, it is inferred that swimming in a tandem configuration seems more beneficial for the upstream fish. It happens due to wake-splitting effect of the rear fish that causes an enhancement of pressure in its wake. For the rear fish, it gains an advantage of drafting under certain conditions and its performance deteriorates at Strouhal numbers greater than 0.40.     

CMP流场的数值模拟及离心力影响分析

化学机械抛光(chemical mechanical polishing,CMP)是一项融合化学分解和机械力学的工艺, 其中包含了流体动力润滑的作用.在已有润滑方程的基础上, 提出并分析了带有离心力项的润滑方程.利用Chebyshev加速超松弛技术对有离心力项的润滑方程进行求解,得到离心力对抛光液压力分布的影响. 数值模拟结果表明,压力分布与不带离心力项的润滑方程得出的明显不同;无量纲载荷和转矩随中心膜厚、转角、倾角、抛光垫旋转角速度等参数的变化趋势相同,但数值相差较大, 抛光垫旋转角速度越大差别越大.      

The instability mechanism of single and multilayer Newtonian and viscoelastic flows down an inclined plane

The instability mechanism of single and multilayer flow of Newtonian and viscoelastic fluids down an inclined plane has been examined based on a rigorous energy analysis as well as careful examination of the eigenfunctions. These analyses demonstrate that the free surface instability in single and multilayer flows in the limit of longwave disturbances (i.e., the most dangerous disturbances) arise due to the perturbation shear stresses at the free surface. Specifically, for viscoelastic flows, the elastic forces are destabilizing and the main driving force for the instability is the coupling between the base flow and the perturbation velocity and stresses and their gradient at the free surface. For Newtonian flows at finite Re, the driving force for the interfacial instability in the limit of longwaves depends on the placement of the less viscous fluid. If the less viscous fluid is adjacent to the solid surface then the main driving force for the instability is interfacial friction, otherwise the bulk contribution of Reynolds stresses drives the instability. For viscoelastic fluids in the limit of vanishingly small Re, the driving force for the instability is the coupling of the base flow and perturbation velocity and stresses and their gradients across the interface. In the limit of shortwaves the interfacial stability mechanism of flow down inclined plane is the same as plane Poiseuille flows (Ganpule and Khomami 1998, 1999a, b). Received: 20 October 2000/Accepted: 11 January 2001     

柔性长鳍波动推进动力学分析

以基于柔性长鳍波动推进的仿生水下机器人试验模型为背景,研究了“尼罗河魔鬼”柔性长鳍的波动推进动力学原理．首先基于鳍条正弦摆动的假设,建立了鳍面波动曲面模型,然后根据抗体理论的基本思想及修正后的流体假设分析了柔性长鳍波动的推力产生机理,并获得其动力学模型,最后通过仿真手段,得到了试验模型的仿生柔性长鳍鳍面波动引起的流体动力及力矩公式,为仿生水下机器人的动力学分析和控制系统设计提供了理论依据．     

Motion of a cylinder in a viscous electroconductive medium with a magnetic field

The method of force sources is used to consider the planar problem of the motion of a circular cylinder in a viscous electroconductive medium with a magnetic field. The conventional and magnetic Reynolds numbers are assumed to be small. Expressions are obtained for the hydrodynamic reaction forces of the medium, acting on the moving cylinder. It is shown that as a result of the flow anisotropy in the medium, caused by the magnetic field, in addition to the resistance forces on bodies moving at an angle to the field, there are deflecting forces perpendicular to the velocity vector. The velocity field disturbances at great distances from the moving cylinder are determined.The problems of viscous electroconductive flow about solid bodies in the presence of a magnetic field constitute one of the divisions of magnetohydrodynamics. Motion of an electroconductive medium in a magnetic field gives rise to inductive electromagnetic fields and currents which interact with the velocity and pressure hydrodynamic fields in the medium [1, 2]. Under conditions of sufficiently strong interaction, the number of independent flow similarity parameters in MHD is considerably greater than in conventional hydrodynamics. This circumstance complicates the theoretical analysis of MHD flow about bodies, and therefore we must limit ourselves to consideration of individual particular flow cases.Here we consider the linear problem of the motion of an infinite circular cylinder in a viscous incompressible medium with finite electroconductivity located in a uniform magnetic field.There are many studies devoted to the flow of a viscous electroconductive medium with a magnetic field about solid bodies (see, for example, [3–5]). Because of this, some of the results obtained here include previously known results, which will be indicated below. In contrast to the cited studies, the examination is made by the method of force sources, suggested in [6]. This method permits obtaining integral equations for the distribution of the forces acting on the surface of the moving body. Their solution is obtained for small Reynolds and Hartmann numbers. Then the nature of the velocity disturbances at great distances from the body are determined. These results are compared with conventional viscous flow about a cylinder in the Oseen approximation.     

Numerical simulation of vortex-induced drag of elastic swimmer models

We present numerical simulations of simplified models for swimming organisms or robots, using chordwise flexible elastic plates. We focus on the tip vortices originating from three-dimensional effects due to the finite span of the plate. These effects play an important role when predicting the swimmer's cruising velocity, since they contribute significantly to the drag force. First we simulate swimmers with rectangular plates of different aspect ratios and compare the results with a recent experimental study. Then we consider plates with expanding and contracting shapes. We find the cruising velocity of the contracting swimmer to be higher than the rectangular one, which in turn is higher than the expanding one. We provide some evidence that this result is due to the tip vortices interacting differently with the swimmer.