幼鱼体长差异性对群游推进效率的影响研究

为探究上下游鱼之间的体长差异性对鱼群群游效率的影响,本文基于Fluent动网格技术,对幼鱼鱼群稳定游动状态下的鱼体周身涡量和推进效率等进行了数值模拟分析。计算结果表明,群游幼鱼的推进效率相比单尾鱼最高可提升5%,上游鱼体长大于下游鱼体长(ΔL>0 cm)时,随体长差异的增大,鱼群尾流结构(d/D)增大,整体推进效率减小;上游鱼体长小于下游鱼体长(ΔL<0 cm)时,随体长差异的增大,鱼群尾流结构(d/D)和整体推进效率减小。当上下游幼鱼的体长差ΔL=0.5 cm时,三角形组合的鱼群整体推力系数和功率系数最高分别可达5.11×10^-3和1.71×10^-3,该组合下的整体推进效率可达17.2%。     

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。     

柱塞泵多目标优化设计及CFD仿真分析

液压泵噪声是液压系统的主要噪声源,针对轴向柱塞泵的流致振动噪声,提出一种改善泵配流特性的设计方案。首先,根据柱塞泵的工作原理对柱塞腔压力特性和泵出口流量特性准确建模并求解。通过分析压力冲击和流量脉动对错配角（φ₀）的响应,得φ₀=4°为佳。利用一种多目标遗传算法（NSGA-Ⅱ）,以减小压力超调量和流量脉动率为目标,对三角槽结构进行了优化;并获得该多目标优化问题的Pareto最优解集,通过对最优解集的分析知,深度角θ₁=16°且宽度角θ₂=85°时较为理想。最后,为了验证模型的正确性,建立流体域计算流体动力学（CFD）模型,对比两种模型计算结果发现吻合较好,能够相互验证。利用CFD分析结果可视化的特点,从柱塞泵流场的角度,进一步分析了泵压力冲击以及流量脉动产生的原因。     

鳗鲡模式游动的鱼体变形曲率波的传播特性研究

将鳗鲡模式游动的七鳃鳗简化成材料性质均匀的变截面黏 弹性梁,通过数值方法求解鱼体在主动弯矩波(作为激励的驱动波)的驱动下匀速游动时身体变形曲率波的传播特性. 结果表明,当主动弯矩 的驱动频率高于鱼体结构基频时,可以观察到曲率波相对于驱动波存在相位滞后,且越靠近尾部滞后现象越明显,这意味着曲率波的波速 小于驱动波的波速,也间接地验证了前人的实验结果. 通过参数研究发现,鱼体变形曲率波与驱动波的波速比与表征流体黏性作用的雷诺数无关,而与表征驱动波和鱼体材料属性的 无量纲激励频率、激励波长及鱼体黏性系数有关. 对于鳗鲡模式游动的鱼类,曲率波与驱动波的波速比随着无量纲激励频率和波长的增大而降低,随着鱼体黏性系数的增大而增大. 进一步研究发现,通过小扰动分析得到的组合相似性参数$\varPi$可以统一描述波速比与激励参数、材料参数之间的关系.      

近源堆沉积土石体细观结构与抗剪强度指标关联分析

细观结构是认知土石体力学行为本质的关键科学问题。本文通过引入物理学和数学等方法,借助颗粒物质力学理论,从几何排列与接触力的空间分布来定量刻画水平固结与山前坡地堆积两种典型环境下土石体的细观结构特征,并建立其与抗剪强度指标的关联。研究表明,（1）两种环境的土石体在细观结构上存在较大差异。在几何排列上,水平固结环境下的土石体具有长程无序和短程有序的特点,坡地堆沉积环境下的土石体表现出了无序的无定形结构;在接触力与单位接触向量的空间分布上,两者较为相似,绝大多数接触力以小于均值接触力的形式存在,其概率密度曲线P（f）呈幂函数衰减;90%以上接触方位角集中在40°~160°和220°~340°范围内。（2）基于径向分布函数、接触力概率密度和单位接触向量分别定义细观结构的特征量K_a,K_s和K_o,发现三个特征量的增大对内摩擦角呈线性促进作用,对黏聚力呈非线性削弱作用。     

鳗鲡模式游动的鱼体变形曲率波的传播特性研究

将鳗鲡模式游动的七鳃鳗简化成材料性质均匀的变截面黏弹性梁,通过数值方法求解鱼体在主动弯矩波(作为激励的驱动波)的驱动下匀速游动时身体变形曲率波的传播特性.结果表明,当主动弯矩的驱动频率高于鱼体结构基频时,可以观察到曲率波相对于驱动波存在相位滞后,且越靠近尾部滞后现象越明显,这意味着曲率波的波速小于驱动波的波速,也间接地验证了前人的实验结果.通过参数研究发现,鱼体变形曲率波与驱动波的波速比与表征流体黏性作用的雷诺数无关,而与表征驱动波和鱼体材料属性的无量纲激励频率、激励波长及鱼体黏性系数有关.对于鳗鲡模式游动的鱼类,曲率波与驱动波的波速比随着无量纲激励频率和波长的增大而降低,随着鱼体黏性系数的增大而增大.进一步研究发现,通过小扰动分析得到的组合相似性参数Π可以统一描述波速比与激励参数、材料参数之间的关系.     

考虑单元劈裂的流-固耦合连续-非连续方法及定向水力压裂模拟

为了更真实地模拟水力压裂过程中的岩石变形、裂缝扩展及流体流动,在自主开发的拉格朗日元与离散元耦合的连续-非连续方法的基础上,发展了一种流-固耦合方法。在该方法中,裂缝可沿四边形单元对角线和单元边界扩展,流体流动满足立方定律。通过与单一裂缝非稳态渗流模型及KGD模型的理论解进行对比,验证了该方法的正确性。由定向射孔水力压裂的模拟结果可以发现,(1)距离射孔越远,流体压力越小;随着时间的增加,裂缝中流体压力降低。(2)随着射孔角度的增加,裂缝起裂和扩展过程中的流体压力及转向距离增加;随着x方向水平应力的增加,裂缝起裂和扩展过程中的流体压力增加;两个方向水平应力之差越大,裂缝转向距离越小。(3)随着时间的增加,裂缝区段数目的增速变慢,这与裂缝体积增加变快有关。     

双柱状颗粒在线性剪切流场中的动力学分析

为研究柱状颗粒在线性剪切流场中的运动状态和受力情况,本文以颗粒长径比为2,颗粒之间的初始距离ΔS_Py=4D为例,基于直接力浸入边界法数值模拟了双柱状颗粒在三维线性剪切流场中的运动过程。根据模拟结果分析了柱状颗粒周围流场参数分布,在考虑壁面对颗粒的影响和颗粒之间相互影响的条件下,研究了颗粒的受力和运动的变化,探索了流体曳力导致柱状颗粒迁移和转动的规律。研究结果表明,双柱状颗粒在线性剪切流场中易向速度大的流体区域运动;前后两颗粒运动状态和轨迹不同,颗粒之间距离较近时,曳力会产生较大的波动;只有当颗粒在壁面附近时,滞后颗粒才能追上领先颗粒,两颗粒发生牵引、翻滚和分离过程。     

复杂凸多面体随机紧密堆积组构性能的数值研究:形状参数的影响

颗粒材料的宏观物理力学性能依赖于颗粒堆积体系的细观组构性能,研究颗粒堆积体系的组构性能有重要意义。然而,当前对颗粒堆积体系组构性能的研究集中于球、椭球和正则多面体等规则几何体,还未有对复杂凸多面体颗粒堆积体系组构性能的系统研究。本文基于旋转椭球面黄金螺旋网格构造了一组复杂凸多面体颗粒模型(Poly_κ-ngs),然后基于松弛算法获得了Poly_κ-ngs多面体的随机紧密堆积结构,最后研究了几何形状参数对Poly_κ-ngs多面体随机紧密堆积体系组构性能的影响。结果表明,长径比κ和顶点数量n_gs均对堆积体系的组构性能有影响,κ是主要影响因素。Poly_κ-ngs多面体随机紧密堆积结构中颗粒的位置分布均匀,长径比κ越接近1,顶点数量n_gs越大时,堆积结构表现出更强的位置长程有序性;颗粒方向分布不均匀,长径比κ越远离1,不均匀程度越高;最高堆积分数随长径比κ的增大先增大后减小,在κ=1时达到峰值;配位数分布服从高斯分布,平均配位数随形状参数的变化和堆积分数不同;面-面接触数量随长径比κ的增大先增大后减小,和堆积分数变化规律一致。本研究为复杂凸多面体颗粒的随机紧密堆积提供了数值模拟方案,得出的结论对含有凸多面体颗粒材料的设计和性能优化具有参考意义。     

海南岛清澜-八门湾潟湖潮汐水动力的模拟研究

基于DELFT3D模型研究了清澜-八门湾潟湖的水位分布和潮汐波内部结构,以及由于人类开垦引起的海岸线变化（以1962年、1985年和2008年为例）对潟湖水动力特性的影响。结果表明,清澜-八门湾潟湖潮汐是由多种分潮耦合而成的复杂驻波,其中K₁,O₁,M₂,S₂和M₄分潮的影响最大。由湾外向湾内传输,由于湾内红树林和浅滩引起的底部损耗增加,M₂,S₂,K₁和O₁分潮幅值减弱;M₄分潮幅值增强,表现出明显的浅水增幅效应;M₂和S₂分潮相位在文教河和文昌江领域表现出明显的干湿效应。不同年代海岸线的研究表明,1985年和2008年间,人类复垦导致潟湖及其潮汐汊道附近的红树林和滩涂区域严重破坏,海岸线缩减,引发了水位降低、纳潮量减少和潟湖潮汐汊道底摩擦弱化,从而削弱了干湿、潮呛和浅水效应。后果提示持续的人类复垦活动将会引发清澜-八门湾潮汐水动力环境的进一步恶化,可能导致未来发生更大的自然灾害。     

Capillary Pressure Curve for Liquid Menisci in a Cubic Assembly of Spherical Particles Below Irreducible Saturation

The capillary pressure–saturation relationship, P _c(S _w), is an essential element in modeling two-phase flow in porous media (PM). In most practical cases of interest, this relationship, for a given PM, is obtained experimentally, due to the irregular shape of the void space. We present the P _c(S _w) curve obtained by basic considerations, albeit for a particular class of regular PM. We analyze the characteristics of the various segments of the capillary pressure curve. The main features are the behavior of the P _c(S _w) curve as the wetting-fluid saturation approaches zero, and as this saturation is increased beyond a certain critical value. We show that under certain conditions (contact angle, distance between spheres, and saturation), the value of the capillary pressure may change sign.     

Two-Phase Flow Properties Prediction from Small-Scale Data Using Pore-Network Modeling

The objective of this work is to evaluate the prediction accuracy of network modeling to calculate transport properties of porous media based on the interpretation of mercury invasion capillary pressure curves only. A pore-scale modeling approach is used to model the multi-phase flow and calculate gas/oil relative permeability curves. The characteristics of the 3-D pore-network are defined with the requirement that the network model satisfactorily reproduces the capillary pressure curve (P_c curve), the porosity and the permeability. A sensitivity study on the effect of the input parameters on the prediction of capillary pressure and gas/oil relative permeability curves is presented. The simulations show that different input parameters can lead to similarly good reproductions of the experimental P_c, although the predicted relative permeabilities K_r are somewhat widespread. This means that the information derived from a mercury invasion P_c curve is not sufficient to characterize transport properties of a porous medium. The simulations indicate that more quantitative information on the wall roughness and the node/bond aspect ratio would be necessary to better constrain the problem. There is also evidence that in narrow pore size distributions pore body volume and pore throat radius are correlated while in broad pore size distributions they would be uncorrelated.     

幂律型浆液扩散压力环向分布模型

为研究幂律型浆液注浆时注浆压力的变化情况,考虑盾尾断面新注入浆液与已注入浆液间阻碍作用,假设壁后注浆时盾尾形成三维环形空隙,提出了幂律型浆液扩散压力环向分布模型,并利用流体力学理论推导了幂律型浆液扩散压力环向分布式,分析了公式适用范围以及各参数的实际意义。与实际工程数据对比,验证了模型和计算式的正确性。结果表明,计算式可以反映注浆时环向分布各个位置压力值的大小;当公式中稠度系数n=1时,该式即为牛顿流体计算式,环向压力扩散模型同样适用,且幂律型流体环向扩散压力小于牛顿流体;受浆液自重影响,注浆孔注浆时向上表现为减压,向下表现为加压;压力环向分布断面呈现出上窄下宽的不规则环形;同一注浆孔幂律型浆液水灰比越大,浆液扩散压力越小。     

Thrust production by a mechanical swimming lamprey

To develop a comprehensive model of lamprey locomotion, we use a robotic lamprey to investigate the formation of the wake structure, the shedding vorticity from the body, and the relationship between thrust production and pressure on the surface of the robot. The robot mimics the motion of living lamprey in steady swimming by using a programmable microcomputer to actuate 13 servomotors that produce a traveling wave along the length of the lamprey body. The amplitude of the phase-averaged surface pressure distribution along the centerline of the robot increases toward the tail, which is consistent with previous momentum balance experiments. This indicates that thrust is produced mainly at the tail. The phase relationship between the pressure signal and the vortex shedding from the tail is also examined, showing a clear connection between the location of vortex structures and the fluctuations of the pressure signal.     

????????????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.     

Time resolved measurements of the flow generated by suction feeding fish

The majority of aquatic vertebrates are suction feeders: by rapidly expanding the mouth cavity they generate a fluid flow outside of their head in order to draw prey into their mouth. In addition to the biological relevance, the generated flow field is interesting fluid mechanically as it incorporates high velocities, is localized in front of the mouth, and is unsteady, typically lasting between 10 and 50 ms. Using manometry and high-speed particle image velocimetry, this is the first study to quantify pressure within and outside the mouth of a feeding fish while simultaneously measuring the velocity field outside the mouth. Measurements with a high temporal (2 ms) and spatial (<1 mm) resolution were made for several feeding events of a single largemouth bass (Micropterus salmoides). General properties of the flow were evaluated, including the transient velocity field, its relationship to pressure within the mouth and pressure at the prey. We find that throughout the feeding event a relationship exists for the magnitude of fluid speed as a function of distance from the predator mouth that is based on scaling the velocity field according to the size of the mouth opening and the magnitude of fluid speed at the mouth. The velocity field is concentrated within an area extending approximately one mouth diameter from the fish and the generated pressure field is even more local to the mouth aperture. Although peak suction pressures measured inside the mouth were slightly larger than those that were predicted using the equations of motion, we find that these equations give a very accurate prediction of the timing of peak pressure, so long as the unsteady nature of the flow is included.     

Fish locomotion: kinematics and hydrodynamics of flexible foil-like fins

The fins of fishes are remarkable propulsive devices that appear at the origin of fishes about 500 million years ago and have been a key feature of fish evolutionary diversification. Most fish species possess both median (midline) dorsal, anal, and caudal fins as well as paired pectoral and pelvic fins. Fish fins are supported by jointed skeletal elements, fin rays, that in turn support a thin collagenous membrane. Muscles at the base of the fin attach to and actuate each fin ray, and fish fins thus generate their own hydrodynamic wake during locomotion, in addition to fluid motion induced by undulation of the body. In bony fishes, the jointed fin rays can be actively deformed and the fin surface can thus actively resist hydrodynamic loading. Fish fins are highly flexible, exhibit considerable deformation during locomotion, and can interact hydrodynamically during both propulsion and maneuvering. For example, the dorsal and anal fins shed a vortex wake that greatly modifies the flow environment experienced by the tail fin. New experimental kinematic and hydrodynamic data are presented for pectoral fin function in bluegill sunfish. The highly flexible sunfish pectoral fin moves in a complex manner with two leading edges, a spanwise wave of bending, and substantial changes in area through the fin beat cycle. Data from scanning particle image velocimetry (PIV) and time-resolved stereo PIV show that the pectoral fin generates thrust throughout the fin beat cycle, and that there is no time of net drag. Continuous thrust production is due to fin flexibility which enables some part of the fin to generate thrust at all times and to smooth out oscillations that might arise at the transition from outstroke to instroke during the movement cycle. Computational fluid dynamic analyses of sunfish pectoral fin function corroborate this conclusion. Future research on fish fin function will benefit considerably from close integration with studies of robotic model fins.     

Effect of pressure-dependent slip on flow curve multiplicity

Various microstructural pictures for slip at polymer/solid interfaces lead to relations which have a region where multiple values of slip velocity are predicted for the same shear stress. This leads to the expectation of multivalued flow curves, which has been verified in specific cases by numerous researchers. We study the effect of pressure dependence on flow curve multiplicity using a simple multivalued slip relation to model the phenomena of hysteresis and spurt flow in polymer extrusion. A continuation technique is used to trace out the boundaries of the region of flow curve multiplicity as pressure drop and die length to diameter (L/D) ratio are changed. Results for Newtonian, shear thinning and viscoelastic constitutive equations show that, despite the multivalued nature of the slip model, multiplicity (and thus hysteresis) is absent at high L/D.  For the sake of completeness, we also carry out time-dependent simulations at constant piston speed taking fluid compressibility into account. These simulations show that oscillations in the pressure drop and exit volumetric flow rate result only if the system is operated in the multiplicity region of the steady state flow curve, in agreement with the results of similar simulations by researchers using various multivalued slip models without pressure dependence. The results demonstrate that a multivalued slip model does not guarantee multiplicity in the flow curve for the constant pressure drop operation, nor oscillations for constant piston speed operation. Received: 18 August 1997 Accepted: 30 March 1998