基于GIS与确定性系数分析方法的汶川地震滑坡易发性评价

鱼类经常采用垂直流向的摆动进行游动, 这种摆动可以用行进波来表示. 应用浸入边界方法模拟了低雷诺数条件下水翼NACA65-010在水中摆动时的流场,并研究了雷诺数对水生动物推进效率的影响. 结果表明：随着雷诺数的增大,推力系数和推进效率增大,而功率系数减小;在Re<20时,推力系数,推进效率和功率系数的变化尤为剧烈. 随雷诺数增加,由于水翼摆动诱导的流场变化也更加复杂,水翼后缘处的涡量场强度逐渐增强. 摆动诱导反卡门涡街产生推力.     

摆动方式对水翼输入功率的影响

利用投影浸入边界法,研究了不同主动摆动方式对水翼输入功率的影响。计算模型采用二维NACA0012翼型,雷诺数Re=800。最大摆角θ0=75°,摆动频率f的变化范围为0.1Hz~0.2Hz,非正弦摆动参数β的变化范围为1~3。首先,研究摆动频率f和非正弦摆动参数β对平均输入功率的影响,研究发现,平均输入功率随着f和β值的增加而增加,但当f0.16 Hz,β2.5时,平均输入功率急剧增加。其次,在摆动频率f=0.16 Hz时,研究不同非正弦摆动参数β下力矩系数、输入功率系数以及升阻力系数随时间的变化规律,研究发现,随着β值的增加,峰值输入功率也逐渐增加,而且β值影响峰值输入功率出现的位置。最后,研究不同β值下,变化的尾流发展对输入功率的影响,认为水翼上下表面产生的后缘涡与升阻力有关系,水翼上表面的负涡对水翼摆动产生阻力,而下表面的负涡对水翼摆动产生升力,从而对水翼摆动的输入功率产生影响。     

自主推进俯仰震荡翼型的数值模拟研究

运用自适应多重网格法和内置边条法研究了俯仰震荡翼型的运动.通过研究翼型的原地摆动与自由游动,提出了一种确定自主推进俯仰震荡翼型推力的方法,得到了推力系数、功率系数和推进效率与Strouhal数的关系.与以往研究不同的是,我们还得到了Strouhal数随雷诺数的变化规律.此外,从自由游动翼型诱导出的涡量场中可以清楚的观察到旋涡的合并,这与实验研究非常吻合.     

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.      

复合材料水翼水动力与结构强度特性数值研究

针对复合材料水翼存在的流固耦合求解问题,结合其自身特有属性,对复合材料水翼结构变形特性进行了数值仿真计算研究.研究建立了复合材料水翼流固耦合数值计算模型,并将数值计算结果与Zarruk等的实验结果进行对比,验证模型的正确性,得出复合材料水翼尖端扭转角随雷诺数的增加而增加的研究结论.基于数值计算模型,系统地研究了不同铺层角对复合材料水翼水动力特性及强度特性的影响,结果表明:不同铺层角复合材料水翼的尖端扭转角,随铺层角的增大而减小,而其尖端位移量随铺层角的增大先减小后增大.为了削弱工程常数的影响对复合材料水翼变形的影响,研究提出了无量纲扭转角和无量纲位移量,进一步探究复合材料水翼结构的弯扭耦合作用对其变形特性的影响.最后利用蔡-吴失效准则进行复合材料水翼强度特性的判断和分析,结果表明:不同铺层角复合材料水翼的蔡-吴系数,随铺层角的增大呈现先减小后增大的趋势,其中0°铺层时的复合材料水翼蔡-吴系数最小,50°铺层时的复合材料水翼的蔡-吴系数最大.     

复合材料水翼水动力与结构强度特性数值研究

针对复合材料水翼存在的流固耦合求解问题,结合其自身特有属性,对复合材料水翼结构变形特性进行了数值仿真计算研究.研究建立了复合材料水翼流固耦合数值计算模型,并将数值计算结果与Zarruk等的实验结果进行对比,验证模型的正确性,得出复合材料水翼尖端扭转角随雷诺数的增加而增加的研究结论.基于数值计算模型,系统地研究了不同铺层角对复合材料水翼水动力特性及强度特性的影响,结果表明:不同铺层角复合材料水翼的尖端扭转角,随铺层角的增大而减小,而其尖端位移量随铺层角的增大先减小后增大.为了削弱工程常数的影响对复合材料水翼变形的影响,研究提出了无量纲扭转角和无量纲位移量,进一步探究复合材料水翼结构的弯扭耦合作用对其变形特性的影响.最后利用蔡$\!$-$\!$-$\!$吴失效准则进行复合材料水翼强度特性的判断和分析,结果表明:不同铺层角复合材料水翼的蔡$\!$-$\!$-$\!$吴系数,随铺层角的增大呈现先减小后增大的趋势,其中0$^\circ$铺层时的复合材料水翼蔡$\!$-$\!$-$\!$吴系数最小,50$^\circ$铺层时的复合材料水翼的蔡$\!$-$\!$-$\!$吴系数最大.      

绕振荡水翼流动及其转捩特性的数值计算研究简

通过对比标准k-ω SST 湍流模型和基于标准k-ω SST 湍流模型修正的γ-Re_θ 转捩湍流模型对绕振荡NACA66 水翼流动的数值计算结果与实验结果,对水翼振荡过程的水动力特性和流场结构变化进行了分析研究. 结果表明：与标准k-ω SST 湍流模型的数值计算结果相比,基于标准k-ω SST 湍流模型修正的γ-Re_θ 转捩湍流模型能有效预测绕振荡翼型流场结构和水动力特性,捕捉流场边界层发生的流动分离和转捩现象;绕振荡水翼的流动过程可分为5 个特征阶段,当来流攻角较小时,在水翼前缘发生层流向湍流的转捩现象,水翼动力特征曲线出现变化拐点;随着来流攻角的增大,顺时针尾缘涡逐渐形成并向水翼前缘发展;当攻角较大时,前缘涡分离导致动力失速,水翼的动力特征曲线出现大幅波动;水翼处于顺时针向下旋转阶段,绕水翼的流动状态逐渐由湍流过渡为层流.     

两串列扑翼的相位差对平均推力影响机理的实验研究

在一个低雷诺数的循环水洞中,实验研究了前后翅翼之间的相位差对两串列扑翼平均推力的影响.利用一个三分量的Kistler 压力传感器来测量扑翼的瞬时力;利用一个数字粒子测速仪系统(TSI DPIV) 来测量扑翼的前缘涡以及其周围的流场. 当相位差从0° 增加到360°,前翅的平均推力随着相位差正弦变化;前翅平均推力的增加是由于后翅的前缘涡和滞止区域增加了前翅的有效攻角. 后翅平均推力曲线有一个明显的V 字形低谷.低谷处较小的平均推力是由于前翅的脱落涡抑制了后翅前缘涡的形成并且减小了其有效攻角.当间距为0.5倍弦长相位差约为290°时,前后翅翼平均推力系数的合值能达到最大值0.667,明显大于两倍的单翼平均推力系数(2×0.255).      

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%。     

Thrust generation and wake structure of wiggling hydrofoil

Marine animals and micro-machines often use wiggling motion to generate thrust. The wiggling motion can be modeled by a progressive wave where its wavelength describes the flexibility of wiggling animals. In the present study, an immersed boundary method is used to simulate the flows around the wiggling hydrofoil NACA 65-010 at low Reynolds numbers. One can find from the numerical simulations that the thrust generation is largely determined by the wavelength. The thrust coefficients decrease with the increasing wavelength while the propulsive efficiency reaches a maximum at a certain wavelength due to the viscous effects. The thrust generation is associated with two different flow patterns in the wake： the well-known reversed Karman vortex streets and the vortex dipoles. Both are jet-type flows where the thrust coefficients associated with the reversed Karman vortex streets are larger than the ones associated with the vortex diploes.     

High fidelity numerical simulation of airfoil thickness and kinematics effects on flapping airfoil propulsion

High-fidelity numerical simulations with the spectral difference (SD) method are carried out to investigate the unsteady flow over a series of oscillating NACA 4-digit airfoils. Airfoil thickness and kinematics effects on the flapping airfoil propulsion are highlighted. It is confirmed that the aerodynamic performance of airfoils with different thickness can be very different under the same kinematics. Distinct evolutionary patterns of vortical structures are analyzed to unveil the underlying flow physics behind the diverse flow phenomena associated with different airfoil thickness and kinematics and reveal the synthetic effects of airfoil thickness and kinematics on the propulsive performance. Thickness effects at various reduced frequencies and Strouhal numbers for the same chord length based Reynolds number (=1200) are then discussed in detail. It is found that at relatively small Strouhal number (=0.3), for all types of airfoils with the combined pitching and plunging motion (pitch angle 20°, the pitch axis located at one third of chord length from the leading edge, pitch leading plunge by 75°), low reduced frequency (=1) is conducive for both the thrust production and propulsive efficiency. Moreover, relatively thin airfoils (e.g. NACA0006) can generate larger thrust and maintain higher propulsive efficiency than thick airfoils (e.g. NACA0030). However, with the same kinematics but at relatively large Strouhal number (=0.45), it is found that airfoils with different thickness exhibit diverse trend on thrust production and propulsive efficiency, especially at large reduced frequency (=3.5). Results on effects of airfoil thickness based Reynolds numbers indicate that relative thin airfoils show superior propulsion performance in the tested Reynolds number range. The evolution of leading edge vortices and the interaction between the leading and trailing edge vortices play key roles in flapping airfoil propulsive performance.     

Boundary layer flow of Maxwell fluid due to torsional motion of cylinder: modeling and simulation

This paper investigates the boundary layer flow of the Maxwell fluid around a stretchable horizontal rotating cylinder under the influence of a transverse magnetic field. The constitutive flow equations for the current physical problem are modeled and analyzed for the first time in the literature. The torsional motion of the cylinder is considered with the constant azimuthal velocity E. The partial differential equations (PDEs) governing the torsional motion of the Maxwell fluid together with energy transport are simplified with the boundary layer concept. The current analysis is valid only for a certain range of the positive Reynolds numbers. However, for very large Reynolds numbers, the flow becomes turbulent. Thus, the governing similarity equations are simplified through suitable transformations for the analysis of the large Reynolds numbers. The numerical simulations for the flow, heat, and mass transport phenomena are carried out in view of the bvp4c scheme in MATLAB. The outcomes reveal that the velocity decays exponentially faster and reduces for higher values of the Reynolds numbers and the flow penetrates shallower into the free stream fluid. It is also noted that the phenomenon of stress relaxation, described by the Deborah number, causes to decline the flow fields and enhance the thermal and solutal energy transport during the fluid motion. The penetration depth decreases for the transport of heat and mass in the fluid with the higher Reynolds numbers. An excellent validation of the numerical results is assured through tabular data with the existing literature.     

Boundary layer ?ow of Maxwell ?uid due to torsional motion of cylinder: modeling and simulation

This paper investigates the boundary layer ?ow of the Maxwell ?uid around a stretchable horizontal rotating cylinder under the in?uence of a transverse magnetic?eld. The constitutive ?ow equations for the current physical problem are modeled and analyzed for the ?rst time in the literature. The torsional motion of the cylinder is considered with the constant azimuthal velocity E. The partial di?erential equations(PDEs)governing the torsional motion of the Maxwell ?uid together with energy transport are simpli?ed with the boundary layer concept. The current analysis is valid only for a certain range of the positive Reynolds numbers. However, for very large Reynolds numbers, the ?ow becomes turbulent. Thus, the governing similarity equations are simpli?ed through suitable transformations for the analysis of the large Reynolds numbers. The numerical simulations for the ?ow, heat, and mass transport phenomena are carried out in view of the bvp4 c scheme in MATLAB. The outcomes reveal that the velocity decays exponentially faster and reduces for higher values of the Reynolds numbers and the ?ow penetrates shallower into the free stream ?uid. It is also noted that the phenomenon of stress relaxation, described by the Deborah number, causes to decline the ?ow ?elds and enhance the thermal and solutal energy transport during the ?uid motion. The penetration depth decreases for the transport of heat and mass in the ?uid with the higher Reynolds numbers. An excellent validation of the numerical results is assured through tabular data with the existing literature.     

Stability of dispersed-particle shape at small Reynolds numbers

The results of experimental studies of the conditions of loss of stability of the shape of a single dispersed-phase inclusion (droplet and bubble) during its motion in a viscous fluid at low Reynolds numbers are presented. It is shown that in the conditions considered the deformation of an initially spherical inclusion occurs due to the development of the Rayleigh-Taylor instability, as a critical value of the Bond number is attained. It is found that the onset of deformation of the phase interface and the instability mechanism depend strongly on the particle motion regime. A range of critical Reynolds numbers, corresponding to the boundaries of the regions of the Rayleigh-Taylor and Kelvin-Helmholtz instabilities, is determined.     

Überlegungen zur Scherviskosität von Suspensionen aufgrund einer Dimensionsanalyse

Dimensional analysis of the motion of solid particles suspended in a fluid phase shows that the macroscopic relative shear viscosity of suspensions generally depends not only on the volume concentration and particle shape but also on two Reynolds numbers and a dimensionless sedimentation number. These dimensionless numbers are formed using parameters characterizing the structure and motion of the suspension at the microscopic level. The analysis was based on the assumptions that the dispersed particles are rigid and sufficiently large that Brownian motion may be neglected, that the continuous fluid phase is Newtonian and that the interactions between particles and between particles and fluid phase are only hydrodynamic. The Reynolds numbers describe the influence of the inertial forces at the microscopic level, and the sedimentation number the influence of gravity. The dimensionless numbers can be neglected if their values are much smaller than one. For each of the dimensionless numbers both the shear rate and the particle size influence the shear viscosity. Thus sedimentation number is large for low shear rates, whereas the Reynolds numbers are large for high shear rates. The viscosity function for one suspension can be transformed into the viscosity function for another suspension with geometrically similar particles but of a different size. The scale-up rules are derived from the requirement that the relevant dimensionless numbers must be constant. The influence of non-hydrodynamic effects at the microscopic level on the shear viscosity can be detected by deviations from the derived scale-up rules.     

Dynamics of dual-particles settling under gravity

As a first step towards understanding particle–particle interaction in fluid flows, the motion of two spherical particles settling in close proximity under gravity in Newtonian fluids was investigated experimentally for particle Reynolds numbers ranging from 0.01 to 2000. It was observed that particles repel each other for Re>0.1 and that the separation distance of settling particles is Reynolds number dependent. At lower Reynolds numbers, i.e. for Re<0.1, particles settling under gravity do not separate.The orientation preference of two spherical particles was found to be Reynolds number dependent. At higher Reynolds numbers, the line connecting the centres of the two particles is always horizontal, regardless of the way the two particles are launched. At lower Reynolds numbers, however, the particle centreline tends to tilt to an arbitrary angle, even of the two particles are launched in the horizontal plane. Because of the tilt, a side migration of the two particles was found to exist. A linear theory was developed to estimate the side migration velocity. It was found that the maximum side migration velocity is approximately 6% of the vertical settling velocity, in good agreement with the experimental results.Counter-rotating spinning of the two particles was observed and measured in the range of Re=0–10. Using the linear model, it is possible to estimate the influence of the tilt angle on the rate of rotation at low Reynolds numbers. Dual particles settle faster than a single particle at small Reynolds numbers but not at higher Reynolds numbers, because of particle separation. The variation of particle settling velocity with Reynolds number is presented. An equation which can be used to estimate the influence of tilt angle on particle settling velocity at low Reynolds number is also derived.     

Numerical study on heavy rigid particle motion in a plane wake flow by spectral element method

Experimental particle dispersion patterns in a plane wake flow at a high Reynolds number have been predicted numerically by discrete vortex method (Phys. Fluids A 1992; 4 :2244–2251; Int. J. Multiphase Flow 2000; 26 :1583–1607). To address the particle motion at a moderate Reynolds number, spectral element method is employed to provide an instantaneous wake flow field for particle dynamics equations, which are solved to make a detail classification of the patterns in relation to the Stokes and Froude numbers. It is found that particle motion features only depend on the Stokes number at a high Froude number and depend on both numbers at a low Froude number. A ratio of the Stokes number to squared Froude number is introduced and threshold values of this parameter are evaluated that delineate the different regions of particle behavior. The parameter describes approximately the gravitational settling velocity divided by the characteristic velocity of wake flow. In order to present effects of particle density but preserve rigid sphere, hollow sphere particle dynamics in the plane wake flow is investigated. The evolution of hollow particle motion patterns for the increase of equivalent particle density corresponds to that of solid particle motion patterns for the decrease of particle size. Although the thresholds change a little, the parameter can still make a good qualitative classification of particle motion patterns as the inner diameter changes. Copyright © 2008 John Wiley & Sons, Ltd.     

垂直湍流液-固流中大颗粒的相对速度

通过量纲分析和实验测量,对于垂直、局部均匀的湍流稀态液一固流中,大颗粒的相对速度,建立了无量纲参数表达式．用分析和实验相结合的方法,确定了表达式中无量纲参数的幂次及有关系数．实验中用激光多普勒分相测量技术,分别测出流体和颗粒的时均速度结果表明,大颗粒相对速度强烈依赖于流体雷诺数,当流体雷诺数较高时,其幂次渐近于1.5。