流体黏性及表面张力对气泡运动特性的影响

基于气泡边界层理论,引入黏性修正,采用边界积分法,考虑黏性效应和表面张力在单气泡以及双气泡耦合作用过程中的影响.首先将建立的数值模型与Rayleigh-Plesset的解析解进行对比,发现二者符合良好,验证了数值模型的有效性;在此基础上,建立考虑流体弱黏性效应的双气泡耦合模型,研究流体黏性和表面张力作用下,气泡表面变形、射流速度、流场能量转换等物理量的变化规律;最后研究雷诺数和韦伯数对于气泡脉动特性的影响规律.结果表明,流体黏性会抑制气泡脉动和气泡射流发展,降低气泡半径和射流速度;表面张力不改变气泡脉动幅值,但缩短了脉动周期,提升气泡势能.     

可压缩多介质黏性流体的数值计算

在多介质复杂流动及其相互作用情况下的界面不稳定性研究中，尽管流体的黏性系数比较小，但对流场特别是因界面不稳定性引起的混合部分产生的影响是不能忽略的。采用算子分裂技术，将考虑热传导和黏性情况下Navier Stokes（NS）方程描述的物理过程分解成3个子过程进行数值计算，即整个流量计算分解成无黏性流量、黏性流量和热流量3部分。无黏性流量计算体部分包含对激波、稀疏波以及接触间断的计算和多介质界面处理技术，黏性流量和热流量的计算，主要考虑牛顿流体黏性应力张量和能量流的影响。     

基于相场法的多孔介质三相相对渗透率的研究

本文基于相场模型研究了多孔介质内的三相流动过程,得到了三相相对渗透率曲线。通过模拟研究了润湿性、黏性比和毛细数对三相相对渗透率的影响。研究结果表明：润湿性对相对渗透率的影响很小;非润湿性流体的相对渗透率随黏性比的增加而增加,中性流体和润湿性流体的相对渗透率随黏性比的增加而降低;非润湿性流体的相对渗透率随毛细数的增加而减小,润湿性流体的相对渗透率随毛细数的增加而增加,中性流体的相对渗透率不随毛细数单调变化。     

流体惯性对弹性界面液滴吸附行为的影响

弹性膜包裹黏性流体组成的液滴(弹性界面液滴)可与功能性壁面形成化学吸附键,产生吸附力,与黏性力和惯性力共同控制流场中液滴的吸附动力学行为。本文基于前期发展的界面追踪(Front tracking method)-界面有限元方法,结合吸附反应动力学模型,发展了可同时考虑流体惯性、界面弹性和流体黏性的弹性界面液滴吸附动力学三维模拟方法。重点分析了流体惯性对弹性界面液滴吸附行为的影响规律。结果表明:雷诺数增大导致流场作用于弹性界面液滴的阻力和升力都增大,因此流体惯性作用导致弹性界面液滴更容易从壁面脱附。     

黏性、表面张力和磁场对Rayleigh-Taylor不稳定性气泡演化影响的理论分析

采用理论分析的方法考察了磁场中非理想流体中Rayleigh-Taylor(RT)不稳定性气泡的演化过程,在与磁场垂直的平面中,综合考虑流体黏性和表面张力的影响,推导了二维非理想磁流体RT不稳定性气泡运动的控制方程组,给出了不同情况下气泡速度的渐近解和数值解,分析了流体黏性、表面张力和磁场对气泡发展的影响,分析结果表明:流体黏性和表面张力能够降低气泡速度和振幅,即能够抑制RT不稳定性;而磁场对RT不稳定性的影响是由非线性部分引起的,并且磁场非线性部分的方向决定了磁场是促进还是抑制RT不稳定性的发展,     

电磁流体表面推进机理与效果分析

电磁流体表面推进是在推进单元周围的导电流体中(海水、等离子体等)激励出电磁体积力,并利用电磁体积力的反作用力达到推进的目的. 基于电磁场和流体力学的基本控制方程,采用有限体积法对电磁流体表面推进的效果进行了数值模拟研究,分析了在不同姿态(攻角)和不同电磁体积力的作用下,航行器周围流场结构的变化规律和推力的变化特点.研究结果表明:沿航行器表面分布的电磁体积力可以有效地改变流体边界层的结构,并能向流体边界层传输动量与能量,从而使航行器获得所需的推力.流体对航行器的黏性阻力和压差阻力的影响随作用参数的增大而减弱 关键词： 表面推进 航行器 推进单元 电磁体积力     

油泵输送黏性流体时的性能换算图

本文通过实验得到了不同比转数油泵在输送黏性流体的水力性能,研究了输送黏性流体时的修正系数与油泵的性能参数、油泵的转速和输送介质黏度之间的相互关系,参照德国KSB公司所提出的相似准则数,绘制了五种比转数油泵输送黏性流体时的流量、扬程和效率性能换算图,给出了性能换算图的使用方法和计算步骤,实例表明用性能换算图得出的计算结果与实测结果具有较高的一致性.     

非水平直圆管中黏性流体层流流量公式的推导及实验验证

通过牛顿粘滞定律和修正后的伯努利方程对非水平直圆管中黏性流体作层流运动时的流量公式进行了推导。指出将泊肃叶公式中的压强视作广义压强,则文中的推导公式与泊肃叶公式完全一致。说明泊肃叶公式可以用来求解非水平直圆管中黏性流体作层流运动时的流量。本文通过实验验证了推导公式。     

非理想流体中Rayleigh-Taylor和Richtmyer-Meshkov不稳定性气泡速度研究

在随气泡顶端运动的坐标系中, 通过将理想流体模型推广到非理想流体的情况, 研究了流体黏性和表面张力对Rayleigh-Taylor (RT)和Richtmyer-Meshkov (RM)不稳定性气泡速度的影响. 首先得到了RT和RM不稳定性气泡运动的控制方程 (自洽的微分方程组); 其次给出了二维平面坐标和三维柱坐标中气泡速度的数值解和渐近解, 并定量分析了流体黏性和表面张力对RT和RM气泡速度和振幅的影响. 结果表明: 从线性阶段到非线性阶段的全过程中, 非理想流体中的气泡速度和振幅小于理想流体中的气泡速度和振幅. 也就是说, 流体黏性和表面张力对RT和RM不稳定性的发展都具有致稳作用. 关键词： Rayleigh-Taylor不稳定性 Richtmyer-Meshkov不稳定性 气泡速度 非理想流体     

微尺度流道内液体表观黏性系数的分子理论

在考虑分子间作用力的基础上，讨论了各向同性壁面对微流道内非极性分子液体表观黏性系数的影响，建立了考虑壁面影响的表观黏性系数分子理论公式.研究结果显示，在靠近壁面处，壁面对表观黏性系数的影响是很大的，这取决于壁面材料和流体介质的分子间作用力.随着离开壁面距离的增加，壁面对表观黏性系数的影响衰减很快.在液态氩为流体介质的条件下，壁面影响大约可涉及到第10层氩分子，即影响深度为4 nm左右. 关键词： 微尺度流道 表观黏性系数 固壁影响 分子理论     

Resonance oscillation damping of a scanning microscope probe by a near-surface viscous liquid layer

Viscous liquid layer motion between a probe with a tip shaped as a paraboloid of revolution and a surface is considered for semicontact-mode operation of a scanning probe microscope. The presence of a viscous liquid layer leads to energy dissipation and is one of the factors responsible for the decrease in the probe oscillation amplitude. The Reynolds equation for viscous liquid motion is used to obtain an analytic solution to the problem. The formula derived for the loss is compared with experimental data obtained for probes and layers with various curvature radii and viscosities.     

基于孔口出流模型的实验研究及理论分析

基于孔口出流模型对底部有小孔的圆柱体容器中液体液面高度随时间的变化规律进行了实验研究,并通过计入液体表面张力引起的孔口收缩和流体黏滞力导致的能量损失对实验结果进行了理论分析,与实验结果一致.     

Investigation of Turbulent Transition in Plane Couette Flows Using Energy Gradient Method

The energy gradient method has been proposed with the aim of better understanding the mechanism of flow transition from laminar flow to turbulent flow. In this method, it is demonstrated that the transition to turbulence depends on the relative magnitudes of the transverse gradient of the total mechanical energy which amplifies the disturbance and the energy loss from viscous friction which damps the disturbance, for given imposed disturbance. For a given flow geometry and fluid properties, when the maximum of the function $K$ (a function standing for the ratio of the gradient of total mechanical energy in the transverse direction to the rate of energy loss due to viscous friction in the streamwise direction) in the flow field is larger than a certain critical value, it is expected that instability would occur for some initial disturbances. In this paper, using the energy gradient analysis, the equation for calculating the energy gradient function $K$ for plane Couette flow is derived. The result indicates that $K$ reaches the maximum at the moving walls. Thus, the fluid layer near the moving wall is the most dangerous position to generate initial oscillation at sufficient high $\operatorname{Re}$ for given same level of normalized perturbation in the domain. The critical value of $K$ at turbulent transition, which is observed from experiments, is about 370 for plane Couette flow when two walls move in opposite directions (anti-symmetry). This value is about the same as that for plane Poiseuille flow and pipe Poiseuille flow (385-389). Therefore, it is concluded that the critical value of $K$ at turbulent transition is about 370-389 for wall-bounded parallel shear flows which include both pressure (symmetrical case) and shear driven flows (anti-symmetrical case).     

Effect of Varying Bulk Viscosity on Generalized Chaplygin Gas

In this paper, viscous generalized Chaplygin gas as a model of dark energy considered. We assume non-constant bulk viscous coefficient and study dark energy density. We consider several cases of density-dependent viscosities. We find that, in the special case, the viscous generalized Chaplygin gas is corresponding to modified Chaplygin gas.     

Stability Analysis of the Viscous Polytropic Dark Energy Model in Einstein Cosmology

The viscous polytropic gas model as one model of dark energy is hot-spot and keystone to the modern cosmology.We study the evolution of the viscous polytropic dark energy model interacting with the dark matter in the Einstein cosmology.Setting the autonomous dynamical system for the interacting viscous polytropic dark energy with dark matter and using the phase space analysis method to investigate the dynamical evolution and its critical stability,we find that the viscosity property of the dark energy creates a benefit for the stable critical dynamical evolution of the interaction model between dark matter and dark energy in the flat Friedmann-Robertson-Walker universe and the viscosity of dark energy will soften the coincidence problem just like the interacting dark energy model.     

Viscous effects on motion and heating of electrons in inductivelycoupled plasma reactors

A transport model is developed for nonlocal effects on motion and heating of electrons in inductively coupled plasma reactors. The model is based on the electron momentum equation derived from the Boltzmann equation, retaining anisotropic stress components which in fact are viscous stresses. The resulting model consists of transport equations for the magnitude of electron velocity oscillation and terms representing energy dissipation due to viscous stresses in the electron energy equation. In this model, electrical current is obtained in a nonlocal manner due to viscous effects, instead of Ohm's law or the electron momentum equation without viscous effects, while nonlocal heating of electrons is represented by the viscous dissipation. Computational results obtained by two-dimensional numerical simulations show that nonlocal determination of electrical current indeed is important, and viscous dissipation becomes an important electron heating mechanism at low pressures. It is suspected that viscous dissipation in inductively coupled plasma reactors in fact represents stochastic heating of electrons, and this possibility is exploited by discussing physical similarities between stochastic heating and energy dissipation due to the stress tensor     

Collision dynamics of two adjacent oscillators

A linear analysis is made of a single collision between two single-degree-of-freedom systems separated by a gap. The contact is modelled by a spring and a viscous damper. The approach is to describe the motion of the pair as being composed of sum and difference displacements. The equation of motion during contact is found and the solution is obtained from the conditions at initial contact. The main parameters are the ratio of strain energy to kinetic energy at initial contact, and the damping of the contact. The contact time and the energy loss are calculated, which gives an expression for the coefficient of restitution for the collision. This coefficient is found to be dependent on the collision velocity, but becomes constant for strong collisions.     

Noninvasive quantification of fluid mechanical energy losses in the total cavopulmonary connection with magnetic resonance phase velocity mapping

A major determinant of the success of surgical vascular modifications, such as the total cavopulmonary connection (TCPC), is the energetic efficiency that is assessed by calculating the mechanical energy loss of blood flow through the new connection. Currently, however, to determine the energy loss, invasive pressure measurements are necessary. Therefore, this study evaluated the feasibility of the viscous dissipation (VD) method, which has the potential to provide the energy loss without the need for invasive pressure measurements. Two experimental phantoms, a U-shaped tube and a glass TCPC, were scanned in a magnetic resonance (MR) imaging scanner and the images were used to construct computational models of both geometries. MR phase velocity mapping (PVM) acquisitions of all three spatial components of the fluid velocity were made in both phantoms and the VD was calculated. VD results from MR PVM experiments were compared with VD results from computational fluid dynamics (CFD) simulations on the image-based computational models. The results showed an overall agreement between MR PVM and CFD. There was a similar ascending tendency in the VD values as the image spatial resolution increased. The most accurate computations of the energy loss were achieved for a CFD grid density that was too high for MR to achieve under current MR system capabilities (in-plane pixel size of less than 0.4 mm). Nevertheless, the agreement between the MR PVM and the CFD VD results under the same resolution settings suggests that the VD method implemented with a clinical imaging modality such as MR has good potential to quantify the energy loss in vascular geometries such as the TCPC.     

Viscous friction in foams and concentrated emulsions under steady shear

We present a model for the viscous friction in foams and concentrated emulsions, subject to steady shear flow. First, we calculate the energy dissipated due to viscous friction inside the films between two neighboring bubbles or drops, which slide along each other in the flow. Next, from this energy we calculate the macroscopic viscous stress of the sheared foam or emulsion. The model predictions agree well with experimental results obtained with foams and emulsions.