Pressure Waves in a Gas–Liquid Medium with a Stratified Liquid–Bubbly Mixture Structure

Evolution and decay of pressure waves of moderate amplitude in a vertical shock tube filled by a gas–liquid medium with a nonuniform (stepwise) distribution of bubbles over the tube cross section are studied experimentally. The gas–liquid layer has the form of a ring located near the tube wall or the form of a gas–liquid column located in the center of the tube. It is shown that the nonuniformity of bubble distribution over the tube cross section increases the attenuation rate of pressure waves.     

考虑时滞效应时耦合化学反应系统的动力学行为

在耦合自催化反应系统中,采用数值分析方法研究了考虑时滞效应和流速扰动时子系统的动力学行为.与原系统相比,该系统呈现出更加丰富的动力学现象.反应过程中出现了结构复杂的混沌吸引子和由在周期解邻域内振荡而产生的概周期运动,并且存在混沌由倍周期分岔演变为新的混沌吸引子的过程.这些结果对于解释耦合化学反应系统中的复杂现象、揭示其反应机理具有一定的指导意义.     

液体发射药电热化学发射内弹道一维两相流模型及数值模拟

在理论分析的基础上 ,结合实验条件建立了液体发射药电热化学发射内弹道一维两相流数学模型 ,该模型包含电弧等离子体生成和运动、等离子体卷吸液体发射药生成小液滴以及膛内多相流体的相互作用。利用该模型对电热化学发射内弹道过程进行了数值模拟 ,并将模拟结果与实验数据进行了分析比较 ,结果表明模拟结果与实验数据基本吻合。     

反应诱导聚合物双层结构弯曲行为的建模与分析

聚合物软材料兼具柔软性和大变形能力,作为一种智能材料在软体机器人等领域应用广泛．化学活性聚合物分子内包含具有较高反应活性的官能团,如环氧、酯键、羟基、羧基等,可以在一定条件下发生化学反应引起材料体积和性质变化．因此研究化学反应如何调控聚合物变形对开发新的功能型聚合物材料有重要指导意义．本文建立化学活性聚合物-弹性基底双层结构的理论模型并分析其在化学反应诱导下的有限弯曲行为．引入化学反应进度作为Helmholtz自由能独立的状态变量,考虑化学反应对聚合物体积变化和模量的影响,以及反应过程独立的能量耗散机制,并基于Neo-Hookean模型建立起各层内的超弹性本构关系．最后利用Newton-Raphson方法对反应完全时的平面应变稳态问题进行数值求解,得到不同几何和反应影响参数下各层内的弯曲变形和应力分布．     

Gas bubble oscillations in a fluid in the presence of 2:1 resonance between the radial and deformation oscillation frequencies

Small nonlinear oscillations of an ellipsoidal bubble in a fluid in the presence of 2:1 frequency resonance between the radial and ellipsoidal modes are considered. The equations of motion are reduced to Hamiltonian form. The quadratic and cubic terms are taken into account in the expansion of the Hamiltonian. The Hamilton function is transformed to the normal form using the invariant normalization method in the first approximation. This makes it possible to construct an analogy between the system considered and the well-known problem of a pendulous spring. The radial and ellipsoidal bubble oscillation modes correspond to the vertical and horizontal coordinates of a material point, respectively. In the absence of resonance the solution of the nonlinear equations differs from the solution of the linear equations by only a small (quadratic in the amplitude) change in the oscillation frequency. In the resonance case the radial and ellipsoidal oscillation modes periodically change places and the energy of one mode is converted into that of the other. The interest in the system in resonance is associated with precisely this fact. The question of the dissipation effect in real media is considered. The decay rate depends significantly on the physical properties of the material and, in certain special cases, can be small enough for the energy transfer effect to manifest itself.     

Mixed convection flow of a micropolar fluid with radiation and chemical reaction

Effects of heat and mass transfer on the mixed convection flow of a magnetohydrodynamic (MHD) micropolar fluid bounded by a stretching surface have been investigated. Homotopy analysis procedure is adopted for computations of a set of coupled nonlinear ordinary differential equations. Numerical values of skin friction coefficient and Nusselt and Sherwood numbers are worked out. A comparative study is provided with the limiting available numerical solution. Copyright © 2010 John Wiley & Sons, Ltd.     

Ellipsoidal Oscillations of a Gas Bubble with Periodic Variation of the Surrounding Fluid Pressure

Ellipsoidal linear and nonlinear oscillations of a gas bubble under harmonic variation of the surrounding fluid pressure are studied. The system is considered under conditions in which periodic sonoluminescence of the individual bubble in a standing acoustic wave is observable. A mathematical model of the bubble dynamics is suggested; in this model, the variation of the gas/fluid interface shape is described correct to the square of the amplitude of the deformation of the spherical shape of the bubble. The character of the air bubble oscillations in water is investigated in relation to the initial bubble radius and the fluid pressure variation amplitude. It is shown that nonspherical oscillations of limited amplitude can occur outside the range of linearly stable spherical oscillations. In this case, both oscillations with a period equal to one or two periods of the fluid pressure variation and aperiodic oscillations can be observed.     

Interaction of a Shock Wave with a Spherical Gas–Liquid Cluster

Experimental studies were performed of the interaction of a plane shock wave with a spherical gas–liquid cluster (polyurethane foam ball saturated with a bubbly liquid) in a vertical shock tunnel filled with a liquid. The cluster was shown to generate a solitary pressure wave of large amplitude.     

Certain hydrodynamic characteristics of the evolution of a solitary spherical bubble in the finite compression regime

The characteristics of unsteady incompressible fluid flow around a gas or vapor bubble compressed to a finite size are theoretically investigated. The velocity on the bubble boundary, the integral flow-rate and kinetic energy, and the form of the pressure distribution in the fluid are analyzed. Certain general qualitative regularities independent of viscosity, surface tension, and the intensity of the heat and mass transfer processes are revealed. The general results obtained are illustrated and specified with reference to a simplified polytropic model.     

Dynamics of a Spherical Microcavity in a Polymeric Liquid

The formation and collapse of a vapor microcavity (microbubble) in aqueous solutions of polyacrylamide with a molecular mass of 0.5–11 million and concentration of 0–6% is studied experimentally. A microbubble less than 100 m in size is formed as a result of local superheating of the liquid by a microheater. Using stroboscopic visualization, the microbubble evolution is investigated in detail. At high polymer concentrations and low molecular masses, a slowing of bubble collapse is noted. On the range of parameters considered, no radical change in microbubble dynamics due to the action of polymeric additives was observed.     

Thermal Growth of a Vapor Bubble Moving in a Superheated Liquid

The problem of thermal growth of a vapor bubble moving in a superheated liquid is solved for two models of the phase interface, namely: a rigid (no-slip condition) and a pliable (slip condition) spherical surface. The second self-similar solution of the problem of the motion of a vapor bubble with a pliable surface is found. On the basis of this solution, an approximate dependence of the nondimensional heat flux into the bubble on the Jacob and Péclet numbers is constructed. For two limiting cases, namely, for a bubble growing at rest and a moving bubble of constant radius, this dependence coincides with the known solutions. The calculation results are compared with the experimental data obtained for vapor bubbles rising in a superheated liquid.     

Two-Phase Models of Formation of Cavitating Spalls in a Liquid

The dynamics of the structure of a liquid layer structure (with microbubbles of a free gas) behind a rarefaction wave front is studied numerically using the two-phase Iordansky–Kogarko–van Wijngaarden model and the frozen mass-velocity field model. An analysis of the initial stage of cavitation by the Iordansky–Kogarko–van Wijngaarden model showed that tensile stresses behind the rarefaction wave front relax quickly and the mass-velocity field in the cavitation zone turns out to be frozen. This effect is used to describe the late stage of the development of the cavitation zone. These models were combined to study the formation of cavitating spalls in a free-surface liquid under shock-wave loading.     

Visco-elastic fluid flow past an infinite vertical porous plate in the presence of first-order chemical reaction

An analysis has been developed to study the unsteady free convection flow of an incompressible visco-elastic fluid on a continuously moving vertical porous plate in the presence of a first-order chemical reaction. The governing equations are solved numerically using an implicit finite difference technique. The obtained numerical solutions are compared with the analytical solutions. The velocity profiles are presented. A parametric analysis is performed to illustrate the influences of the visco-elastic parameter, the dimensionless chemical reaction parameter, and the plate moving velocity on the steady state velocity profiles, the time dependent friction coefficient, the Nusselt number, and the Sherwood number.     

气液两相流压力波传播速度研究

将双流体模型用于绝热无相的管道气液两相流,依据小扰动线化分析原理,导出了压力波波数Ｋ方程通过对不同空隙率下肉体上压力波小随角频率变化的计算,研究了虚拟质量力和狭义相间阻力对压力波波速及其人色散性的影响。对泡状流和弹状流压力波波速的计算结果与前人的测量结果作了比较,两者符合良好。     

横向气流穿过垂直下落液滴场的二维流动数值计算

对Cl2/He混合气体横向穿过垂直下落的BHP(按重量25%的KOH,25%的H2O2及50%的H2O)液滴场的化学反应流动作了数值计算.模拟的流场是气体/液滴两相流场,在气相方程中,考虑了液滴与Cl2反应产生及释放O2(1Δ)的质量源项及表示液滴对气流阻碍作用的动量源项.由于气相动量小,液滴在下落过程横向偏移小,下落...     

Dynamics of a small bubble in a compressible fluid

An efficient (simplified) method for solving problems of spherically symmetric dynamics of a small gas bubble in a compressible fluid is proposed. The method is based on the joint use of the full problem statement (the gas dynamics equations for the gas and the fluid) and its relevant simplifications. Some approximate statements are discussed. In the proposed method, the rarefaction and compression of the gas during the slow motion of the bubble surface is assumed to be uniform over the bubble volume. At the same time the fluid in the thin zone adjacent to the bubble is considered to be slightly compressible. Otherwise the gas dynamics equations are used for the gas and the fluid. The dynamics of the fluid in the thick external zone are described by the linear acoustics only. The proposed simplified method and two others used in literature are estimated by comparison of their numerical results with those obtained in full statement. Copyright © 2000 John Wiley & Sons, Ltd.     

Disengagement of the gas phase in bubble columns

The technique of gas disengagement is popularly used to assess the bubble size distribution in bubble columns. The technique involves the dynamic measurement of dispersion height when the gas supply is stopped. In this paper a mathematical model has been proposed for the process of dynamic gas disengagement. It has been shown that the initial faster disengagement is due to the presence of internal liquid circulation and not due to the presence of very large bubbles. Further, slower disengagement has been attributed to the transition from heterogeneous dispersion to homogeneous dispersion. The new model also explains the effects of superficial gas velocity, column diameter, column height and liquid phase physical properties on the gas disengagement.     

Numerical calculation for the coefficients of stokes harmonic waves of high order

This paper has reformulated the mathematical model and boundary conditions in the semi-physical plan (x,ψ)by using W.H.Hui’s method and suggested two new ways of numerical calculation for the coefficients of Stokes harmonic waves of high order. By transforming the perturbation parameter? into a new one we we rejind Cokelet’s results (1977) of phase speed and semi-waveheight expressions.     

填充床中气体渗流与化学波耦合问题的研究

研究等温和显著气固反应条件下填充床内反应气体浓度(物质)波推进与混合气体渗流的相互作用,指出化学反应对流动的影响包括两个方面,反应过程中混合气体质量的变化和密度的变化。混合气体流动将反过来影响反应进程。分析表明,按耦合模型和非耦合模型得到的速度场完全不同;按耦合模型,反应气体的浓度(物质)波阵面的推进对混合气体的流场有显著影响,因此按耦合模型计算的混合气体流场强烈地依赖于时间;忽略化学反应引起的混合气体密度变化的耦合模型,将导致一个质量消失的汇(或质量生成的源),因此将引起混合气体渗流速度的明显变化,并可能导致物理上不合理的结果;按耦合模型和非耦合模型计算的浓度场也有很大差别;当反应气体与惰性气体摩尔质量相差较大时,不能忽略反应过程中混合气体密度的变化;研究表明对于显著气固反应不能忽略化学反应与气体渗流的相互作用。