爆炸波作用下三相饱和土本构研究

基于对爆炸荷载作用下三相饱和土宏观试验现象的综述，并结合以往对饱和土中气体运动规律的分析，对一维爆炸波作用下三相饱和土本构关系进行了描述。     

三相介质饱和土自由场中爆炸波的传播规律

饱和土是由固体上颗粒、水和少量封闭的空气组成的三相介质。介绍了爆炸波作用下三相介质饱和土有限元基本方程的建立过程，推导了引进人工粘性项的动力有限元方程，得到在饱和土自由场中爆炸波的传播规律；不同空气含量饱和土中爆炸波随深度的衰减规律。文中还给出了有限元与有限差分间断分解结果的比较，以说明本文理论方法的有效性。     

非饱和粘性土中气体渗透特征

由于气体的进入形成了非饱和带,饱和土层中的气体渗透实质上是非饱和渗透问题。本文借助基于多孔介质气体渗流理论并考虑气体压缩性的、描述非饱和土中气体渗流运动的本构方程,试验研究了上海地区饱和粘性土层中的气体渗透规律。结果表明,饱水粘性土层中,气体渗透存在起始压力值,数值为100～200kPa, 与上海地区粘土的非饱和进气值较为接近;气体渗透速度与外界所施加的"压力平方差梯度"之间存在明显的分段特征。后者可能与在充满结合水的微孔隙通道中,只有一部分结合水在气压梯度的作用下发生流动,从而形成气体通道,施加的外力越大,形成的气体通道也越大,气体渗流也越明显。非饱和粘性土中气体渗透规律的研究,对于饱和粘土中的气压法施工,具有重要的理论与工程实践意义。     

饱和土中爆炸波传播问题的数值模拟

应用流体弹塑性力学的框架描述了爆炸荷载作用下饱和土介质的本构模型,包括体积压缩关系和强度关系。应用LS-dyna软件分析了三相饱和土介质中的爆炸波传播及其与结构的相互作用问题,并与先前完成的试验结果进行了对比,两者吻合较好。饱和土中爆炸波传播的一些特殊现象被重现,表明本构模型的描述是可行的。     

内源瞬态荷载作用下弹性介质与饱和多孔介质中衬砌隧道的动力响应对比分析

衬砌隧道周围介质不同，其内源瞬态荷载作用下的动力响应不同，该文对弹性介质与饱和多孔介质中作用有内源荷载的衬砌隧道的动力响应进行对比分析。首先，令饱和多孔介质孔隙率n=0，将衬砌周围饱和介质退化为弹性介质，动力响应解答相应退化为弹性介质中的解答，退化解与已有的隧道周围为弹性介质时的动力响应解答完全一致。其次，对不同孔隙率的饱和多孔介质和理想弹性介质中衬砌隧道的动力响应进行计算，结果表明：与弹性介质相比，饱和多孔介质中衬砌内表面的动力响应较大，轴向衰减速度较快；随着孔隙率的减小，饱和土中衬砌内表面的动力响应逐渐减小，位移和应力时程曲线均趋近于弹性介质。     

Basset力对液体中易溶性气泡运动的影响

为从理论上分析Basset力对液体中易溶性气泡运动过程的影响, 综合考虑气泡上浮速度和传质速率间的耦合关系, 构建了适于描述易溶性气泡上浮过程的动态耦合模型. 利用该模型对氨气鼓泡吸收过程的计算分析表明, Basset力对易溶性气泡运动行为影响显著. 为此, 又提出了衡量Basset力对气泡运动全过程影响大小的评价指标(无量纲$\eta $数)及其算法. 基于上述模型和指标对水中气泡上浮过程的分析表明: 气体溶解度系数$H$对Basset力影响气泡运动的强度具有决定性作用, 当气泡内气体的$H<10^{-4}$时, 可以忽略Basset力的影响, 而当$H>10^{-4}$时, Basset力的影响迅速增强, 不应忽略; 且气泡半径越小的, Basset力的影响越显著; 而气泡所处深度与Basset力的影响强度关系不大.      

水平刚性面下方水下爆炸气泡垂向运动的理论研究

为了研究边界面对水下爆炸气泡脉动的影响,根据势流理论建立了水平刚性面下方在浮力作用下作垂向运动的水下爆炸气泡的理论模型,编制计算程序进行求解。对水下爆炸气泡脉动运动的特点、流场的速度和压力的分布、气泡引起的载荷形式进行了分析。结果表明此模型能够反映水下爆炸气泡和周围流体介质的运动规律,并能进行定量的计算。     

STUDY ON SUBSIDENCE OF THE BUBBLES INSIDE THE VIBRATING LIQUID UNDER THE CONDITION OF VIBRATION

本文从流体动力学角度探究气泡下沉现象的机理,对垂直振动圆柱容器中的气泡下沉行为进行了系统研究。根据附加质量以及气泡压缩性概念,建立出现下沉效应的可压缩性气泡数学模型,并通过分离变量法分析气泡下沉的临界位移以及运动速度。研究表明,正弦激励振幅以及频率是影响气泡下沉条件的重要因素,决定了临界位移与运动速度的大小,且振幅和频率越大,临界位移越小,气泡越容易下沉。     

振动条件下液体内部气泡下沉现象研究

本文从流体动力学角度探究气泡下沉现象的机理,对垂直振动圆柱容器中的气泡下沉行为进行了系统研究。根据附加质量以及气泡压缩性概念,建立出现下沉效应的可压缩性气泡数学模型,并通过分离变量法分析气泡下沉的临界位移以及运动速度。研究表明,正弦激励振幅以及频率是影响气泡下沉条件的重要因素,决定了临界位移与运动速度的大小,且振幅和频率越大,临界位移越小,气泡越容易下沉。     

基于文克勒模型的饱和土中单桩对瑞利波动力响应研究

基于Boit饱和多孔介质理论,采用文克勒地基梁模型,研究了表面透水的饱和土介质中顶部固接的桩对瑞利波的横向响应。分别计算了软土和硬土两种情况下不同深度桩的响应、饱和土的孔隙率对桩响应的影响、土介质相对刚度对桩响应的影响、fc-z空间桩响应的分布、桩顶约束条件的影响,并与桩顶自由的结果进行了比较。结果表明,饱和土介质中桩对瑞利波的横向响应受波的频率、土介质性质、桩土刚度比、桩顶约束条件等因素的影响。整体来看,瑞利波对桩的横向影响自上而下大致呈衰减趋势,在地表处影响最大;瑞利波对桩的影响深度随频率的降低而扩大;桩、土之间的水平相对位移幅度与土的孔隙率、相对刚度比均呈正相关关系;同频率下,相对软土地基来说硬土地基中桩受瑞利波的影响较深;顶部固接比顶部自由的桩对瑞利波的响应小。计算结果对工程抗震设计具有参考价值。     

T型微通道反应器内气液两相流动机制及影响因素

基于液滴或气泡的多相微流控是近年来微流控技术中快速发展的重要分支之一.本文利用高速显微摄影技术和数字图像处理技术对T型微通道反应器内气液两相流动机制及影响因素进行实验研究.实验采用添加表面活性剂的海藻酸钠水溶液作为液相,空气作为气相.研究T型微通道反应器内气液两相流型的转变过程,并根据微通道内气泡的生成频率和生成气泡的长径比对气泡流进行分类.研究发现当前的进料方式下,可以观测到气泡流和分层流2种流型,且依据气泡生成频率和微通道内气泡的长径比可将气泡流划分为分散气泡流、短弹状气泡流和长弹状气泡流3种类型,并基于受力分析确定3种气泡流的形成机制分别为剪切机制、剪切-挤压机制和挤压机制.考察不同液相黏度和表面张力系数对不同类型气泡流范围的影响规律.结果表明:液相黏度相较于表面张力系数而言,对气泡流生成范围影响更大.给出不同类型气泡流流型转变条件的无量纲关系式,实现微通道生成微气泡过程的可控操作.      

电场作用下气泡上升行为特性的数值计算研究

利用电场控制气泡形态及运动,强化气液相间传热传质是电流体动力学的重要研究内容之一. 然而目前多数研究集中在非电场下的气泡动力学上,对于电场下的气泡行为特性及电场的作用机制仍需开展深入研究. 本研究对电场作用下单个气泡在流体中上升过程的动力学行为进行了数值模拟研究. 在建立二维模型的基础上求解电场方程与Navier-Stokes方程,并采用水平集方法捕捉了上升气泡的位置及形状. 模拟结果的准确性与有效性通过与前人实验和数值结果进行对比得到了验证. 通过改变雷诺数、邦德数和电邦德数等不同参数研究了电场下液体黏度、表面张力和电场力对气泡运动变形的影响. 计算结果表明,电场对气泡的动态特性有显著影响. 非电场情况下液体黏度和表面张力较大时气泡基本维持球状,反之气泡发生变形并逐步达到稳定状态. 此外,电场作用使气泡在初始上升阶段发生剧烈形变,随着不断上升,气泡形变程度不断减小,且气泡的上升速度和长径比均出现振荡. 垂直电场使气泡的上升速度有较大的提高,且随着电邦德数的增大,难以达到相对稳定的状态.      

同轴气流式液体射流分裂液滴粒径研究

针对同轴气流式液体射流分裂液滴粒径预测模型缺乏的现状,结合射流线性稳定性理论,建立了基于临界模数的同轴气流式黏性液体射流分裂液滴粒径表达式,在此基础上,分别研究了气流旋拧(气流同时存在轴向和周向运动)及流体物性(气体可压缩性、液体黏性、气液密度比和表面张力)对液滴粒径的影响规律.研究发现:周围气流轴向引射作用和同轴旋转...     

nfluence of exciting parameters on the vibration of single cavitation bubble driven by intensive sound

根据考虑了液体可压缩性的改进的微气泡动力学方程,采用改进的初始半径对单泡超声空化现象进行了数值计算研究. 结果表明,微气泡振动对一些参量很敏感：微气泡振动半径与初始半径的比值随振动频率的增大而减小;提高声场声压会加剧气泡崩塌程度,但过高的声压又不能使微气泡崩塌;微气泡崩塌速率随气泡初始半径的增加而增大,在一定范围内能保证空化泡稳定振动,在初始半径为1.6\,$\mu$m 处空化程度最强,如果继续增大初始半径则空化程度减弱、甚至消失;微气泡崩塌程度随黏滞系数和表面张力的增大而减弱,过大的黏滞系数和表面张力会使微气泡崩塌难以发生. 计算结果与他人的实验数据相比,发现液体的可压缩性使单泡空化强度增强, 对最佳空化区域范围的确定有较大的影响.     

Buoyancy-driven motion of a two-dimensional bubble or drop through a viscous liquid in the presence of a vertical electric field

The effect of an electric field on the buoyancy-driven motion of a two-dimensional gas bubble rising through a quiescent liquid is studied computationally. The dynamics of the bubble is simulated numerically by tracking the gas–liquid interface when an electrostatic field is generated in the vertical gap of the rectangular enclosure. The two phases of the system are assumed to be perfect dielectrics with constant but different permittivities, and in the absence of impressed charges, there is no free charge in the fluid bulk regions or at the interface. Electric stresses are supported at the bubble interface but absent in the bulk and one of the objectives of our computations is to quantify the effect of these Maxwell stresses on the overall bubble dynamics. The numerical algorithm to solve the free-boundary problem relies on the level-set technique coupled with a finite-volume discretization of the Navier–Stokes equations. The sharp interface is numerically approximated by a finite-thickness transition zone over which the material properties vary smoothly, and surface tension and electric field effects are accounted for by employing a continuous surface force approach. A multi-grid solver is applied to the Poisson equation describing the pressure field and the Laplace equation governing the electric field potential. Computational results are presented that address the combined effects of viscosity, surface tension, and electric fields on the dynamics of the bubble motion as a function of the Reynolds number, gravitational Bond number, electric Bond number, density ratio, and viscosity ratio. It is established through extensive computations that the presence of the electric field can have an important effect on the dynamics. We present results that show a substantial increase in the bubble’s rise velocity in the electrified system as compared with the corresponding non-electrified one. In addition, for the electrified system, the bubble shape deformations and oscillations are smaller, and there is a reduction in the propensity of the bubble to break up through increasingly larger oscillations.     

Bubbles in liquids with phase transition

In the forthcoming second part of this paper a system of balance laws for a multi-phase mixture with many dispersed bubbles in liquid is derived where phase transition is taken into account. The exchange terms for mass, momentum and energy explicitly depend on evolution laws for total mass, radius and temperature of single bubbles. Therefore in the current paper we consider a single bubble of vapor and inert gas surrounded by the corresponding liquid phase. The creation of bubbles, e.g. by nucleation is not taken into account. We study the behavior of this bubble due to condensation and evaporation at the interface. The aim is to find evolution laws for total mass, radius and temperature of the bubble, which should be as simple as possible but consider all relevant physical effects. Special attention is given to the effects of surface tension and heat production on the bubble dynamics as well as the propagation of acoustic elastic waves by including slight compressibility of the liquid phase. Separately we study the influence of the three phenomena heat conduction, elastic waves and phase transition on the evolution of the bubble. We find ordinary differential equations that describe the bubble dynamics. It turns out that the elastic waves in the liquid are of greatest importance to the dynamics of the bubble radius. The phase transition has a strong influence on the evolution of the temperature, in particular at the interface. Furthermore the phase transition leads to a drastic change of the water content in the bubble. It is shown that a rebounding bubble is only possible, if it contains in addition an inert gas. In Part 2 of the current paper the equations derived are sought in order to close the system of equations for multi-phase mixture balance laws for dispersed bubbles in liquids involving phase change.     

The influence of thermodynamic gas parameters on laser-induced bubble dynamics in water

The oscillating properties of laser-induced cavitation bubbles in water are investigated by means of a fiber-optic sensor based on optical beam deflection. The experimental results show two important points. One is that the smaller the bubble radius the more quickly the bubble surface moves. Thus, the variations of the temperature and the pressure inside the bubble will be close to those of an adiabatic process. The other is that the high-energy vapor inside the newborn bubble diffuses and coagulates rapidly through violent expansion and thermal conduction. Thus, the gas content of the bubble reduces significantly in the first oscillation. Numerical simulation is made for the bubble model with consideration of liquid viscosity, surface tension, and gas content. Through modification of the polytropic index and the gas content parameter, two parameters of this model, the numerical results fit the experimental results well.     

A physically based correlation for the effects of power law rheology and inclination on slug bubble rise velocity

A study has been made of the motion of long bubbles in inclined pipes containing viscous Newtonian and non-Newtonian liquids. A semi-theoretical expression for the rise velocity of air bubbles in water is derived on the hypothesis that the dominant factor is the momentum exchange of the bubble underflow, i.e. the bubble nose shape. The correlation calls on empirical inputs from established literature on bubble rise speeds at high Reynolds number. The effects of increasing Newtonian viscosity are analysed with reference to the momentum exchange and it is shown how viscosity reduces the inclination dependence of the bubble Froude number. Results from an experimental survey in seven different non-Newtonian liquids in three different diameter pipes are presented. These data are correlated so as to decouple the effects of surface tension and viscosity. An empirical relation is proposed for the effective shear rate in the fluid travelling around the bubble nose. Our correlation is compared to literature data from a broad range of Reynolds numbers with excellent agreement except at shallow angles.     

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.     

多孔介质中稳定泡沫的流动计算及实验研究

将多孔介质简化为一簇变截面毛管束,根据多孔介质的颗粒直径、颗粒排列方式、孔喉尺度比及束缚水饱和度,计算出变截面毛细管的喉道半径和孔隙半径. 在考虑多孔介质喉道和孔隙中单个气泡的受力和变形基础上,利用动量守恒定理,推导出单个孔隙单元内液相的压力分布和孔隙单元两端的压差计算公式,最终得到多孔介质的压力分布计算公式. 利用长U型填砂管对稳定泡沫的流动特性进行了实验研究. 研究结果表明:稳定泡沫流动时多孔介质中的压力分布呈线性下降,影响泡沫在多孔介质中流动特性的因素包括:多孔介质的孔喉结构、泡沫流体的流量和干度、气液界面张力、气泡尺寸,其中孔喉结构和泡沫干度是影响泡沫封堵能力的主要因素.关键词: 稳定泡沫;多孔介质;变截面毛管;流动;表观粘度;压力分布;实验研究