VOF方法模拟回形水块自由跌落入水过程

应用VOF方法研究了回形水块自由跌落入水的全过程,采用逐次超松弛法和共轭剩余法结合差分法求解二维Navier-Stokes方程,用施主-受主法求解流体体积函数控制方程。通过数值计算得到了回形水块自由跌落入水时的流场和自由面分布,并成功地模拟了界面变形、界面破碎、气泡的生成及溃灭、水凹以及水涌的生成过程,结果表明:逐次超松弛法较共轭剩余法更为精确。     

竖直楔形流道内二维浮力气泡上升的数值模拟

为解决Level Set方法中气泡体积非守恒性以及非正交网格引起的流场数值耗散问题,该文引入体积修正法和窄带重新初始化方法,并结合贴体坐标下的SIMPLE算法首次对楔形流道内气泡的运动机制进行了模拟研究,分析了气泡在竖直楔形流道内上升过程的形态演变,获得了竖直速度、水平速度、气泡纵横比以及气泡倾斜角等参数的变化规律。     

有限水域内球形气泡振动特性及影响参数研究

该文基于不可压NS方程,推导了有限水域条件下气泡的无量纲动力学方程,得到了与惯性力、黏性力、表面张力和不可凝结气体相关的四个无量纲数,通过数值求解的方法给出了无量纲参数对气泡振动的影响规律及其起作用的参数空间。最后,在小厚度小振幅条件下讨论了气泡振动的特性及适用范围,在此基础上给出了自由面附近气泡溃灭时间的理论预测公式。     

双层堤基的非稳定渗流

论述下为砂层、上为粘性土覆盖层双层堤基的非稳定渗流, 计算了砂层和覆盖层因体积胀缩、水动延滞所引起的水头滞后. 对于某些特殊的土质条件, 导出了解析解; 对于普遍的情况, 则应用数值法拉普拉斯逆变换求出了数值解. 研究结果可用于堤防的渗流计算, 以供堤防设计和管理等工作参考.、、     

二维分层流的格子Boltzmann数值模拟

基于格子Boltzmann方法，采用改进的自由能模型，对二维分层流和“气泡”迁移现象进行了数值模拟。计算模型中，边界条件处理采用了法向反弹假定，这种处理方法具有二阶精度且简单易行。在给定的条件下，对分层流进行了数值模拟，流速的计算值与其理论解非常吻合，最大相对误差不超过0.3％。常规自由能模型不能满足伽利略不变性，在模拟气泡运动时，圆形气泡往往会变为椭圆形；而改进的自由能模型满足伽利略不变性，在模拟气泡时其形状不会失真。最后，给出了一个“气泡”迁移的数值仿真，在计算了200 000个时间步后，气泡仍然保持原来的形状。数值试验的结果验证了模型的有效性与可靠性。     

空泡在管道中的动力学行为研究

该文将实验与数值计算相结合研究了空泡在管道中的动力学行为,使用高速相机记录了空泡在受限制管道空间内的成长及溃灭过程,采用BEM的数值仿真方法模拟了空泡在管道中的成长及溃灭过程,并将仿真结果与实验结果进行了对比。分析了空泡的演化过程及对应的压力分布情况,最后讨论了管道对空泡周期和体积的影响。     

带耗散自由面势流方程的Crank-Nicolson有限差分方法数值模拟

该文建立了带耗散自由面势流方程组Crank-Nicolson有限差分方法。通过坐标变换将不规则的水槽区域变换为一个规则的正方形区域,在计算区域构造交错网格,对非线性势流方程组建立Crank-Nicolson隐格式的有限差分方法,设计了流场变量的耦合迭代的算法。数值求解了在无耗散情况下的自由面数值解与解析解的误差,数值解吻合Frandsen的解析解。数值模拟了在不同耗散系数下的自由运动,水平激励和垂直激励下的自由面波高和速度场。数值实验表明自由振动和垂直激励振动随着耗散系数的增加波高和流场速度都减小。在水平激励下的自由面波高和速度场随耗散系数的增加而减小,当波高和速度衰减到一定程度后,自由面保持稳定的波高和速度场,自由面波高出现了耗散现象。     

平口管口处气泡行为特征数值模拟

为研究平口管口处单个气泡行为特征,采用流体体积模型(Volume of Fluid, VOF)方法,数值模拟研究气泡的膨胀脱离过程,对比分析了浸没方式、管径、气体速度、液体密度和表面张力对气泡膨胀脱离时间和脱离直径的影响规律。研究结果表明:在3种不同管口浸没方式下,气泡膨胀脱离时间均随管口气体速度增大而降低,但降低速率逐渐减小;在底部和顶部浸没方式下,气泡膨胀脱离时间和脱离直径随表面张力增大而增加,随液体密度增大而降低;对于侧面浸没方式,气泡膨胀脱离时间和脱离直径随表面张力增大整体呈上升趋势,而受液体密度影响不大。气泡膨胀脱离时间与气泡脱离直径呈正相关。所建立的数学模型能够真实地模拟气泡形成及运动特征,从而提供一个有效的研究方法。     

单管冻结温度场演变规律分析及试验验证

为了研究土中水相变（潜热）热传导过程中冻结温度场演变规律，构建了稳定线性冷源作用下单管冻结相变温度场模型，由变量替换法求得冻结温度场随冻结时间演变规律近似解析解，同时利用指数积分函数解析式表达了冻结区和未冻结区的温度场分布，并将理论解析解与冻结槽试验结果进行比对。研究表明：理论与试验结果误差在合理范围内，验证了瞬态冻结温度场分布近似解析解的有效可靠性；单管冻结锋面半径和冻结时间有着平方根关系；冻结温度场在时空维度上服从对数分布，且曲线曲率随冻结时间的增加而逐渐减小。研究成果对认识稳定线性冷源作用下冻结管周围不良地基土温度场演变规律和指导冻结法施工技术有重要意义。     

计算域中存在直立岛式结构物时波浪传播的数值模拟

针对计算域中存在直立岛式结构物的复连通区域,提出了岛体边界条件,利用缓坡方程建立了在计算域中存在直立岛式结构物时波浪传播的数值模拟模型。模型适用于变水深水域问题,克服了解析解只能适用于等水深水域的不足。直立圆柱和无穷小厚度岛式防波堤周围波浪传播的数值算例表明,数值解与解析解吻合良好,说明所建立的数值模拟模型能有效地模拟复连通区域内的波浪传播。矩形固定式防波堤周围波浪场的数值模拟说明该模型对工程实际有较强的适应性。     

Investigation of cavitation bubble collapse near rigid boundary by lattice Boltzmann method

The dynamics of the bubble collapse near a rigid boundary is a fundamental issue for the bubble collapse application and prevention. In this paper, the bubble collapse is modeled by adopting the lattice Boltzmann method(LBM) and is verified, and then the dynamic characteristics of the collapsing bubble with the second collapse is investigated. The widely used Shan-Chen model in the LBM multiphase community is modified by coupling with the Carnahan-Starling equation of state(C-S EOS) and the exact difference method(EDM) for the forcing term treatment. The simulation results of the bubble profile evolution by the LBM are in excellent agreements with the theoretical and experimental results. From the two-dimensional pressure field evolution, the dynamic characteristics of the different parts during the bubble collapse stage are studied. The role of the second collapse in the rigid boundary damage is discussed, and the impeding effect between two collapses is demonstrated.     

A HIGH-SPEED PHOTOGRAPHIC STUDY OF ULTRASONIC CAVITATION NEAR RIGID BOUNDARY

This article investigated an existing steady pattern of collapse and rebound （disintegration and aggregation） of cavitation bubbles near rigid boundary in acoustic field. A deformation process of cavitation bubble was accomplished in two acoustic cycles, namely, a spherical bubble collapsed towards the boundary to its minimum volume and then rebounded and grew into a toroidal bubble （or two individual bubbles） in one acoustic cycle, and the toroidal bubble （or two individual bubbles） collapsed towards the center of ring to its minimum volume, and then rebounded into a spherical bubble in the next acoustic cycle. Inertia force plays a key role in the transition between these two states. The microjet produced during the collapse of spherical bubble and the shock wave produced during the collapse of toroidal bubble （or two individual bubbles） impacts the boundary alternately. A cavitation bubble operating in this pattena can thus be an effective corrosion mechanism of rigid boundary.     

基于格子玻尔兹曼方法的近壁区空化泡生长溃灭过程模拟

基于格子玻尔兹曼热流耦合模型,兼顾温度场与流场的相互影响,对近壁区空化泡的生长和溃灭过程开展了系统研究,分析了泡壁无量纲距离和初始输入温度对空化泡演化过程和溃灭强度的影响,并对比了被动标量模型与热流耦合模型的差异。结果表明:空化泡受壁面限制,生长过程中无法保持圆形,其最大半径与泡壁无量纲距离之间存在幂函数关系,当泡壁无量纲距离大于1.6时,则为线性关系;空化泡最大半径与输入能量为线性关系,与初始输入无量纲温度之间存在幂函数关系;热流耦合模型考虑了高温对表面张力的影响,与被动标量模型模拟结果相比,其溃灭阶段获得的射流体积更大,微射流更集中,但溃灭强度更小。     

THE BUBMAC METHOD-A NUMERICAL METHOD BUBBLE DYNAMICS IN VORTEX CORE

This paper presents a numerical method called BUBMAC method,whichis based on the Marker And Cell(MAC) technique,to numerically simulate the problemof bubble dynamics in vortex core. With the incorporation of the azimuthal velocity intothe momentum equation, the model takes into account the complete interaction betweenthe bubble dynamics and the vortex flow field.The three momentum equations (N-Seqs.) are solved numerically by the finite-difference methed, and the motion of bubblesurface is described by tracing massless marker particles distributed only on the surface ofthe bubble. With some important medifications to the original MAC method, the numeri-cal accuracy of the method is conisderably increased. The validation of the proposed BUB-MAC method is checked by comparing the numerical results with some available analyticalsolutions in the cases of spherical bubble evolution and with numerical results in the casesof nonspherical collapse of bubble and bubble evolution in tip-vortex core. The compar-iso     

LEVEL SET METHOD FOR NUMERICAL SIMULATION OF A CAVITATION BUBBLE COLLAPSING NEAR A RIGID WALL

1.INTRODUCTION Collapseofthecavitationbubbleneararigi wallisaparticularlyimportantsubject,becausei affectstheperformanceofthehydraulicmachiner suchasthepropellersofnavalvesselandsubma rineetc.,producesnoiseandvibration,andcause materialdamageatthesurfaceofhydraulicma chinerybythehighvelocityandpressureresulte fromthatcollapse.Uptonow,itisdifficulttoin vestigatethewholecollapseevolutionofthebubbl withjetimpactonlybyanalyticalandtheoretica methods,andtheexperimentalornumericalmeth odshavebee…     

Numerical study of spike characteristics due to the motions of a non-spherical rebounding bubble

The boundary integral method(BIM) is used to simulate the 3-D gas bubble, generated within the two bubble pulsation periods in proximity to a free surface in an inviscid, incompressible and irrotational flow. The present method is well validated by comparing the calculated shapes of the bubble and the free surface with both the experimental results and the numerical ones obtained by the Axisymmetric BIM code. The expansion, the collapse of the gas bubble and the further evolution of the rebounding non-spherical bubble are simulated. The various variation patterns of the free surface spike and the bubble centroid for different standoff distances, the buoyancy parameters and the strength parameters are obtained to reveal the nonlinear interaction between the bubble and the free surface. The amplitude of the second maximum bubble volume and the four typical patterns of the bubble jet and the free surface spike are examined in the context of the standoff distance. The large buoyancy is used to elevate the spray dome rather than the free surface spike.     

基于伪势模型的表面张力对空化泡溃灭的影响

采用可调节表面张力的大密度比、大黏滞系数比格子玻尔兹曼伪势空化模型模拟了近壁区空化泡溃灭过程,并进一步分析了表面张力和初始空化泡内外压差对空化泡溃灭过程中流场分布的影响,探究了表面张力变化对空化泡溃灭时产生的微射流和溃灭压力演化的影响。结果表明:在近壁区空化泡溃灭过程中,随着表面张力减小,加剧了气液交界面的变形,导致微射流更为集中。同时空化泡在溃灭过程中蓄积的表面能减小,在溃灭时刻迅速释放后,减弱了空化泡溃灭强度,空化泡溃灭最大微射流流速和最大压力均随着表面张力的减小而减小,导致空化泡溃灭时间缩短,增加了壁面引起的Bjerknes力,加剧空化泡朝向壁面溃灭的趋势。     

空泡在固壁附近潰灭的数值模拟

实验表明,静止流场中的近壁空泡在泡内外压差的驱动下将发生变形收缩,直至完全溃灭消失。在这一过程中,伴随着空泡形状的变化,流场中出现多个高压脉冲和高速射流。本文通过直接求解原始变量的N-S方程,用VOF方法计算气液两相交界面的运动过程,对这一现象进行了数值模拟和分析。结果表明理论分析和实验现象相吻合。     

气力提升管内气泡形变的实验研究

气泡的形变作为气力提升装置的主要影响因素,其形变规律直接影响气力提升装置的提升性能。为此,该文中运用高速摄像仪拍摄了气液两相流流场结构,基于图像处理获取了气泡运动过程中的形状参数,通过气泡匹配技术得到了气泡的运动速度。实验结果表明,改变运行参数会引起气泡碰撞和融合发生机率的变化,进而影响到气泡初始生成体积;气泡运动速度的周期性变化会改变气泡的直径和变形,进而改变管内流型,最终导致气力提升装置的提升性能发生变化。     

High-speed experimental photography of collapsing cavitation bubble between a spherical particle and a rigid wall

The synergetic effects between cavitation bubbles and silt particles on the damages of materials are essential problems in fluid machineries. For studying the underlying microscopic mechanisms, in the present paper, the dynamic behaviors of a single cavitation bubble between a spherical particle and a rigid wall are experimentally investigated with a high-speed camera. The results indicate that the existence of the particle can affect the bubble shape during collapse and significantly accelerate the collapse velocity of the bubble. The influences of the particle size, the distance between the bubble and the particle and the distance between the bubble and the rigid wall on the phenomena are qualitatively and quantitatively analyzed. These parameters can prominently affect the collapse velocity of the bubble(especially its maximum value).