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。     

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。     

含沙静止液体中双空泡溃灭时的动力特性

本文采用脉动压力传感器及高频频谱分析仪,实测了含有塑料沙甘油水溶液中双空泡溃灭时,作用于固体边壁上的压强及其频谱。结果发现:静止液体中含沙可使作用于固体边壁上的空泡溃灭压强较不含沙时减少20—60％,并且存在一个使溃灭压强为最大的临界含沙浓度。液体中的含沙使空泡溃灭时产生的压力波振荡延续更长的时间,并使压力波的主频率发生变化。当泡间产生微射流时,作用在边壁上的压强较无微射流时为大。     

滨海软土黏滞系数非线性特性及其表达

通过对滨海软土三轴蠕变相关试验数据进行分析,发现黏滞系数不仅与时间有关,还与土所受到的偏压和围压有关。应变率与时间的关系呈现了非常明显的双对数线性关系,因此,黏滞系数是关于时间的幂函数;黏滞系数线性地依赖于偏压的变化,而与围压呈指数关系。最后建立了黏滞系数与时间、偏应力、围压之间的非线性关系表达式,并探讨了其相关参数的意义及取值范围。建立的黏滞系数表达式适用于描述非线性特征明显、应力水平较高的软土黏滞系数随时间、应力的变化规律,可推广到滨海软土直剪蠕变试验中。研究成果可为建立变黏滞系数的滨海软土及其他地区软土的非线性流变模型奠定基础。     

颗粒对空化泡溃灭行为影响的实验研究

本文基于高速摄影实验平台对空化泡在两个球形颗粒之间的溃灭和射流等动力学行为进行了详细的实验研究。实验过程中,单个空化泡由输出能量可精确控制的高能量激光发生器通过聚焦透镜在水中产生,两个球形颗粒与空化泡之间的相对位置由高精度三维位移平台进行精确控制,并采用高速摄影机对空化泡在两个球形颗粒间的生长、溃灭、射流及后续的再次溃灭等完整过程进行详细记录。通过对实验数据的详细分析,发现球形颗粒之间的间距与空化泡的尺寸是影响空化泡动力学行为的主要因素。     

自由液面附近空化泡的溃灭特性

研究自由液面附近空化泡的溃灭特性对认识水利水电工程中的空蚀破坏机理具有重要意义。为进一步认识自由液面附近空化泡的溃灭特性,开展了电火花激发空化泡试验。结果表明:(1)空化泡与自由液面距离较近时总是背离自由液面溃灭,背离的临界条件为空化泡与自由液面的距离约是空化泡最大半径的5倍。空化泡尺寸越大或空化泡与自由液面距离越近,其背离现象越明显。(2)自由液面附近双空化泡相向溃灭融合,且融合体背离自由液面移动。(3)在自由液面和空气泡的共同影响下,空化泡的溃灭方向是二者分别对空化泡作用的矢量和。通过本文研究,对自由液面附近空化泡的溃灭有了定量认识,可以在一定条件下预测空化泡的溃灭方向。     

润湿性微纳米圆管中去离子水的流动特征

在传统的流体力学中,流体在固壁处流速为零,即无滑移边界条件。在微纳米条件下,由于固液界面作用力增大,无滑移边界条件不再适用。该文首先从微米石英圆管中的流动实验现象出发,给出润湿性边界黏滞层厚度随应力变化的边界模型。在此基础上推导出层流条件下,微纳米管中的流量公式,并对流量公式进行渐近分析,同时也分析流动微纳米尺度下的流动特征。最后用微米圆管中去离子水的流动实验,对新建的理论模型进行验证。研究表明:润湿性条件下,边界黏滞层厚度随驱动力的增大而减小;新建立的微纳米尺度下的边界模型和流量公式非常符合实验结果,其中一次方的流量模型应用时更为简单,在压力梯度较小时,指数项起作用产生了非线性流动的效果。     

液流中边壁附近的空泡溃灭研究

本文利用高速摄影方法观测了液流中的空泡溃灭过程,分析了空泡在流动系统中溃灭时的泡壁稳定性,和由于空泡射流挟带其表面分裂出的微小泡冲击边壁,从而使微小泡在高压强作用下溃灭,造成壁面破坏的可能性。本文还研究了液体粘性变化和液体挟带颗粒对空泡微射流速度及溃灭历时的影响     

空化与空蚀研究的现状和动向

空化现象是液体从液相变成气相的相变过程。当液体的温度不变而压力降低时,液体内形成空穴。空穴在流场中初生、发育、溃灭与再生,循环不已。这一过程称为空化。在学科性质上属于两相流范畴。这是一种瞬息变化的随机过程,其瞬态微观现象是十分复杂的。空化与空蚀的研究已有近百年的历史。首先     

不同空泡脱落模式下空化多尺度特性研究

云状空化是一种以相变为核心,包含多种尺度空泡生长演变行为的强瞬态多相流动现象。该文采用双向耦合欧拉-拉格朗日多尺度空化模型,模拟了空化数σ=1.2时,绕8°攻角三维NACA66水翼空化流动,通过VOF(volume of fluid)界面捕捉法求解欧拉体系下大尺度空穴演化,利用拉格朗日体系下的离散空泡模型(DBM)追踪亚网格尺度离散空泡运动及生长溃灭过程。结果表明：绕水翼云状空化流动,包含附着型片状空泡生长与发展、回射流产生与推进、云状空泡脱落和宏观片空泡再次生长以及压力波推进4个阶段。离散空泡数量及尺度随大尺度空穴周期性演变而变化,主要分布在强湍流脉动区。在各空化发展阶段,离散空泡集中分布在水翼中后部,微空泡直径概率密度函数均符合单峰Gamma分布,最高空泡数密度泡径为50μm。在湍动程度较小的附着型空穴生长阶段,微空泡数密度相对于空泡直径呈-5/3幂率特征;在湍动程度较大的回射流发展阶段、云状空穴脱落溃灭及压力波推进阶段,较小尺度的微空泡数密度相对于空泡直径呈-2/3幂率特征,较大尺度的微空泡数密度相对于空泡直径呈-6幂率特征。     

VIBRATION OF A SINGLE PROTEIN BUBBLE IN BINGHAM LIQUID

The vibration of a single protein bubble may take place under the action of high pressure difference. In this process, the bubble wall may experience a finite deformation. The equation describing the dynamics of the protein bubble with viscoelastic film in Bingham liquid is derived. A numerical solution to this equation is carried out to study the effect of liquid pressure, the characteristic parameters of Bingham liquid and the viscosity of the protein film on the finite deformation of the bubble. The results show that the vibration of the protein bubble wall is caused by the action of pressure difference, the elastic stress in finite deformation and the dissipation of viscosity of the protein film and Bingham liquid. The vibration is nonlinear. Decreasing the pressure difference between gas and Bingham liquid on both sides of the protein bubble will lead to a change of vibration performance. The frequency and amplitude are reduced, tegether with the speed of vibration damping. In addition, the deformation rate of the bubble is smaller when the amplitude of vibration is reduced, which means shorter time to reach a balance state. On the other side, the increase of the magnitude of viscosity of the protein film or the plastic viscosity of Bingham liquid can restrain the vibration of the protein bubble wall in the course of finite deformation, as a result, the load bearing capacity of bubble is enhanced.     

虹吸管气液两相流压降特性试验

通过系列试验量测了不同安装高度、不同水位差时虹吸管水平管段的压降、含气率及过流量,探讨和分析了虹吸管气液两相流压降的变化规律及气液两相流流型不同时影响管道压降的因素。结果表明,气液两相流管道压降与液相满流时压降规律相同,即压降值随管道水头的增大而增大;但是,与液相有压管流不同,水位差一定时压降值随安装高度的增大而减小。当虹吸管内为气泡流时,气泡存在对沿程阻力系数λ影响很小,可忽略不计。管道实测压降小于计算值的原因是气液两相压降减小率等于流速减小率。当虹吸管内为过渡流和气团流时,气液两相压降减小率与流速减小率相差较大,实测压降的减小不仅与流速减小有关,含气率的大小对气液两相压降的影响也不可忽略,含气率的增大使气液两相流动阻力增大,即压降增大。     

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).     

THE DYNAMIC EQUATION FOR A SPHERICAL BUBBLE IN UNIFORM SIZED SOLID PARTICLE-LIQUID HOMOGENEOUS SUSPENSION

The momentum equation for solid-liquid two-phase flow proposed by Elghobashi and Abou-Arab is taken as the governing equations for the liquid phase,and the Lagrangian dynamic equation for a spherical particle moving in the liquid flow as the basic equation for the relative motion of the particle in the liquid phase in the paper.The dynamics equation for a bubble in solid-liquid twophase flow is proposed,as well as the simplified equations under two extreme conditions with the relationship between the bubble collapse characteristic time τ_C~* and particle response time τ_(rel)~* being τ_C~*>> τ_(rel)~* and τ_C~* <<τ_(rel)~*.     

水沙动力学适用牛顿流体的基本方程

通过改进蔡树棠两相流基本方程的相间阻力项,建立能够进行二维解析研究的水沙动力学基本方程.该研究给出沙相压力和沙相粘性切应力的学科定义,以及它们的本构关系.对于牛顿流体,当输沙稳定时,挟沙水流的能坡与清水并无差别.沙相压力的二阶导数不为零是泥沙扩散条件,也是凹岸冲、凸岸淤自然现象的力学本质.蔡树棠方程隐含着流体的粘性切应力按运动组成可以分解或叠加的物理性质.正是这个结构性特点,给了解答浑水粘性这个古老的力学难题的方法,使我们能够完整地推导出牛顿流体沙相运动学粘滞系数vs公式;推导并证明了浑水综合粘滞系数μm=εv+ svνs,而不是习惯使用的μm=v+svvs.探讨了影响vs值的一些复杂因素,如颗粒表面束缚水的影响.本文研究能够证明:对分散相应用连续介质原理不仅可行,且是遵从压力传递规律惟一的正确方法.而揭示粘性切应力的结构性,在多相流和流变学领域具有深远的意义.     

Numerical analysis of bubble dynamics in the diffuser of a jet pump under variable ambient pressure

Recent studies have shown that the collapse of cavitation bubbles in a jet pump can generate an extremely high pressure with many potential applications. The dynamics of the bubble is governed by the Rayleigh-Plesset equation. With the bubble dynamics equation and the heat and mass transfer model solved with the Runge-Kutta fourth order adaptive step size method, the oscillations of the bubble in the diffuser of the jet pump are assessed under varied conditions. To obtain the pressure variation along the diffuser, the Bernoulli equation and the pressure measured in experiment are coupled. The results of simulation show that a transient motion of the bubbles can be obtained in the diffuser quantitatively, to obtain the pressure and temperature shock in the bubble. Moreover, increasing the outlet pressure coefficient would result in a more intense bubble collapsing process, which can be used in the subsequent studies of the cavitation applications. The predictions are compared with experiments with good agreement.     

考虑浆体黏度的泥石流流速计算方法

泥石流浆体黏度通过影响泥石流内、外部的阻力特征影响泥石流的糙率系数,泥石流糙率系数与浆体黏度之间的关系还没有基于观测数据的定量表述。通过对云南东川蒋家沟泥石流观测资料的分析,研究泥石流糙率系数与浆体黏度之间的关系,并通过回归分析得到考虑浆体黏度的泥石流糙率系数计算公式,最后基于此构建了考虑浆体黏度的泥石流流速计算公式。结果表明,随着泥石流浆体黏度的增加,泥石流糙率系数逐渐增加,浆体黏度在宏观上表现为增阻作用,考虑浆体黏度的泥石流流速计算公式改进了现有公式在计算浆体黏度较高的泥石流流速中的不足。研究结果可为进一步研究浆体黏度对泥石流运动阻力的影响提供参考,也为工程实践中的流速计算和增阻消能设计提供新的思路。     

EXPERIMENTAL STUDY OF EFFECTS OF AIR CONTENT ON CAVITATION AND PRESSURE FLUCTUATIONS

This article studies the effects of air content on propeller cavitation and pressure fluctuations. The cavitation is observed while the pressure fluctuations on the hull are measured. When adjusting the air content, the sheet cavitation range does not change distinctly, but the pressure fluctuations see obvious differences. The amplitudes of the pressure fluctuations increase with the decrease of the air content. The results indicate that the air content has little effect on the sheet cavitation range but has an important effect on the bubble cavitation and the tip vortex cavitation. When the air content decreases, the water tensile force increases, which results in the instability of the bubble cavitation and the tip vortex cavitation and the increase of the pressure fluctuations. To minimize the scale effects, the experiments should be run at a high Reynolds number with a high nuclei content. The high Reynolds number is often realized by increasing the flow velocity and the propeller rotation speed, and the high nuclei content is often made by increasing the dissolved air content.