近固壁微米尺度空泡溃灭的数值研究

考虑空泡表面张力、液体黏性和气体可压缩性,采用VOF多相流模型对近固壁微米尺度空泡在静止流场溃灭过程进行了数值研究.获得了近固壁空泡溃灭过程的流场细节,分析了空泡与固壁的无量纲距离γ对空泡溃灭过程动力特性的影响,并揭示了不同γ条件下的固壁空蚀破坏机理.计算结果表明:随着γ的减小,泡心向固壁移动的趋势明显,射流形成前空泡上部高压区内压力减小,空泡溃灭时间延长,最大射流速度减小.模拟结果验证了空泡溃灭将产生冲击波和高速微射流,二者均会在固壁面产生脉冲压力,其是造成壁面损伤的两种主要原因.参数γ对固壁的空蚀破坏机理有重要影响.与微射流机制相比,以冲击波机制为主的空蚀破坏更显著.微射流冲击固壁的作用半径为10μm左右,将引起固壁"点"蚀坑的出现.当γ=2.0时,冲击波扫掠壁面的范围相对较广,有效作用半径约为1 mm,其导致固壁产生较大圆形蚀坑,且中心空蚀严重.     

INVESTIGATION OF BUBBLE-BUBBLE INTERACTION EFFECT DURING THE COLLAPSE OF MULTI-BUBBLE SYSTEM

通过直接数值模拟方法对多泡在压力驱动下的溃灭过程进行了研究。气相满足理想气体正压模型,液相为不可压 流体,采用基于压力的方法求解多泡的两相流场。数值研究表明,在多泡流场中,中心气泡的溃灭过程明显不同于单泡,存在总体溃灭延迟现象和后期加速现象。随着周围气泡数的增多或气泡间距的减小,中心气泡的溃灭时间增长,溃灭时的压力峰值增大。结合理论定性分析发现,气泡运动不仅受远场压力的驱动,还受周围气泡诱导压力场的影响。周围气泡的诱导压力经历先减小后增大的过程,从而使受其影响的中心气泡产生先延迟后加速的特征。     

Fluent环境中近壁面微空泡溃灭的仿真计算

基于FLUENT软件环境,采用VOF模型和非稳态方法求解Navier-Stokes方程,模拟了近壁面的空泡溃灭过程,同时计算了空泡溃灭处与壁面的距离对射流强度的影响.结果表明：在近壁面,空泡将形成非对称溃灭,因水锤作用,引发高速水射流在壁面产生高压而形成空蚀破坏;基于FLUENT环境的计算结果与已有的实验和计算结果相符,为研究空泡溃灭和空蚀机制提供了类比的数值计算方法.     

水中高压脉动气泡水射流形成机理及载荷特性研究

具有脉动特性的气泡(如水下爆炸气泡、螺旋桨空泡和气枪气泡)动力学行为很大程度上取决于其边界条件. 实验已证实,近自由液面气泡在坍塌过程中常常产生背离自由液面的水射流现象,而近刚性边界气泡在坍塌阶段产生朝向壁面的高速水射流,严重威胁水中结构的局部强度. 前人基于 Rayleigh-Plesset 气泡理论和 “Bjerknes” 力来预测气泡射流方向,然而理论方法难以透彻的揭示气泡射流的初生、发展和砰击过程中丰富的力学机理. 本文首先采用水下高压放电技术产生气泡,并通过高速摄影对不同边界条件下气泡的运动特性进行实验研究. 然后,采用边界积分法模拟气泡非球状坍塌过程. 研究表明,边界条件改变了气泡周围的流场压力梯度方向,进而影响气泡射流初生位置;射流在发展阶段,气泡附近流场的局部高压区和射流之间存在“正反馈效应”,从而揭示了气泡射流速度在短时间内即可增加到百米每秒的力学机理. 射流砰击会在流场中造成局部高压区,随着气泡回弹,射流速度和砰击压力逐渐减小. 本文还探讨了无量纲距离参数对气泡运动及射流砰击载荷的影响,旨为近场水下爆炸等相关领域提供参考.      

水中高压脉动气泡水射流形成机理及载荷特性研究

具有脉动特性的气泡(如水下爆炸气泡、螺旋桨空泡和气枪气泡)动力学行为很大程度上取决于其边界条件.实验已证实,近自由液面气泡在坍塌过程中常常产生背离自由液面的水射流现象,而近刚性边界气泡在坍塌阶段产生朝向壁面的高速水射流,严重威胁水中结构的局部强度.前人基于Rayleigh-Plesset气泡理论和"Bjerknes"力来预测气泡射流方向,然而理论方法难以透彻的揭示气泡射流的初生、发展和砰击过程中丰富的力学机理.本文首先采用水下高压放电技术产生气泡,并通过高速摄影对不同边界条件下气泡的运动特性进行实验研究.然后,采用边界积分法模拟气泡非球状坍塌过程.研究表明,边界条件改变了气泡周围的流场压力梯度方向,进而影响气泡射流初生位置;射流在发展阶段,气泡附近流场的局部高压区和射流之间存在"正反馈效应",从而揭示了气泡射流速度在短时间内即可增加到百米每秒的力学机理.射流砰击会在流场中造成局部高压区,随着气泡回弹,射流速度和砰击压力逐渐减小.本文还探讨了无量纲距离参数对气泡运动及射流砰击载荷的影响,旨为近场水下爆炸等相关领域提供参考.     

固壁空蚀数值研究

空蚀是空泡在固壁附近溃灭对固壁材料产生破坏的现象。本文将空泡界面假设为自由面，并由VOF(Volume of Fluid)中界面构造精度较高的Youngs方法求解，通过直接计算原始变量的Navier-Stokes方程，数值模拟了空泡距固壁不同位置时溃灭对固壁造成的空蚀破坏。计算发现空泡溃灭产生高压脉冲相对于高速射流对空蚀形成起主导作用；空泡在流场中位置不同，高压脉冲对固壁上的空蚀破坏结果不同，并给出了距离界限。     

淹没磨料射流的空泡运动分析

通过对淹没条件下磨料射流的空泡运动分析研究,建立了淹没磨料射流的空泡运动方程,揭示了淹没磨料射流中空泡的溃灭特性,数值模拟了淹没磨料射流的磨料体积浓度以及空泡所处流场压力对空泡运动及溃灭的影响规律.分析表明:淹没磨料射流中磨料的存在增大流体的粘性系数,增大空泡溃灭历时,减弱射流的空蚀破坏能力;流场压力的改变对空泡溃灭过程影响显著,压力越高,空泡溃灭历时越短.     

喷管对水下爆轰气泡形态和压力特征影响的实验研究

针对具有不同类型喷管的爆轰管在水下爆轰中形成的燃气射流问题,搭建了爆轰实验平台,研究了单次爆轰过程中尾部喷管对水下气泡形态与压力特征的影响。采用数字粒子图像测速技术对高速摄影机拍摄得到的气泡脉动图片进行流场可视化分析,得到各喷管工况下的气泡速度场。为了确认爆轰管内是否形成稳定爆轰波,并观察爆轰波在气液两相界面上的透反射特性,爆轰管尾部安装有2个动态压力传感器,同时在距离喷管一定距离处设置一个水下爆炸传感器,以监测水中传播的压力波。结果表明：扩张喷管工况下的气泡脉动过程与直喷管工况基本一致,但扩张喷管提高了燃气射流速度,气泡膨胀体积更大;因为燃气射流的持续性,收敛喷管工况下的气泡脉动过程具有明显差异,气泡膨胀体积较小,但气泡二次脉动时长相较于一次脉动时长衰减更小;扩张喷管提高了气泡脉动强度,扩张喷管工况下的气泡脉动压力与透射冲击波压力远大于直喷管工况下的气泡脉动压力与透射冲击波压力;收敛喷管工况下的气泡脉动压力与透射冲击波压力都较小,但收敛喷管燃气射流的持续性减缓了气泡脉动压力的衰减速度。相比于直喷管,扩张喷管工况下的气泡脉动时间、气泡脉动压力与透射冲击波压力都更大。收敛喷管工况下的气泡...     

垂直壁面附近上升单气泡的弹跳动力学研究

采用高速摄影技术结合阴影法,对静止水中垂直壁面附近上升单气泡运动进行实验研究,对比气泡尺度及气泡喷嘴与壁面之间的初始无量纲距离(S~*)对气泡上升运动特性的影响,分析气泡与壁面碰撞前后,壁面效应与气泡动力学机制及能量变化规律.结果表明,对于雷诺数Re≈580～1100,无量纲距离S~*2～3时,气泡与壁面碰撞且气泡轨迹由无约束条件下的三维螺旋转变成二维之字形周期运动;当S~* 2～3时,壁面效应减弱,有壁面约束的气泡运动与无约束气泡运动特性趋于一致.气泡与壁面碰撞前后,壁面效应导致横向速度峰值下降为原峰值的70%,垂直速度下降50%;气泡与壁面碰撞前,通过气泡中心与壁面距离(x/R)和修正的斯托克斯数相关式可预测垂直速度的变化规律.上升气泡与壁面碰撞过程中,气泡表面变形能量单向传输给气泡横向动能,使得可变形气泡能够保持相对恒定的弹跳运动.提出了气泡在与壁面反复弹跳时的平均阻力系数的预测模型,能够很好地描述实验数据反映出的对雷诺数Re、韦伯数We和奥特沃斯数Eo等各无量纲参数的标度规律.     

垂直壁面附近上升单气泡的弹跳动力学研究

采用高速摄影技术结合阴影法,对静止水中垂直壁面附近上升单气泡运动进行实验研究,对比气泡尺度及气泡喷嘴与壁面之间的初始无量纲距离(S^*)对气泡上升运动特性的影响,分析气泡与壁面碰撞前后,壁面效应与气泡动力学机制及能量变化规律.结果表明,对于雷诺数Re≈580～1100,无量纲距离S^*<2～3时,气泡与壁面碰撞且气泡轨迹由无约束条件下的三维螺旋转变成二维之字形周期运动;当S^*> 2～3时,壁面效应减弱,有壁面约束的气泡运动与无约束气泡运动特性趋于一致.气泡与壁面碰撞前后,壁面效应导致横向速度峰值下降为原峰值的70%,垂直速度下降50%;气泡与壁面碰撞前,通过气泡中心与壁面距离(x/R)和修正的斯托克斯数相关式可预测垂直速度的变化规律.上升气泡与壁面碰撞过程中,气泡表面变形能量单向传输给气泡横向动能,使得可变形气泡能够保持相对恒定的弹跳运动.提出了气泡在与壁面反复弹跳时的平均阻力系数的预测模型,能够很好地描述实验数据反映出的对雷诺数Re、韦伯数We和奥特沃斯数Eo等各无量纲参数的标度规律.     

Shock-induced collapse of a vapor nanobubble near solid boundaries

The collapse of a nano-bubble near a solid wall is addressed here exploiting a phase field model recently used to describe the process in free space. Bubble collapse is triggered by a normal shock wave in the liquid. The dynamics is explored for different bubble wall normal distances and triggering shock intensities. Overall the dynamics is characterized by a sequence of collapses and rebounds of the pure vapor bubble accompanied by the emission of shock waves in the liquid. The shocks are reflected by the wall to impinge back on the re-expanding bubble. The presence of the wall and the impinging shock wave break the symmetry of the system, leading, for sufficiently strong intensity of the incoming shock wave, to the poration of the bubble and the formation of an annular structure and a liquid jet. Intense peaks of pressure and temperatures are found also at the wall, confirming that the strong localized loading combined with the jet impinging the wall is a potential source of substrate damage induced by the cavitation.     

Characteristics of impact-driven high-speed liquid jets in water

This paper describes a preliminary investigation of the characteristics of high-speed water jets injected into water from an orifice. The high-speed jets were generated by the impact of a projectile launched by a horizontal single-stage powder gun and submerged in a water test chamber. The ensuing impact-driven high-speed water jets in the water were visualized by the shadowgraph technique, and the images were recorded by a high-speed digital video camera. The processes following such jet injection into water, the jet-induced shock waves, shock wave propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud re-generation were observed. Peak over-pressures of about 24 and 35 GPa measured by a Polyvinylidence difluoride (PVDF) piezoelectric film pressure sensor were generated by the jet impingement and the bubble impingement, respectively. The peak over-pressure was found to decrease exponentially as the stand-off distance between the PVDF pressure sensor and the nozzle exit increases.     

On the pressure of cavitation bubbles

Shock wave emission upon the collapse of a cavitation bubble attached to a rigid wall is investigated using high-speed photography with 200 million frames/s and 5 ns exposure time. At a distance of 68 μm from the bubble wall, the shock pressure is 1.3 ± 0.3 GPa. The shock pressure decays proportionally to r^−1.5 with increasing distance from the bubble. An estimation of the peak pressure at the bubble wall reveals a pressure of about 8 GPa. A major part of the shock wave energy is dissipated within the first 100 μm from the bubble wall.     

The Peculiar Dynamics of Cavitation Bubbles

Details from cavitation bubble dynamics are reported: jet formation, counterjet formation, shock wave radiation and light emission. Multiple shock wave radiation from single bubble collapse with jet formation could be time resolved by high speed photography with 20 million frames per second. An explanation of counterjet formation is given. Pictures of the light emission (sonoluminescence) in acoustic cavitation are presented.     

Air entrapment during a high-speed liquid jet entry in a deep water surface

The fluid dynamic phenomena of a high speed liquid jet impact on a deep water surface have been studied using Imacon high-speed photography. Both framing and streak techniques are applied to investigate the initial impact stage and penetration velocity. The cavitation caused by air entrapment between two colliding liquid surfaces has been found. The bubble collapse experiences different stages in relation to the contact area, liquid shock wave, release wave and fluid convection.     

An experimental study of liquid jets formed by bubble-shock wave interaction in dilute polymer solutions

 This paper describes experiments in which a shock wave, emanating from the collapse of a cavitation bubble, causes a second bubble to collapse, thereby producing a liquid jet. A comparison of the jets formed by bubble collapse in dilute solutions of polyacrylamide and polyethylene oxide, and in their Newtonian counterparts, shows that in the polymer solutions liquid jet development is markedly suppressed. The implications of these findings are discussed in the context of cavitation damage. Received: 3 September 1998/Accepted: 23 July 1999     

Vortex ring modelling of toroidal bubbles

During the collapse of a bubble near a surface, a high-speed liquid jet often forms and subsequently impacts upon the opposite bubble surface. The jet impact transforms the originally singly-connected bubble to a toroidal bubble, and generates circulation in the flow around it. A toroidal bubble simulation is presented by introducing a vortex ring seeded inside the bubble torus to account for the circulation. The velocity potential is then decomposed into the potential of the vortex ring and a remnant potential. Because the remnant potential is continuous and satisfies the Laplace equation, it can be modelled by the boundary-integral method, and this circumvents an explicit domain cut and associated numerical treatment. The method is applied to study the collapse of gas bubbles in the vicinity of a rigid wall. Good agreement is found with the results of Best (J. Fluid Mech. 251 79–107, 1993), obtained by a domain cut method. Examination of the pressure impulse on the wall during jet impact indicates that the high-speed liquid jet has a significant potential for causing damage to a surface. There appears to be an optimal initial distance where the liquid jet is most damaging.     

The Art, Craft and Science of Modelling Jet Impact in a Collapsing Cavitation Bubble

One of the key characteristics of the asymmetric collapse of a cavitation bubble near a rigid boundary is the development of a high speed liquid jet that penetrates the interior of the bubble, impacting on the other side to yield a toroidal bubble. After the formation of the toroidal bubble, a vigorous splash may occur that can lead to pressures on the boundary an order of magnitude greater than the impact pressures associated with the jet. Qualitative agreement with available experimental data is found although, as the bubble approaches minimum volume, shock waves are also observed which further complicate our full understanding of the mechanisms for damage.     

水下爆炸中气泡射流壁压特性实验研究

水下爆炸气泡射流载荷是中近场水下爆炸壁压载荷的重要组成部分, 将水下爆炸气泡射流简化为一段高速水柱来研究水下爆炸气泡射流载荷特性是研究水下爆炸气泡射流载荷的主要手段。本文基于腔内爆炸提出了一种新的高速水射流实验方法,并给出了实验装置设计、实验方法以及实验系统。基于实验系统,开展了不同工况下高速水射流的实验研究,研究了腔口位置、腔深对水射流形态的影响,并对水射流的形态形成因素进行了分析。使用压电型壁压传感器测得了水射流冲击壁压,给出了水射流冲击壁压的特性及其特点。实验结果表明:腔口位置与腔深是影响水射流端面形态的重要因素;生成的高速水射流冲击壁压峰值满足水锤理论。基于腔内爆炸的高速水射流实验方法能够应用于包括水下爆炸气泡射流在内的高速水射流形态、壁压特性的研究。