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。     

THE COUNTER-JET FORMATION IN AN AIR BUBBLE INDUCED BY THE IMPACT OF SHOCK WAVES

The interaction of an air bubble (isolated in water or attached to a boundary) with shock waves induced by electric sparks is investigated by high-speed photography.The interaction is closely related to the counter-jet induced by the impact of shock waves.The formation of a counter-jet in an air bubble is related to the liquid jet formed in the same air bubble,but the mechanism is different with that of the counter-jet formation in a collapsing cavitation bubble.The formation of a counter-jet in an air bubb...     

Experimental study of the interaction between the spark-induced cavitation bubble and the air bubble

Experiments are carried out by using high-speed photography to investigate the interaction between the spark-generated cavitation bubble and the air bubble in its surrounding fluid. Three problems are discussed in detail： the impact of the air bubble upon the development of the cavitation bubble, the evolution of the air bubble under the influence of the cavitation bubble, and the change of the fluid pressure during the development of a micro jet of the cavitation bubble. Based on the experimental results, under the condition of no air bubble present, the lifetime of the cavitation bubble from expansion to contraction increases with the increase of the maximum radius. On the other hand, when there is an air bubble present, different sized cavitation bubbles have similarity with one another generally in terms of the lifetime from expansion to contraction, which does not depend on the maximum radius. Also, with the presence of an air bubble, the lifetime of the smaller cavitation bubble is extended while that of the bigger ones reduced. Furthermore, it is shown in the experiment that the low pressure formed in the opposite direction to the cavitation bubble micro jet makes the air bubble in the low pressure area being stretched into a steplike shape.     

EFFECT OF A PROPELLER AND GAS DIFFUSION ON BUBBLE NUCLEI DISTRIBUTION IN A LIQUID

A multi-bubble dynamics code accounting for gas diffusion in the liquid and through the bubble wall was developed and used to study the modification of a bubble nuclei population dynamics by a propeller.The propeller flow field was obtained using a Reynolds-Averaged Navier-Stokes(RANS) solver and bubble nuclei populations were propagated in this field.The numerical pro-cedure enabled establishment of the possibility of production behind the propeller of relatively large visible bubbles starting from typical ocean nuclei size distributions.The resulting larger bubbles are seen to cluster in the blade wakes and tip vortices.Parametric investigations of the initial nuclei size distribution,the dissolved gas concentration,and the cavitation number were conducted to ide-ntify their effects on bubble entrainment and the resultant void fractions and bubble distribution modifications downstream from the propeller.Imposed synthetic turbulence-like fluctuations unto the average RANS flow field were also used to study the effect avera-ging in the RANS procedure has on the results.     

NUMERICAL SIMULATION OF BUBBLE FLOW INTERACTIONS

Bubble flow interaction can be important in many practical engineering applications. For instance, cavitation is a problem of interaction between nuclei and local pressure field variations including turbulent oscillations and large scale pressure variations. Various types of behaviours fundamentally depend on the relative sizes of the nuclei and the length scales of the pressure variations as well as the relative importance of bubble natural periods of oscillation and the characteristic time of the field pressure variations. Similarly, bubbles can significantly affect the performance of lifting devices or propulsors. We present here some fundamental numerical studies of bubble dynamics and deformation, then a practical method using a multi-bubble Surface Averaged Pressure (DF-Multi-SAP©) to simulate cavitation inception and scaling, and connect this with more precise 3-D simulations. This same method is then extended to the study of two-way coupling between a viscous compressible flow and a bubble population in the flow field.     

掺气水流的气泡尺寸检测

近年研究表明,高速水流泄水建筑物的掺气减蚀效果,不仅与掺气浓度有关,还应考虑气泡密度;认为0.2 mm或0.5 mm以下的微小气泡在掺气减蚀中起主要作用,可能只要很小掺气浓度即可达到掺气减蚀效果。因此在掺气减蚀研究中,必须加强对气泡尺寸的检测。分析总结了针式掺气流速仪应用经验,建议气泡当量尺寸直接采用探针检测得出的中值尺寸d50。虽然预计d50、气泡间距比实际偏大,气泡密度偏小,但是测量成果对于研究掺气减蚀机理,制定掺气减蚀的新标准,有重大意义。     

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.     

THE MOTION OF A BUBBLE IN WAVES

The motion of a single spherical bubble due to buoyancy in the ideal fluid with waves is studied in this article. Assuming that the bubble has no effect on the wave field, equations of a bubble motion are attained. The equations are solved. It is found that the nonlinear effect increases with the increase of the bubble radius. Since the difference between the frequencies of gravity waves and the eigenfrequency of bubbles is great for small bubbles, their oscillations can not be coupled. During the rising of a bubble, the oscillation of the bubble decays very fast, so it can be ignored. The rising time and the motion orbit are given. When the wave is high and the bubble is small, the spiral orbit may appear.     

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。     

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).     

掺气减蚀研究的新方向

利用针式掺气流速仪测量掺气浓度场、流速场，气泡尺寸及其概率分布的研究成果表明，原型水流韦伯数高，形成微小气泡的能力比模型强，气泡上浮慢，接近底部的小尺寸气泡概率及掺气浓度比模型大，0．2mm或0．5mm以下的微小气泡可能在掺气减蚀中起着主要作用，完善掺气检测仪器，加强掺气水流中气泡尺寸及其概率分布的观测，比较不同气泡尺寸的掺气减蚀作用，建立多级气泡尺寸的掺气水流数学模型，是今后掺气减蚀研究的新方向。     

掺气减蚀保护作用的新概念

利用针式掺气流速仪测量原型和模型掺气浓度场、流速场，气泡尺寸及其概率分布的研究成果表明，原型水流韦伯数高，形成微小气泡的能力比模型强，气泡上浮慢，接近底部的小尺寸气泡概率及掺气浓度比模型大。初步研究认为：0.2mm或0.5mm以下的微小气泡在掺气减蚀中起着主要作用，可能只要很小掺气浓度即可达到掺气减蚀的效果。因此以小尺寸气泡的掺气浓度，作为判断掺气减蚀保护作用的指标将更为准确。     

EXPERIMENTAL INVESTIGATIONS ON BUBBLE COLLAPSE NEAR BOUNDARIES

Collapse of a spark-generated bubble near rigid or deformable surfaces is studied experimentally with a high speed cammera. The formation of a bubble wall jet has been observed and discussed. Results from experimental studies on the effect of liquid viscosity on bubble collapse near the rigid wall are also presented.     

非牛顿流体的黏弹性对气泡溃灭的影响

气穴现象广泛存在于工程机械中。为了研究了壁面附近气泡的黏弹性对固体壁面的影响情况,以5%的聚丙烯酰胺(PAM)的水溶液为例,建立了数学模型和控制方程,并采用边界积分法求解控制方程,利用三次样条函数描绘了气泡表面。同时还探讨了气泡摆动、气泡喷射位置、流体对气泡喷射速度的扰动等气泡动力学现象。研究结果表明,黏弹性可以抑制射流的形成,进而减轻汽蚀。     

明渠自掺气水流气泡形成过程的试验研究

采用高速摄像技术对陡槽内自掺气水流进行了观测,拍摄到了掺气起始区气泡在水流内部形成的完整过程,对气泡形成的过程进行了概化分析。水面附近涡体运动产生的脉动压强和水面相互作用,导致水面发生上凸下凹的变形。当这种变形达到一定程度后,受顺水流向或横向脉动压力、横向切应力的作用,深入水体凹坑的边壁在某一部位闭合,下部包裹空气形成气泡而发生自掺气。随着紊动强度进一步增大,水面变形加剧,表现为高高跃出水面的柱状突起和水滴,这些水柱的倾倒、聚并也会造成空气进入水中形成气泡。明渠水流自掺气是水体内部紊动与水面相互作用的结果。     

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。     

Cavitation bubbles collapse characteristics behind a convex body

Cavitation bubbles behind a convex body were experimentally studied by a high speed camera and a hydrophone synchronously. The experiments were conducted in a circulating water tunnel with five various contraction ratios： β = 0.497, β= 0.6, β= 0.697, β= 0.751, and β= 0.799. The distributions of the cavitation bubble collapse positions behind the five different convex bodies were obtained by combining the images taken by the high speed camera. According to the collapse positions, it was found that no cavitation bubble was collapsed in the region near the wall until the ratio of the water head loss over the convex body height was larger than 20, which can be used to predict if the cavitation damage would occur in the tunnel with orifice energy dissipaters.     

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…     

掺气减蚀技术及掺气设施研究进展

为优化掺气设施体型并对其水力特性进行研究,在已有理论研究成果与工程实践经验基础上,从水流掺气减蚀机理、掺气水流运动特性及掺气设施体型及布置三方面评述了水流掺气减蚀技术研究进展。针对目前掺气减蚀技术研究中存在的不足,认为今后应进一步加强多泡情况下空化泡-空气泡-边壁微观作用机制,近壁(底板、侧壁)水体掺气浓度、气泡特征的沿程演变规律,复杂条件下掺气设施体型优化等方面的研究。