LES NUMERICAL SIMULATION OF CAVITATION BUBBLE SHEDDING ON ALE 25 AND ALE 15 HYDROFOILS

A cavitation calculation scheme is developed and applied to ALE 15 and ALE 25 hydrofoils, based on the Bubble Two-phase Flow (BTF) cavity model with a Large Eddy Simulation (LES) methodology. The Navier-Stokes equations including cavitation bubble clusters are solved through the finite-volume approach with a time-marching scheme. Simulations are carried out in a 3-D field with a hydrofoil ALE 15 or ALE 25 at an angle of attack of 8° and cavitation number σ = 2.3 with a 2 × 10⁶, meshing system. With the time-marching, the cavitation bubble gradually grows to a steady lump shape and then produces an irregular small bubble behind the main cavitation bubble, finally shedding from the leading edge of the cloud cavitation structure. The calculated results including velocity field and pressure field are consistent with experiment data at the same Reynolds number and cavitation number. The vortex and reverse flow are observed on the hydrofoil surface.     

Numerical analysis of cavitation cloud shedding in a submerged water jet

Focused on the unsteady behavior of high-speed water jets with intensive cavitation a numerical analysis is performed by applying a practical compressible mixture flow bubble cavitation model with a simplified estimation of bubble radius. The mean flow of two-phase mixture is calculated by unsteady Reynolds averaged Navier-Stokes(URANS) for compressible flow and the intensity of cavitation in a local field is evaluated by the volume fraction of gas bubbles whose radius is estimated with a simplified RayleighPlesset equation according to pressure variation of the mean flow field. High-speed submerged water jet issuing from a sheathed sharp-edge orifice nozzle is treated. The periodically shedding of cavitation clouds is captured in a certain reliability compared to experiment data of visualization observation and the capability to capture the unsteadily shedding of cavitation clouds is demonstrated. The results demonstrate that cavitation takes place near the entrance of nozzle throat and cavitation cloud expands consequentially while flowing downstream. Developed bubble clouds break up near the nozzle exit and shed downstream periodically along the shear layer. Under the effect of cavitation bubbles the decay of core velocity is delayed compared to the case of no-cavitation jet.     

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.     

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。     

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.     

INTERACTION OF A CAVITATION BUBBLE AND AN AIR BUBBLE WITH A RIGID BOUNDARY

The motion of a spark-induced cavitation bubble and an air bubble near a rigid boundary is experimentally studied by using high-speed photography. Several dimensionless parameters are used to describe the geometrical configuration of the bubble-bubble-boundary interaction. The bubble-bubble interaction can be considered in two different conditions. The cavitation bubble will collapse towards the air bubble if the air bubble is relatively small, and away from the air bubble if the air bubble is relatively large. The two zones are identified in the bubble-boundary interaction, and they are the danger zone and the safety zone. The relative position, the bubble-boundary distance and the bubble-bubble distance play important roles in the bubble-bubble-boundary interaction, which can be considered in several conditions according to the responses of the bubbles. Air jets are found to penetrate into the cavitation bubbles. The cavitation bubble and the air bubble (air jet) move in their own way without mixing. The motion of a cavitation bubble may be influenced by an air bubble and/or a rigid boundary. The influence of the air bubble and the influence of the boundary may be combined, like some thing of a vector.     

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.     

旋成面叶栅汽蚀绕流的变分原理

叶栅汽蚀绕流时所产生的汽穴不仅使流场发生了变化，而且其未知的形状使流场的求解变得十分困难。本文通过映射方法解决叶栅流场求解域的不定问题，分别以两个新型通用函数 （角函数和矩函数） 为泛函的自变量，建立了任意旋成面叶栅局部汽蚀绕流的变分原理，旨在为用有限元法和其它变分直接解法求解汽蚀汽穴及叶栅汽蚀绕流流场提供新的理论基础。     

PREDICTION OF CAVITATION INCEPTION IN PIPELINES

Based on the analysis of the averaged axisymmetric turbulent flow and the motionand growth of gas nuclei in the flow,this paper presents a method for predicting cavitation in-ception in pipelines.Mathematic model is proposed and the fluctuating pressure in the flow andother random factors with respect to gas nuclei are treated using the Monte-Carlo method.Sothat the bridge between macroscopic aspect of cavitation in the flow and the microscopic event ofindividual nucleus cavitation is set up.Numerical investigation and experimnental test are carriedout for the flow past an orifice in a uniform pipe with circular section.     

紊动剪切流中气核振荡对空化的作用

为了解压力脉动对泡动力学和空化现象的作用，可将压力脉动的不同随机量与气核在流场中的动态响应联系一起进行分析。结果认为：在水流强烈的紊动下，由于压力脉动的影响，气核交替地膨胀和收缩，脉动负峰值可使瞬时压力降低。气核发育时间明显缩短，促使空化提前发生。     

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.     

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。     

Experimental study of flow patterns and pressure drops of heavy oil-water-gas vertical flow

A stainless steel apparatus of 18.5 m high and 0.05 m in inner diameter is developed, with the heavy oil from Lukeqin Xinjiang oil field as the test medium, to carry out the orthogonal experiments for the interactions between heavy oil-water and heavy oil-water-gas. With the aid of observation windows, the pressure drop signal can be collected and the general multiple flow patterns of heavy oil-water-gas can be observed, including the bubble, slug, churn and annular ones. Compared with the conventional oil, the bubble flows are identified in three specific flow patterns which are the dispersed bubble (DB), the bubble gas-bubble heavy oil, and the bubble gas-intermittent heavy oil (B_g – I_o). The slug flows are identified in two specific flow patterns which are the intermittent gas-bubble heavy oil (I_g – B_o) and the intermittent gas-intermittent heavy oil (I_g – I_o). Compared with the observations in the heavy oil-water experiment, it is found that the conventional models can not accurately predict the pressure gradient. And it is not water but heavy oil and water mixed phase that is in contact with the tube wall. So, based on the principle of the energy conservation and the kinematic wave theory, a new method is proposed to calculate the frictional pressure gradient. Furthermore, with the new friction gradient calculation method and a due consideration of the flow characteristics of the heavy oil-water-gas high speed flow, a new model is built to predict the heavy oil-water-gas pressure gradient. The predictions are compared with the experiment data and the field data. The accuracy of the predictions shows the rationality and the applicability of the new model.     

脉动压力及压力梯度对不平整突体空化初生的影响

本文对六种不平整突体在正负压力梯度状况下的初生空化数进行了试验研究和理论分析，并根据气泡动力学方程建立了脉动压力与气泡振荡半径之间的关系，讨论了脉动压力及压力梯度对不平整突体空化初生的影响，试验结果表明，同一突体在负压力梯度状况下的初生空化数高于正压力梯度状况下的初生空化数；对于非流线型突体，在负压力梯度状况下空化初生时的壁面脉动压力强度低于正压力梯度状况下的脉动压力强度，脉动压力峰值约为七至十二倍的脉动压力均方根值。     

渐扩式泄洪洞出口水流模拟及空化 特性预测研究

为了获得泄洪洞出口段在高速水流情况下的水流特征,分析空化及其对结构可能存在的空蚀破坏影响,本文采用了VOF（Volume Of Fluid）方法,并结合Realizable k-ε紊流模型,对泄洪洞水流进行了三维数值计算。计算获得了压强、流速及水深的三维分布参数,并将部分计算结果与设计值验证,吻合较好。通过引入自定义函数方式计算了整个流场的空化数,获得了泄洪情况下泄洪洞高速水流空化数分布规律及易发生空化空蚀的区域。     

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.     

高水头船闸阀门顶缝空化切片试验研究

针对日益严峻的高水头单级船闸阀门顶缝空化问题,采用能够真实反映缝隙流特性的1:1切片模型试验,研究阀门顶缝空化特性及门楣自然通气防空化机理。研究表明,随着空化的发展,缝隙段依次发生喉口跌坎空化、主流中心空化和阀门面板空化,缝隙段负压区不断延伸,直至整个缝隙段达到稳定的-10 m水柱负压,压力脉动很小;门楣自然通气通过增加缝隙段压力,消除主流中心空化和阀门面板空化,抑制喉口跌坎空化。当采用门楣自然通气措施后,缝隙段压力稳定在-2 m水柱左右,空化消失,缝隙段水流脉动压力增大;缝隙段压力与单宽通气量近似二次多项式关系,通气量极值对应的缝隙段压力约-2 m水柱,此时缝隙段压力与通气量达到平衡状态,当缝隙段压力逐渐升高时,门楣通气量逐渐降低,直至自然通气停止。     

溢洪道底板空蚀发生位置及抗空蚀措施分析

以肯斯瓦特水利枢纽工程为例,通过水工模型试验,测试其设计水位工况下和泄洪消能标准水位工况下溢洪道泄洪时底板的流速及压力分布,并根据空化理论模型对其底板沿程空化数进行了计算。结果表明:可以通过溢洪道流速特性和底板压力分布来分析过流面的空化空蚀特点;在实际工程中,可采用优化过流面的体形、设置掺气减蚀设施、控制溢流面的不平整度以及使用抗空蚀材料等方法来防止空化和空蚀的发生。     

万家寨水电站水中含沙对空化压力的影响

对万家寨水电站含沙量对水力机械内空化压力的影响进行了研究，并利用自行研制的测量装置实测了黄河万家寨水电站含沙水样的初生空化与，临界空化压力．实测结果表明，随着含沙量的增大，初生空化与临界空化压力值均有所提高，且接近线性关系．这一成果可用于所有含沙河流上的水电站水轮机与水泵站水泵合理确定安装高程时的参考．这样做可以减少空蚀与泥沙磨损给运行带来的经济损失．