Turbulence and cavitation models for time-dependent turbulent cavitating flows

Cavitation typically occurs when the fluid pressure is lower than the vapor pressure at a local thermodynamic state,and the flow is frequently unsteady and turbulent.To assess the state-of-the-art of computational capabilities for unsteady cavitating flows,different cavitation and turbulence model combinations are conducted.The selected cavitation models include several widely-used models including one based on phenomenological argument and the other utilizing interface dynamics.The kε turbulence model with additional implementation of the filter function and density correction function are considered to reduce the eddy viscosity according to the computed turbulence length scale and local fluid density respectively.We have also blended these alternative cavitation and turbulence treatments,to illustrate that the eddy viscosity near the closure region can significantly influence the capture of detached cavity.From the experimental validations regarding the force analysis,frequency,and the cavity visualization,no single model combination performs best in all aspects.Furthermore,the implications of parameters contained in different cavitation models are investigated.The phase change process is more pronounced around the detached cavity,which is better illus-trated by the interfacial dynamics model.Our study provides insight to aid further modeling development.     

一种修正的低温流体空化流动计算模型

为了更准确地预测低温流体的空化流动特性, 基于Kubota空化模型, 对蒸发和凝结源项进行修正, 建立了一种考虑热力学效应的空化模型. 分别采用原始和修正的Kubota空化模型, 计算了绕对称回转体液氮的空化流动, 通过与实验结果的比较对修正的空化模型进行了评价. 结果表明, 与原Kubota空化模型比较, 修正的空化模型由于考虑了热力学效应, 计算获得的蒸发量减小, 凝结量增大, 空穴长度减小, 空穴界面形态呈模糊状态.计算结果与实验结果更加一致, 说明修正的空化模型能准确的描述低温流体空化过程的质量传输过程, 能够更准确模拟低温流体中的空化流动特性.      

冲蚀磨损与冲蚀、空蚀交互磨损的对比研究

基于计算流体力学(computational fluid dynamics,CFD)方法,数值对比研究了在模拟水轮机工况中不同转速时,冲蚀与冲蚀、空蚀交互作用时,转盘表面的流场(压力场、流体浓度分布),然后在转盘式磨损装置上,进行了汽液固三相冲蚀与空蚀交互磨损试验.结果表明:对于冲蚀磨损来说,随着转速增加,压力增大,在冲蚀作用下加入空蚀磨损,压力增高,磨损加剧;而对于交互磨损来说,随着转速增加,转盘表面的空蚀磨损区域从空化孔附近开始沿着转盘旋转的反方向偏移,并且磨损程度加剧;数值计算的气泡较多且压力梯度较高区域和试验转盘磨损区域基本一致,数值计算结果和试验结果吻合得较好.     

下游泵送螺旋槽密封空化试验及性能分析

基于自主设计的可视化试验装置及膜温和膜厚测量方法,对下游泵送螺旋槽密封空化特征及性能参数进行试验研究.探讨了油压和转速对不同螺旋槽密封液膜中空穴发生位置、空穴分布及空穴边界的影响,拟合了不同螺旋槽密封空穴边界的试验关系式,并对不同空化模型的理论泄漏量和膜厚与相应试验值进行了对比分析.结果表明:内槽型和中槽型密封的空穴均发生在螺旋槽内,但两者的空穴区形状明显不同;油压的增加有助于抑制液膜中空穴的发生,而转速的增加反之;尤其是内槽型,油压对其空穴发生影响更为显著;在低转速或高油压时,Reynolds和JFO两空化模型均可用于预测泄漏量和膜厚;在高转速或低油压时,JFO空化模型预测值更准确,而Reynolds空化模型预测值偏大.     

考虑空化影响的水润滑橡胶合金轴承冲蚀磨损研究

用计算流体力学方法,数值模拟水润滑轴承空化-冲蚀交互作用时气液固三相流场的动力学特性(压力场、速度场、气含率分布),然后在水润滑轴承摩擦磨损装置上,进行试件磨损试验,并观察试件表面形貌.结果表明:考虑空化影响后,水润滑轴承整个流场压力分布更接近实际;数值模拟所得的流场压力、速度、气含率最大值,均出现在发生空化的位置附近,其余位置基本不变,说明交互磨损比单一磨损严重.观察试件表面磨痕,存在短程犁沟、空蚀针孔、麻点状气蚀坑和蚀坑,磨痕呈现规律性,磨痕与轴转速的方向基本一致.试验结果和数值计算吻合较好,证明了理论分析的正确与合理.上述仿真与试验初步探讨了水润滑轴承空化与冲蚀交互作用的磨损机理与影响因素.     

A numerical method for the computation of the effects of an air vessel on the pressure surges in pumping systems with air entrainment

This paper describes an improved numerical method and computational procedure for the implementation of typical air vessel responses and their influence on the pressure transient for unsteady flow in a pipeline system with air entrainment. The proposed numerical method and computational procedure is without the necessity of an excessive iterative procedure as required previously by the conventional approach. The effects of air in the transient fluid system with the air vessel were then studied through the improved numerical computational method. Free and dissolved gases in the transported fluid, and cavitation at vapour pressure, are included. © 1998 John Wiley & Sons, Ltd.     

Multiphase fluid dynamics and transport processes of low capillary number cavitating flows

To better understand the multiphase fluid dynamics and associated transport processes of cavitating flows at the capillary number of 0.74 and 0.54, and to validate the numerical results, a combined computational and experimental investigation of flows around a hydrofoil is studied based on flow visualizations and time-resolved interface movement. The computational model is based on a modified RNG k-ε model as turbulence closure, along with a vapor-liquid mass transfer model for treating the cavitation process. Overall, favorable agreement between the numerical and experimental results is observed. It is shown that the cavi- tation structure depends on the interaction of the water-vapor mixture and the vapor among the whole cavitation stage, the interface between the vapor and the two-phase mixture exhibits substantial unsteadiness. And, the adverse motion of the interface relates to pressure and velocity fluctuations inside the cavity. In particular, the velocity in the vapor region is lower than that in the two-phase region.     

基于相变松弛法则的水下爆炸空化模型研究

水下爆炸过程中存在着大量的空化现象,空化的产生、演化及其溃灭过程对于水下冲击波传播、爆炸气泡运动以及水下结构物冲击损伤都会产生重要影响。本文基于多相可压缩流体理论模型,考虑空化发生过程中汽-液两相流体亚平衡状态下两相之间发生的热力学-化学平衡机制,分析汽-液两相介质之间的质量和热量交换,从而实现对相变过程的自动捕捉。该系统的控制方程采用分步法处理,首先利用二阶MUSCL-Hancock格式和HLLC黎曼求解器来求解齐次双曲型方程,再采用牛顿迭代法求解相变方程。数值测试结果表明,本文的计算模型对于空化相变过程具有较好的捕捉能力。最后将该模型应用到水下近水面爆炸空化的数值模拟当中,研究发现空泡的溃灭压力峰值约为冲击波压力峰值的15%,有效作用时间是冲击波载荷有效作用时间的2倍以上。本文的空化相变模型能够为水下爆炸空化现象的机理研究提供重要支撑。     

Surrogate model‐based strategy for cryogenic cavitation model validation and sensitivity evaluation

The study of cavitation dynamics in cryogenic environment has critical implications for the performance and safety of liquid rocket engines, but there is no established method to estimate cavitation‐induced loads. To help develop such a computational capability, we employ a multiple‐surrogate model‐based approach to aid in the model validation and calibration process of a transport‐based, homogeneous cryogenic cavitation model. We assess the role of empirical parameters in the cavitation model and uncertainties in material properties via global sensitivity analysis coupled with multiple surrogates including polynomial response surface, radial basis neural network, kriging, and a predicted residual sum of squares‐based weighted average surrogate model. The global sensitivity analysis results indicate that the performance of cavitation model is more sensitive to the changes in model parameters than to uncertainties in material properties. Although the impact of uncertainty in temperature‐dependent vapor pressure on the predictions seems significant, uncertainty in latent heat influences only temperature field. The influence of wall heat transfer on pressure load is insignificant. We find that slower onset of vapor condensation leads to deviation of the predictions from the experiments. The recalibrated model parameters rectify the importance of evaporation source terms, resulting in significant improvements in pressure predictions. The model parameters need to be adjusted for different fluids, but for a given fluid, they help capture the essential fluid physics with different geometry and operating conditions. Copyright © 2008 John Wiley & Sons, Ltd.     

Research progresses and prospects of unsteady hydrodynamics characteristics for cavitation

空化作为一种重要的复杂水动力学现象,具有明显的三维流动特征与剧烈的非定常特性,在水力机械、船舶推进器、水利工程中广泛存在,且通常会带来不利的影响,长期以来一直是水动力学领域研究的重点与难点课题之一.本文首先从实验测量和数值模拟两个角度,综述了空化水动力学非定常特性研究的发展概况, 分析了当前存在的问题.在空化实验研究中,主要介绍了空化水洞、空化流场测量以及多物理场同步测量等方面所取得的进展.在数值模拟方法中, 对目前的空化模型和湍流模型进行了分类介绍,并重点讨论了大涡模拟、验证和确认等在空化流模拟中的应用.之后以附着型空化为主, 同时兼顾云状空泡、空蚀、涡空化等,梳理了其研究中存在的几个关键科学问题,包括空化演变、空化流动的三维结构、失稳机制、空化不稳定性及其与低频压力脉动的联系、空化与旋涡的相互作用、空化与弹性水翼的流固耦合、空化对尾流场影响等.最后展望了空化水动力学的研究方向和未来发展趋势.     

DYNAMICAL BEHAVIORS OF DOUBLE CAVITATION BUBBLES UNDER ULTRASONIC HONING

为了探讨超声珩磨作用下磨削区的空化机理,基于速度势叠加原理,考虑超声珩磨速度和珩磨压力,建立了磨削区两空化泡的动力学模型. 数值模拟了磨削区空化泡初始半径、两空化泡间距、超声声压幅值、珩磨压力、珩磨头转速对磨削区两空化泡动力学特性的影响. 研究表明,考虑两空化泡之间的相互作用时,要想获得良好的空化效果,可将两空化泡初始半径之比控制在3 倍以内;选择较高的超声波声压幅值与较低的珩磨压力,并且使超声波声压幅值与珩磨压力和液体静压力之差介于0.66~1.89MPa 之间;增大珩磨头转速空化泡溃灭也略有加速;通过试验测量材料表面粗糙度的方法间接验证了理论分析的合理性.     

超声珩磨作用下两空化泡动力学特性

为了探讨超声珩磨作用下磨削区的空化机理,基于速度势叠加原理,考虑超声珩磨速度和珩磨压力,建立了磨削区两空化泡的动力学模型. 数值模拟了磨削区空化泡初始半径、两空化泡间距、超声声压幅值、珩磨压力、珩磨头转速对磨削区两空化泡动力学特性的影响. 研究表明,考虑两空化泡之间的相互作用时,要想获得良好的空化效果,可将两空化泡初始半径之比控制在3 倍以内;选择较高的超声波声压幅值与较低的珩磨压力,并且使超声波声压幅值与珩磨压力和液体静压力之差介于0.66~1.89MPa 之间;增大珩磨头转速空化泡溃灭也略有加速;通过试验测量材料表面粗糙度的方法间接验证了理论分析的合理性.      

Evaluating cavitation regimes in an internal orifice at different temperatures using frequency analysis and visualization

Experiments on a water cavitating orifice were conducted to investigate the influence of pressure and temperature on flow regime transition due to cavitation. The thermal effects could be important in cases with cryogenic cavitation or hot fluid injection. The investigations were based on CCD observations and a pressure fluctuations frequency analysis.The high-speed photographic recordings were used to analyze the cavitation evolution and individuate the frequency content of the two-phase flow by processing the pixel-intensity time-series data.The cavitating structures showed different behaviors and characteristics with variations in operating conditions, as the pressure inside the orifice and the flow temperature .The flow regime map for the cavitating flow was obtained using experimental observations to analyze the occurrence of the different two-phase flow regime transitions at various operating conditions.As the pressure at the orifice inlet increased, at the same downstream pressure, cavitation inception occurred. The decrease of the cavitation number brought a significant increase in cavitation zone extension. As the pressure drop inside the orifice increased, the cavitation was characterized by an evident increase in cavitation zone length to the outlet of the orifice. With a further cavitation number decrease, the transition to jet cavitation was evident.The temperature influenced both the cavitation intensity and the cavitation number at which different two-phase flow regime transitions occurred, which tended to increase with temperature.The vapor fraction was estimated using an image processing algorithm.The frequency content given by the pressure fluctuations was analyzed and compared with the frequency spectra obtained from the visual observations. The behavior of the different cavitating flows could be correlated to the frequency spectrum of the pressure fluctuations measured upstream and downstream of the orifice. The cavitation number reduction and consequent increase in cavitating area width were related to a corresponding significant increase in the amplitude of typical frequency components. The transition to jet cavitation was characterized by a significant increase in the first peak in the frequency spectrum; weaker spectral peaks were also present at high cavitation numbers.     

Numerical simulation of multiphase cavitating flows around an underwater projectile

The present simulation investigates the multiphase cavitating flow around an underwater projectile. Based on the Homogeneous Equilibrium Flow assumption, a mixture model is applied to simulate the multiphase cavitating flow including ventilated cavitation caused by air injection as well as natural cavitation that forms in a region where the pressure of liquid falls below its vapor pressure. The transport equation cavitating model is applied. The calculations are executed based on a suite of CFD code. The hydrodynamics characteristics of flow field under the interaction of natural cavitation and ventilated cavitation is analyzed. The results indicate that the ventilated cavitation number is under a combined effect of the natural cavitation number and gas flow rate in the multiphase cavitating flows.     

Flow regime transitions due to cavitation in the flow through an orifice

This paper presents both experimental and theoretical aspects of the flow regime transitions caused by cavitation when water is passing through an orifice. Cavitation inception marks the transition from single-phase to two-phase bubbly flow; choked cavitation marks the transition from two-phase bubbly flow to two-phase annular jet flow.

It has been found that the inception of cavitation does not necessarily require that the minimum static pressure at the vena contracta downstream of the orifice, be equal to the vapour pressure liquid. In fact, it is well above the vapour pressure at the point of inception. The cavitation number [σ = (P₃ − P_v)/(0.5 pV²); here P₃ is the downstream pressure, P_v is the vapour pressure of the liquid, ρ is the density of the liquid and V is the average liquid velocity at the orifice] at inception is independent of the liquid velocity but strongly dependent on the size of the geometry. Choked cavitation occurs when this minimum pressure approaches the vapour pressure. The cavitation number at the choked condition is a function of the ratio of the orifice diameter (d) to the pipe diameter (D) only. When super cavitation occurs, the dimensionless jet length [L/(D - d); where L is the dimensional length of the jet] can be correlated by using the cavitation number. The vaporization rate of the surface of the liquid jet in super cavitation has been evaluated based on the experiments.

Experiments have also been conducted in which air was deliberately introduced at the vena contracta to simulate the flow regime transition at choked cavitation. Correlations have been obtained to calculate the critical air flow rate required to cause the flow regime transition. By drawing an analogy with choked cavitation, where the air flow rate required to cause the transition is zero, the vapour and released gas flow rate can be predicted.     

A numerical model for cloud cavitation based on bubble cluster

The cavitation cloud of different internal structures results in different collapse pressures owing to the interaction among bubbles. The internal structure of cloud cavitation is required to accurately predict collapse pressure. A cavitation model was developed through dimensional analysis and direct numerical simulation of collapse of bubble cluster. Bubble number density was included in proposed model to characterize the internal structure of bubble cloud. Implemented on flows over a projectile, the proposed model predicts a higher collapse pressure compared with Singhal model. Results indicate that the collapse pressure of detached cavitation cloud is affected by bubble number density.     

射流对绕水翼云空化流动抑制机理研究

为理解绕水翼云空化流动的发展机理和探究水翼吸力面开孔射流的影响,采用密度 修正的RNG $k$-$\varepsilon $湍流模型和Schnerr-Sauer空化模型对原始NACA66(mod) 水翼和采用射流后的 水翼的云空化非定常过程进行模拟和对比分析;采用在水翼吸力面近壁区设立监测线的方法对近壁区的流场进行监测,得到 近壁区汽相体积分数、回射流速度、压力及压力梯度的时空分布云图;开展了云空化流场特性的涡动力学分析,进而分析水 翼云空化的发生机理和射流抑制空化的抑制机理. 结果表明:游离型空泡在下游溃灭时产生强烈的局部高压,其向上游传播 导致前缘空穴的一次回缩,而空穴的二次回缩受回射流的影响. 回射流的发展区域受限于较高的压力梯度,高的压力梯度一 直存在,但回射流在一个周期内的首次出现需要时间的积累. 在水翼吸力面射流使得射流孔附近压力升高,弥补了由于空化 和绕流造成的压降,压力梯度增大,抗逆压能力增强,对回射流起到阻挡作用;另一方面,射流使得回射流区域面积和回射 流的强度也有所减小,从而对云空化的发展起到抑制的效果. $Q$准则的涡结构云图相比于汽相体积分数云图能显示复杂的 流动结构,前缘附着型空穴和尾缘游离型空穴内存在旋涡,回射流对空穴存在剪切作用造成空穴脱落. 而射流对空穴和回射 流的剪切和阻挡使云空化发展得到抑制.      

Characterization of incipient cavitation in axial valve by hydrophone and visualization

As the effects of cavitation in valves are devastating, the choice of the correct valve for a given operating range is crucial. For this, the valve characteristic is needed, whereby one side of the operating range depends on the determination of the incipient cavitation.In this paper, the visualization method for incipient cavitation detection is presented. For the purpose of comparison, pressure oscillations inside the pipeline were simultaneously measured with a hydrophone. The effect of operating pressure was studied for two different openings of the valve.For each operating point of incipient cavitation, corresponding points were measured for developed cavitation and no-cavitation state, based on a constant-portion change of volumetric flow rate with regard to the incipient cavitation volumetric flow rate. The visualization and hydrophone signals were compared.The visualization method proved efficiency over hydrophone measurements because it is more sensitive to cavitation and the signal is independent of the operating pressure. The main drawback is the preparation of the observation window.