Determination of the shedding frequency of cavitation cloud in a submerged cavitation jet based on high-speed photography images

To accurately determine the shedding frequency of the cavitation cloud in a submerged cavitation jet,the spectral analysis and the proper orthogonal decomposition(POD)for high-speed photography images are performed.The spectrums of 6 different kinds of image signals(the area-averaged gray level,the line-averaged gray level,the point gray level,the cavitation length,width,and area)are calculated and compared.The line-averaged gray level is found to be optimal in determining the shedding frequency but an accurate frequency can only be obtained in the stable-frequency zone where the cavitation cloud sheds.In repeated experiments,the plateau-shape distribution of the main frequency is established with a deviation of 10.8%.A revised Reynolds number Re'is defined and the shedding frequency can be correlated to Re'by a power law when the cavitation number is less than 0.02.This relationship is validated by the experimental data in literature.The first mode of the POD characterizes the ensemble-average of the cavitation cloud while the second mode is the major part of the cavitation cloud transient components.The modes 2-5 are organized in pairs,which confirms the periodic feature of the cavitation cloud in the submerged cavitation jet.Near the nozzle exit,the modes 2-5 are symmetrically distributed in the jet shear layer.The shedding frequency of the cloud cavitation can also be precisely determined by performing the spectral analysis of the weighting coefficients of the mode 2.This paper shows that the two parameters,namely,the line-averaged gray level and the weighting coefficients of the mode 2,can be confidently used to calculate the shedding frequency of the cavitation cloud in a submerged cavitation jet.     

淹没水射流空化初生的动力判据

传统的空化初生判据没有考察空泡动力效应的影响和空泡的非等温过程，通过对Rayleigh-Plesset方程的适当无因次化，建立了一种既考虑空泡的动力学效应又适合各种绝热指数的简便的空化初生动力判据。计算了不同环境压力对空化初生的影响。     

绕扭曲翼型三维非定常空泡脱落结构的数值分析

该文以一种三维扭曲翼型为研究对象,采用均衡流空化模型和基于滤波器的湍流修改模式模拟了绕翼型非定常空化流动。计算结果显示,在一定条件下绕翼型的空泡流动存在两种脱落结构,即主体脱落和二次脱落。进一步的流场分析表明回射流和侧面射流是这两种空泡脱落结构产生的主要原因。计算结果与试验的比较说明,该文采用的数值模拟方法可成功模拟实验观测的空泡脱落现象。     

Numerical analysis of cavitation shedding flow around a three-dimensional hydrofoil using an improved filter-based model

The cavitation shedding flow around a 3-D Clark-Y hydrofoil is simulated by using an improved filter-based model(FBM) and a mass transfer cavitation model with the consideration of the maximum density ratio effect between the liquid and the vapor. The unsteady cloud cavity shedding features around the Clark-Y hydrofoil are accurately captured based on an improved FBM model and a suitable maximum density ratio. Numerical results show that the predicted cavitation patterns and evolutions compare well with the experimental visualizations, and the prediction errors of the time-averaged lift coefficient, drag coefficient and Strouhal number St for the cavitation number σ= 0.8, the angle of attack α= 8° at a Reynolds number Re= 7 ×10~5 are only 3.29%, 2.36% and 9.58%, respectively. It is observed that the cavitation shedding flow patterns are closely associated with the vortex structures identified by the Q-criterion method. The predicted cloud cavitation shedding flow shows clearly three typical stages:(1) Initiation of the attached sheet cavity, the growth toward the trailing edge.(2) The formation and development of the re-entrant jet flow.(3) Large scale cloud cavity sheds downstream. Numerical results also indicate that the non-uniform adverse pressure gradient is the main driving force of the re-entrant jet, which results in the U-shaped cavity and the 3-D bubbly structure during the cloud cavity shedding.     

水下回转航行体的云状空化回射流运动特征研究

该文针对典型的细长回转航行体的水下航行过程,基于SHPB(霍普金森压杆)发射系统开展了机理性实验.相应基于单一流体/多相混合模型,结合空化模型及修正的RNG k-ε湍流模式对该问题进行了数值模拟.并在实验与数值模拟结果的基础上,研究了回射流诱导的云状空化不稳定性问题,探讨了该现象中空化非稳态演化的物理机制,分析了空泡末端回射流的产生原因以及它对空泡演化的诱导作用.进一步从压力梯度角度给出了回射流运动的动力学模型和空泡长度的预测表达式,并通过数值结果验证了该模型和公式的有效性.     

Numerical investigation of unsteady sheet/cloud cavitation over a hydrofoil in thermo-sensitive fluid

The sheet/cloud cavitation is of a great practical interest since the highly unsteady feature involves significant fluctuations around the body where the cavitation occurs. Moreover, the cavitating flows are complicated due to the thermal effects. The present paper numerically studies the unsteady cavitating flows around a NACA0015 hydrofoil in the fluoreketone and the liquid nitrogen with particular emphasis on the thermal effects and the dynamic evolution. The numerical results and the experimental measurements are generally in agreement. It is shown that the temperature distributions are closely related to the cavity evolution. Meanwhile, the temperature drop is more evident in the liquid nitrogen for the same cavitation number, and the thermal effect suppresses the occurrence and the development of the cavitating flow, especially at a low temperature in the fluoroketone. Furthermore, the cavitating flows are closely related to the complicated vortex structures. The distributions of the pressure around the hydrofoil is a major factor of triggering the unsteady sheet/cloud cavitation. At last, it is interesting to find that one sees a significant thermal effect on the cavitation transition, a small value of σ/2ɑ is required in the thermo-sensitive fluids to achieve the similar cavitation transition that occurs in the water.     

Numerical analysis of the unsteady behavior of cloud cavitation around a hydrofoil based on an improved filter-based model

The unsteady cavitation evolution around the Clark-Y hydrofoil is investigated in this paper, by using an improved filter-base model(FBM) with the density correction method(DCM). To improve the prediction accuracy, the filter scale is adjusted based on the grid size. The numerical results show that a small filter scale is crucial for the unsteady simulations of the cavity shedding flow. The hybrid method that combines the FBM and the DCM could help to limit the overprediction of the turbulent viscosity in the cavitation region on the wall of the hydrofoil and in the wake. The large value of the maximum density ratio ? l /? v, clip promotes the mass transfer rate between the liquid phase and the vapor phase, which results in a large sheet cavity length and the vapor fraction rise inside the cavity. The cavity patterns predicted by the improved method are verified by the experimental visualizations. The time-average lift, the drag coefficient and the primary oscillating frequency St for the cavitation number ?= 0.8, the angle of attack, ?= 8o, at a Reynolds number Re= 7 ?105 are 0.735, 0.115 and 0.183, respectively, and the predicted errors are 3.29%, 3.36% and 8.93%. The typical three stages in one revolution are well-captured, including the initiation of the sheet/attached cavity, the growth toward the trailing edge(TE) with the development of the re-entrant jet flow, and the large scale cloud cavity shedding. It is observed that the cloud cavity shedding flow induces the vortex pairs of the TE vortices in the wake and the shedding vortices. The positive vorticity vortex of the re-entrant jet and the TE vortices interacts and merges with the negative vorticity vortex of the leading edge(LE) cavity to produce the shedding flow.     

深水垫情况下水垫塘淹没射流特征分析

本文基于自由射流扩散规律的研究成果,在对常规水垫塘内水流流态进行分析研究的基础上,结合模型试验成果对深水垫情况下水垫塘内水流流态进行了分析,对淹没射流特征进行了研究。研究结果认为,深水垫情况下的水垫塘内水流流态的规律性比较明显,淹没冲击射流流态与淹没水跃流态平顺衔接过渡,总体上可认为是一种射流流态,符合射流的线性扩散规律。     

喷嘴参数对射流结构影响的数值模拟

根据现有实验条件,建立了物理模型和数学模型.选用k-ε紊流模型,并采用SIMPLEC算法进行数值计算.计算结果表明,角形喷嘴射流没有等速核段,轴心速度衰减与锥形喷嘴射流基本段轴心速度衰减相似,射流断面速度分布具有自模性.从角形喷嘴功能要求及射流动力特性出发,扩散角在30°附近有一较佳值;扩散段长度应达到一定值;圆柱段长度在8mm附近有一较佳值.     

波浪荷载对半圆型潜堤作用的数值分析

基于动量源数值造波原理,运用Fluent软件模拟了半圆型潜堤表面波压的分布及变化规律,采用π定理分析了波陡、水深及波长对半圆型潜堤表面波浪力的影响,并通过统计潜堤表面各点所受压力（最大波压力与最小波压力的差值）最大值的分布,给出了半圆型潜堤表面压力最大值出现的位置及概率.分析结果表明,在水深较深时,波陡的改变会使波压力在堤顶至背浪侧发生明显变化;水深波长比的改变,会引起半圆堤两侧波压力不同的变化;波压力的最大值一般发生在堤顶处及其偏向背浪侧附近;半圆堤面上的动波压力最大值主要出现在两侧堤脚附近,随着波高的增加,堤面上各点的动波压力由近似对称分布向两侧受力不同转变.     

二维水翼局部空泡脱落特性数值分析

空化是水动力学中特有的一种物理现象,研究空化现象对改善水力机械和舰船推进器性能具有很现实的工程意义,发展空泡流动的数值模拟技术是现在空化研究领域中的热点.本文通过Fluent软件提供的二次开发接口,把基于状态方程的空化模型添加到Fluent软件中,并采用非线性汽、液湍流黏性计算模式实现了二维水翼定常、非定常空泡流动的数值模拟.计算的定常空泡翼型表面的压力分布与试验吻合较好,模拟了非定常空泡从初生、发展、断裂、脱落以及在下游溃灭的整个过程.揭示了空泡内部流动结构和非定常周期性脱落的原因.通过数值模拟与试验对比,研究了空泡脉动的数的规律.     

Numerical analysis of cavitation within slanted axial-flow pump

In this paper, the cavitating flow within a slanted axial-flow pump is numerically researched. The hydraulic and cavitation performance of the slanted axial-flow pump under different operation conditio...     

航行体有攻角出水全过程数值模拟

水下航行体高速出水时,空泡溃灭会产生较大的弯矩载荷,是其结构强度的主要设计依据.本文基于计算流体方法与结构动力学方法的单向耦合,建立了航行体有攻角出水全过程数值模拟方法,并针对抽象的典型工况进行了较大规模的并行计算,最终得到了航行体关键截面的全时程载荷响应.进一步通过分析航行体空泡形状和表面压力的演化特征,解释了航行体出水载荷的直接形成原因.     

三维挑流水舌及其碰撞的LBM数值模拟研究

为了研究挑流水舌空中掺气破碎以及水舌碰撞形成的复杂两相流运动,采用格子玻尔兹曼方法对泄洪过程中的挑流水舌以及两股水舌的碰撞进行数值模拟研究.研究表明:(1)格子Boltzmann方法能够对挑流掺气水舌进行数值模拟,得到掺气水舌的运动界面以及水舌风的空间分布.水舌内缘挑距、外缘挑距以及横向宽度与文献中实测数据对比误差分别...     

长距离有压输水工程泵站水锤的数值分析

综合考虑了长距离有压输水系统中的泵、泵后阀门、空气阀、调压井、异性串联管、下游水库等各种内、外边界条件,采用特征-差分方法,针对实际工程的泵站水锤问题,通过数值方法对事故停泵后重新启泵的时间间隔、输水系统管线糙率等要素进行了敏感性分析。结果表明,合理的启泵时间间隔有助于改善有压输水系统及其部件在水力过渡过程中的水动力特性,并缩减调压井尺寸及相应工程造价;管线糙率对水头损失和水锤波衰减存在影响,对全系统在水力过渡过程中的水动力特性影响较小。     

取水工程附近河床冲淤规律数值模拟研究

取水口处的泥沙淤积问题关系到取水工程能否安全运行,从水流、河势角度论证了取水口布置的合理性,并减少取水对航道及防洪影响的必要性。以长江九江湖口水道某电厂取水口为例,采用平面二维水沙数学模型研究了取水口附近局部流态、河床冲淤、取水工程处悬沙含量等的变化规律,并对附近河床近期变化进行了预测。计算条件采用丰、中、枯典型年和典型系列年水沙条件,系列年考虑三峡工程和上游水库调度的影响。通过分析计算结果,选择了较优取水口方案,并推荐了适宜的取水口标高。研究结果表明,同一河段取水工程处河床冲淤规律受来水来沙条件和取水工程位置处的局部地形的共同影响。     

Implicit large eddy simulation of unsteady cloud cavitation around a plane-convex hydrofoil

The present paper focuses on the erosive cavitation behavior around a plane convex hydrofoil. The Zwart-Gerber-Belamri cavitation model is implemented in a library form to be used with the OpenFOAM. The implicit large eddy simulation(ILES) is applied to analyze the three-dimensional unsteady cavitating flow around a plane convex hydrofoil. The numerical results in the cases under the hydrodynamic conditions, which were experimentally tested at the high speed cavitation tunnel of the école Polytechnique Fédérale de Lausanne(EPFL), clearly show the sheet cavitation development, the shedding and the collapse of vapor clouds. It is noted that the cavitation evolutions including the maximum vapor length, the detachment and the oscillation frequency, are captured fairly well. Furthermore, the pressure pulses due to the cavitation development as well as the complex vortex structures are reasonably well predicted. Consequently, it may be concluded that the present numerical method can be used to investigate the unsteady cavitation around hydrofoils with a satisfactory accuracy.     

基于温度场分析的新型水下清淤装置数值研究

为了解新型水下清淤取样冻结装置在施工过程中的温度场发展变化规律,以及最终冻结效果的可靠性,采用有限元分析软件建立温度场数值模型,模拟水下清淤装置在冻结过程中的温度场发展动态变化过程,得到最终数值计算的温度场云图及等温线图。主要得到以下结论:积极冻结10 d可使-1℃等温线基本完成交圈,积极冻结20 d可使-10℃等温线完成交圈,此后冻土帷幕的发展速度减缓,冻土区域趋于平滑。通过分析设置在冻结装置上的5条观测路径及测温点,得到结论如下:积极冻结30 d后即可达到清淤所需强度,此时,各路径上的观测点温度均已达到-10℃以下,可以保证冻结效果的可靠性。清淤装置的冻结管排布方式对温度场发展会有较大的影响,顶部冻结板会影响温度场垂直方向的发展,其影响范围约为1.5 m。以上研究结果有助于对日后的工程实践提供相应的指导和帮助。     

收缩—扩散型喷嘴长度影响空化效果的数值分析

喷嘴长度是影响收缩—扩散型喷嘴空化效果的重要参数。运用连续方程、动量方程和Rayleigh-Plesset方程,对流经收缩—扩散型喷嘴的泡液流进行稳态求解。计算结果表明泡液流中很小的入口空隙率对流动特性有强烈影响,并随着无量纲喷嘴长度的变化对流动特性的影响有着显著的相似性。随着喷嘴长度的增长,喷嘴入口空隙率临界值以指数规律下降,气泡半径呈线性增长。     

水垫塘多层淹没射流数值模拟及消能研究

本文对纵向多层淹没射流水垫塘流态进行了数值模拟，将能量方程耦合于ＳＩＭＰＬＥ算法中使计算结果与实测值更为接近．文中分析、比较了单层和多层射流入塘后的水流结构及消能机理．研究表明，由于多层射流的能量耗散加快且分布均匀化和“动水垫”效应，使其在充分发挥水垫消能作用，提高消能率和降低塘底动水荷载及冲剧等方面明显优于单层射流．