某二维水翼空化数值模型对比

以二维水翼计算模型为研究对象,采用数值模拟的方法,通过不同的空化模型和湍流模型对计算模型进行数值模拟研究,得到了翼型在空化过程中空泡的运动规律。结果表明:该翼型在不同空化模型下,发生空化时,空化过程包含了空泡的产生、发展、脱落和溃灭几个过程,空化的运动过程总具有明显的周期性;Schnerr-Sauer模型的计算结果与实际的水翼空化流动过程相吻合,Schnerr-Sauer空化模型可以很好地模拟绕水翼的空化流动过程。     

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.     

Experimental measurement of tip vortex flow field with/without cavitation in an elliptic hydrofoil

In this paper, recent measurements of tip vortex flow with and without cavitation carried out in Cavitation Mechanism Tunnel of China Ship Scientific Research Center(CSSRC) are presented. The elliptic hydrofoil with section NACA 662-415 was adopted as test model. High-speed video(HSV) camera was used to visualize the trajectory of tip vortex core and the form of tip vortex cavitation(TVC) in different cavitation situations. Laser Doppler velocimetry(LDV) was employed to measure the tip vortex flow field in some typical sections along the vortex trajectory with the case of cavitation free. Stereo particle image velocimetry(SPIV) system was used to measure the velocity and vorticity distributions with and without cavitation. Series measurement results such as velocity and vorticity distributions, the trajectory of tip vortex core, the vortex core radius, cavity size and cavitation inception number were obtained. The results demonstrated that the minimum pressure coefficient in the vortex core obtained by flow field measurement was quite coincident with the tip vortex cavitation inception number obtained under the condition of high incoming velocity and low air content. And TVC would decrease the vortex strength comparing with the case without cavitation.     

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.     

空化对振动水翼附加质量和阻尼特性影响研究

抽水蓄能机组需频繁调节工况以适应电网需求。由此水力激振频率范围加宽,共振风险增加。附加质量和阻尼特性是评估共振工况点和共振幅值的关键参数。在空化发生时,叶片绕流流体从液相变成汽相,其复杂的流固耦合效应对叶片附加质量和阻尼特性产生影响。本文将叶片简化为NACA 0009水翼,探索了空化发生时基于双向流固耦合的附加质量和水力阻尼特性预测方法。结果表明,模拟得到的静水中固有频率和不同空化数下的空化脱落频率与实验结果吻合良好,相对误差分别在7.12%和8.76%以内。在空化数σ=1.04~2.02范围内,固有频率和水力阻尼随空化数的降低分别增大和减小,相对于实验的平均误差分别为9.69%和13.65%。内部流动分析表明反向射流导致前缘空化周期性脱落,吸力面的绕流速度下降则是空化发生后水力阻尼下降的主要原因。研究成果对抽水蓄能机组设计阶段预判空化时的叶片振动评估具有重要指导意义。     

Numerical analyses of ventilated cavitation over a 2-D NACA0015 hydrofoil using two turbulence modeling methods

The present paper studies the ventilated cavitation over a NACA0015 hydrofoil by numerical methods. The corresponding cavity evolutions are obtained at three ventilation rates by using the level set method. To depict the complicated turbulent flow structure, the filter-based density corrected model(FBDCM) and the modified partially-averaged Navier-Stokes(MPANS) model are applied in the present numerical analyses. It is indicated that the predicted results of the cavitation shedding dynamics by both turbulence models agree fairly well with the experimental data. It is also noted that the shedding frequency and the super cavity length predicted by the MPANS method are closer to the experiment data as compared to that predicted by the FBDCM model. The simulation results show that in the ventilated cavitation, the vapor cavity and the air cavity have the same shedding frequency. As the ventilated rate increases, the vapor cavity is depressed rapidly. The cavitation-vortex interaction in the ventilated cavitation is studied based on the vorticity transport equation(VTE) and the Lagrangian coherent structure(LCS). Those results demonstrate that the vortex dilatation and baroclinic torque terms are highly dependent on the evolution of the cavitation. In addition, from the LCSs and the tracer particles in the flow field, one may see the process from the attached cavity to the cloud cavity.     

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.     

绕水翼超空化流发展及其旋涡特性的实验研究

为深入了解超空化流场结构特性,利用数字粒子图像测速(DPIV)和高速全流场显示技术,观测了绕hydronautics水翼的超空化流动涡量场。采用空化流场中的空化泡作为示踪粒子来显示流动结构。结果表明:在超空化的不同发展阶段,流动涡量场呈现不同的分布特征:随着空化数的降低,上下涡带逐渐靠拢,并向后延伸、拉直;下涡带随空化区域内汽相和水汽混合相分布的不同而发生明显变化:空穴形成阶段的下涡带基本稳定;在两相共存阶段,汽相和水汽混合相的频繁转换使得下涡带的起始位置和形状发生剧烈的变化;而在超空化完全发展阶段,下涡带又趋于稳定,且起始位置移至水翼后部。在整个超空化阶段,汽相和水汽混合相之间存在着较大的速度梯度;与来流方向相比,垂直于来流方向的速度脉动要大得多。     

Determination of frequencies of oscillations of cloud cavitation on a 2-D hydrofoil from high-speed camera observations

A method is presented to determine significant frequencies of oscillations of cavitation structures from high-speed camera recordings of a flow around a 2-D hydrofoil. The top view of the suction side of an NACA 2412 hydrofoil is studied in a transparent test section of a cavitation tunnel for selected cloud cavitation regimes with strong oscillations induced by the leading-edge cavity shedding. The ability of the method to accurately determine the dominant oscillation frequencies is confirmed by pressure measurements. The method can resolve subtle flow characteristics that are not visible to the naked eye. The method can be used for noninvasive experimental studies of oscillations in cavitating flows with adequate visual access when pressure measurements are not available or when such measurements would disturb the flow.     

基于转捩SST模型的钝尾缘水翼绕流数值计算与分析

边界层转捩是影响水翼绕流水动力学特性的关键因素之一。为了探究γ-Reθt转捩模型在低湍流度来流下水翼边界层转捩及尾涡脱落预测中的适用性,本文基于耦合γ-Reθt转捩模型和SST k-ω模型的转换SST模型,对0°攻角的NACA0009钝尾缘水翼绕流进行了数值计算。分析了转捩SST模型对网格尺度的敏感性,预测了边界层和尾迹区流场及不同雷诺数下水翼尾涡脱落特征,并与实验数据和SST k-ω模型计算结果进行了对比。结果表明:随近壁面第一层网格尺度y+的增大,水翼边界层转捩位置沿流向后移,最大y+值小于1时,预测的转捩位置趋于一致;与SST k-ω模型相比,转捩SST模型可以求解层流边界层,避免了在边界层和尾迹区对涡黏系数的过高预测,得到的边界层厚度和水翼尾迹区流动参数与实验结果更为接近;转捩SST模型可以较准确预测水翼尾涡脱落频率随雷诺数增大而增加的变化趋势,水翼尾涡脱落频率预测值与实验结果的最大相对误差为6.2%。     

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.     

The effect of water quality on tip vortex cavitation inception

The inception of tip vortex cavitation is very sensitive to water quality. In order to quantify the effect of water quality on the inception of tip vortex cavitation, we develop a motion model to describe the migration and growth of nuclei in water. An analytical solution of migration of nuclei in a vortex flow is obtained so that the capture times of various nuclei can be given out directly. A criterion is built to determine the critical nucleus in a certain nuclei spectra distribution. Tensile strength of the critical nucleus is used to quantify the effect of water quality and correct the tip vortex cavitation inception number. Finally this change of cavitation inception number is compared with experimental results to validate our model.     

不同掺气孔径下水流空化特性试验研究

空化数反映了压力变化对水流空化特性的影响.该文基于浙江工业大学水力学实验室设计的直流式水洞,利用先进的量测设备对空化区中空化特性进行了试验研究.通过对不同流速不同掺气孔径下圆柱突体上下游空化数的对比分析,得出了不同掺气孔径下空化数的变化趋势有关的结论.该结论可供设计优化掺气设施时参考.     

基于B样条网格的三维水翼水动力性能研究

该文应用低阶速度势面元法建立了预报三维水翼水动力性能的数值方法,重点对翼梢和尾涡进行了研究。文中采用基于B样条的网格划分方法,在水翼前缘处的弦向网格线与外轮廓线正交,在尾缘处与尾涡离开的速度方向相一致。翼梢上尾涡分离点的位置采用迭代的方法进行确定。对于三维升力问题而言,横向流动的影响作用很大,本文对现有的库塔条件形式进行了改进,在考虑横流影响的前提下,满足随边荷载为零的条件。通过对计算结果与试验结果进行比较表明,将基于B样条的网格划分方法及改进的库塔条件应用于已有的面元法程序中,能够很好地改善计算结果的精度。     

温度对液氢绕水翼非定常空泡流影响的数值研究

该文基于均质多相流理论,采用RNG k-模型和修正系数Zwart空化模型对液氢绕NACA0015水翼非定常空化流动进行数值模拟。模拟了非定常空泡云的生长、断裂、涡状空化团脱落和破裂的周期性过程,模拟中考虑了热力学效应,并分析了温度对空泡流场特性的影响。研究结果表明:在相同空化数和来流速度下,空泡周期随温度增高而变长,升、阻力系数的波动周期随温度增高而增加,但其时均值的随温度增高而减小。     

基于CFD混流式水轮机转轮空化及涡旋流动数值模拟

采用RNK湍流模型和压力速度耦合的SIMPLEC算法,基于欧拉-欧拉方法中均匀多相流假设的混合两相流体无滑移模型,选用Schnerr and Sauer空化模型,对混流式水轮机转轮进行三维空化湍流数值模拟。利用ANSYS CFX,通过转轮的汽含率云图、流线图、turbo面分析以及转轮进出口压力、速度曲线分析,获得了该水轮机在大开度、小开度下的转轮、叶片以及泄水锥的空泡、涡旋黏度等主要流动特征。计算结果表明:大开度和小开度对转轮的空化影响不同,存在一定规律,即大开度下空化严重而范围比较集中;小开度下空化较轻但发生范围较广;大开度下涡旋黏度偏大,则空泡不宜脱流,即不容易在叶片上产生漩涡;小开度则涡旋黏度小,即叶片上漩涡明显。