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。     

某二维水翼空化数值模型对比研究

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。     

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.     

三维水翼云空泡非定常特性数值分析及试验验证

采用Singhal空化模型和修改的κ-εRNG湍流模型,数值模拟了三维扭曲水翼非定常空泡流动及其云空泡的动态脱落,空泡形态、流场等定性结果与相应的试验结果进行了比较。预报的回射流和侧射流的流动特性,以及云空泡的动态脱落的主要特征与试验结果相一致。计算结果表明,对于外形为凸形的片状空泡,侧射流是片状空泡脱落的主要原因。     

绕轴对称体三维非定常空化流动的数值与实验研究

采用数值和实验相结合的方法研究了绕轴对称体三维非定常空化流动现象。实验中,采用高速摄像的技术和动态测力系统对绕半球型和平头型轴对称体的非定常空化流场及其动力特性进行了分析,观察了在不同空化数下,绕半球型和平头型轴对称体的空泡形态;测量了轴对称体受到的阻力,并对阻力信号进行了时频分析,得到了在非定常空化阶段,轴对称体动力特征频率。基于实验现象,发展了一种基于密度修正的分域空化流动的计算方法,并与实验结果进行了对比,结果表明:该方法可以较好的模拟绕轴对称体三维非定常空化流动现象。绕不同头型轴对称体的空化流场结构均存在明显的三维非定常流动特性,并且,空化流场形态与动力特征频率存在高度的相关性,不同头型轴对称体的非定常空化流场结构存在较大的差异,半球型轴对称体空泡流动的脉动主要是空泡尾部的高频小脱落引起的,而平头型轴对称体的空泡流脉动成分主要是大尺度的漩涡空泡团的周期性脱落。     

基于Level set方法的不同湍流模型在水翼空化流计算中的应用研究

该文以二维翼型NACA0015为研究对象,基于Level set方法耦合Zwart空化质量传递模型,采用四种湍流模型分别模拟了水翼诱导的非定常空化流过程,计算结果与实验的对比表明:标准k-ε模型无法准确预测尾部空泡的断裂和脱落,通过滤波器模型修正湍流黏性后可以正确预测空泡演变规律;PANS模型参数fk取值0.2时,可以得到满意的空泡演变规律;SAS-SST湍流模型在处理大分离流动上存在明显优势,对尾部空泡周期性脱落规律预测较好。空泡体积变化和监测点压力脉动频率的关系表明空化的发生和演变是空化区域内压力脉动产生的主要原因。     

二维水翼片空泡脱落及云空化数值模拟

二维空化水翼的非定常特性和云空化演化一直是空泡流领域研究的热点之一。该文采用雷诺平均NS方程(RANS)、分离涡模拟(DES)以及大涡模拟(LES)对绕二维水翼的空泡流进行了数值模拟,重点研究与评估这些数值模拟方法预报空泡流非定常特性的能力,给出了不同流动状态下的二维水翼的水动力系数。通过与二维水翼空化实验数据比较表明,修正湍流黏性系数的RANS数值模型可较准确地预报水动力系数,而DES和LES模型可准确地预报水动力系数和空泡脱落频率,是模拟云空化脱落与演化的有效途径。不同空化数条件下云空泡流场的大涡模拟结果表明,水翼片空泡尾部大尺度旋涡的生成与发展是导致片空泡断裂的主要原因,而一旦片空泡发生断裂则在片空泡下部形成显著的回射流。     

不同空泡脱落模式下空化多尺度特性研究

云状空化是一种以相变为核心,包含多种尺度空泡生长演变行为的强瞬态多相流动现象。该文采用双向耦合欧拉-拉格朗日多尺度空化模型,模拟了空化数σ=1.2时,绕8°攻角三维NACA66水翼空化流动,通过VOF(volume of fluid)界面捕捉法求解欧拉体系下大尺度空穴演化,利用拉格朗日体系下的离散空泡模型(DBM)追踪亚网格尺度离散空泡运动及生长溃灭过程。结果表明：绕水翼云状空化流动,包含附着型片状空泡生长与发展、回射流产生与推进、云状空泡脱落和宏观片空泡再次生长以及压力波推进4个阶段。离散空泡数量及尺度随大尺度空穴周期性演变而变化,主要分布在强湍流脉动区。在各空化发展阶段,离散空泡集中分布在水翼中后部,微空泡直径概率密度函数均符合单峰Gamma分布,最高空泡数密度泡径为50μm。在湍动程度较小的附着型空穴生长阶段,微空泡数密度相对于空泡直径呈-5/3幂率特征;在湍动程度较大的回射流发展阶段、云状空穴脱落溃灭及压力波推进阶段,较小尺度的微空泡数密度相对于空泡直径呈-2/3幂率特征,较大尺度的微空泡数密度相对于空泡直径呈-6幂率特征。     

基于不同空化模型NACA66水翼三维空化特性数值研究

为了比较Zwart-Gerber-Belamri(ZGB)空化模型、Schnerr-Sauer(S-S)空化模型和改进后的Schnerr-Sauer(S-S)空化模型对水翼空化流动数值计算的影响,该文以攻角为6°的NACA66型三维水翼为研究对象,基于密度修正(DCM)的SST k-ω湍流模型,采用湍动能修正后的空化模型并对其非定常空化流场进行数值计算。获得了不同空化模型计算得到的翼型升阻力系数、吸力面空泡形态演变、速度场的分布规律以及翼型表面平均压力分布规律。结果表明,在非稳定空化状态下,ZGB和改进后的S-S均模拟出非定常空化的准周期性,而原始S-S预测非定常准周期性不明显。三种空化模型均捕捉到空化流场中的回射流结构,ZGB模型和改进后的S-S模型较原始S-S模型能捕捉到更强烈的云状空化,且空化向尾缘发展,随后向下游脱落,这与已有文献中的实验结果基本一致。     

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。     

Numerical investigation of unsteady cavitating turbulent flows around twisted hydrofoil from the Lagrangian viewpoint

Unsteady cavitating turbulent flow around twisted hydrofoil is simulated with Zwart cavitation model combined with the filter-based density correction model(FBDCM).Numerical results simulated the entire process of the 3-D cavitation shedding including the re-entrant jet and side-entrant jet dynamics and were compared with the available experimental data.The distribution of finite-time Lyapunov exponent(FTLE) was used to analyze the 3-D behavior of the re-entrant jet from the Lagrangian viewpoint,which shows that it can significantly influence the particle trackers in the attached cavity.Further analysis indicates that the different flow behavior on the suction side with different attack angle can be identified with Lagrangian coherent structures(LCS).For the area with a large attack angle,the primary shedding modifies the flow pattern on the suction side.With the decrease in attack angle,the attached cavity tends to be steady,and LCS A is close to the upper wall.A further decrease in attack angle eliminates LCS A in the boundary layer.The FTLE distribution also indicates that the decreasing attack angle induces a thinner boundary layer along the foil surface on the suction side.     

Shedding frequency of sheet cavitation around axisymmetric body at small angles of attack

Cavity shedding of cavitating flows around an axisymmetric body belongs to the unsteady cavitating flows in the condition of steady incoming current.The periodic characteristics of unsteady cavitating flows around an axisymmetric body at small angles of attack are investigated experimentally and numerically.The evolution and shedding process of the three-dimensional sheet cavitation are computed numerically by the Reynolds averaged Navier-Stokes equations and the RNG k-?model.The modification approach for eddy viscosity coefficient in the transition area of the two-phase flow is adopted to reproduce the shedding process of cavitating flows.The computed frequency of the cavity shedding coincides with the experimental data for the cases of unsteady cavitating flows around axisymmetric bodies with four headforms.Given the cavitation number,the shedding process of the cavitating flow depends heavily on the headform of the axisymmetric body.If the angle of attack of the axisymmetric body is greater than a critical value,the violent shedding of the sheet cavitation seems to be depressed.     

The effect of free surface on cloud cavitating flow around a blunt body

In this study, the effect of the free surface on the cloud cavitating flow around a blunt body is investigated based on the water tank experiment and the CFD method. Numerical results are in good agreement with experimental data, and the mesh independence of the methods is verified. The cavity evolution process includes the cavity growth, the re-entrant jet, the cavity shedding, and the collapse, which can all be observed from the water tank experiment. The effects of the free surface on the cavity length, the thickness, and the cavity evolution period are analyzed by comparing the difference between the cavitating flows on the upper and lower sides of the body. This study also examines the effect of the distance between the free surface and the model through a series of water tank experiments and numerical simulations. The cavity stability and asymmetry, as well as the thickness and the velocity of the re-entrant jet inside the cavity, which varies with the submerged depth, are discussed with consideration of the effect of the free surface. The effect of the free surface on the cavitating flow around the blunt body is enhanced with the decrease of the submerged depth.     

对二维翼空化流动的数值模拟

应用二维不可压缩N-S方程结合改进VOF空化模型模拟了NACA66二维翼的空化流动.数值计算中采用SST k-ω湍流模型计算了不同空泡数下定常稳态空化发生时水翼吸力面的压力分布,并与实验结果进行对比,发现空化末端压力梯度计算值明显大于实验值,而且压力有明显突增,通过修正空化模型中的经验参数较好地解决了这个问题.应用SST-SAS湍流模型及修正的经验参数较好地模拟了低空泡数时非定常云空泡的脱落现象,该数值方法对非稳态空化的发展,脱落过程及其频率的预报结果与实验结果吻合良好.     

修正的SST k-ω模型在云状空化流动计算中的应用研究

采用基于Rayliegh-Plesset方程的空化模型及实验结果,评价了修正的SST k-ω湍流模型在云状空化流动计算中的应用。采用不同的修正系数,分别计算了绕Clar k-y型水翼的云状空化流动,获得了随时间变化的空化形态和升、阻力等流场及动力特性。通过与实验结果的对比表明,修正后的模型可以更准确地捕捉云状空化区域的空穴形态和空泡脱落的非定常细节;密度函数中指数n的选取对计算所得的空穴长度、升阻力和主要频谱分布均有影响,并给出了n的合理取值范围,为后续水工模型的空化数值计算提供了参考依据。     

Three-dimensional large eddy simulation and vorticity analysis of unsteady cavi- taring flow around a twisted hydrofoil

Large Eddy Simulation (LES) was coupled with a mass transfer cavitation model to predict unsteady 3-D turbulent cavitating flows around a twisted hydrofoil. The wall-adapting local eddy-viscosity (WALE) model was used to give the Sub-Grid Scale (SGS) stress term. The predicted 3-D cavitation evolutions, including the cavity growth, break-off and collapse downstream, and the shedding cycle as well as its frequency agree fairly well with experimental results. The mechanism for the interactions between the cavitation and the vortices was discussed based on the analysis of the vorticity transport equation related to the vortex stretching, volumetric expansion/contraction and baroclinic torque terms along the hydrofoil mid-plane. The vortical flow analysis demonstrates that cavitation promotes the vortex production and the flow unsteadiness. In non-cavitation conditions, the streamline smoothly passes along the upper wall of the hydrofoil with no boundary layer separation and the boundary layer is thin and attached to the foil except at the trailing edge. With decreasing cavitation number, the present case has σ= 1.07, and the attached sheet cavitation becomes highly unsteady, with periodic growth and break-off to form the cavitation cloud. The expansion due to cavitation induces boundary layer separation and significantly increases the vorticity magnitude at the cavity interface. A detailed analysis using the vorticity transport equation shows that the cavitation accelerates the vortex stretching and dilatation and increases the baroclinic torque as the major source of vorticity generation. Examination of the flow field shows that the vortex dilatation and baroclinic torque terms increase in the cavitating case to the same magnitude as the vortex stretching term, while for the non-cavitating case these two terms are zero.     

Numerical study on influence of structural vibration on cavitating flow around axisymmetric slender body

The unsteady behaviors of cloud cavitating flow would lead to structural vibration and deformation that conversely affect its development. The present paper aims to preliminarily discuss the influences of structural vibration on the development of the cavitating flow. Simulations of a slender body are carried out under different vibration amplitudes and frequencies. The results show that the structural vibration causes alternate variation of local attack angle at the head of the body, and thus changes the development of cavitation and re-entrant jet. On the downstream side, the length and thickness of the cavity are larger than that on the upstream side due to larger area of negative pressure. For a large vibration amplitude, alternate variations of the local attack angle change the adverse pressure gradient at the closure of the cavity, and then affect the development of the re-entrant jet, so that the phenomena of local shedding of the cavitation happen, compared with global shedding in the case of no structural vibration. For a frequency larger than 0.05, transverse speed of the vibration is suggested to be a dominant factor in controlling the behavior of the cavitating flow besides the local attack angle, since it causes local cavitating phenomena.     

双圆柱在不同排列方式下绕流流态模拟

双圆柱绕流伴随着流动分离、旋涡生成与脱落、旋涡间相互干扰等复杂问题,其流动形态和流动特征受圆柱相对位置影响。使用FLUENT流体软件,选取间距比1.75,2.5和4,在二维层流模型下,模拟了双圆柱串列、30°夹角错置、60°夹角错置和双圆柱并列绕流,分析了双柱绕流流态、旋涡脱落形态、升力、阻力系数随圆柱相对位置改变而变化的规律,并对比已有的试验成果和模拟成果,为桥梁建设和圆柱绕流理论研究提供了基础数据。     

NUMERICAL SIMULATIONS OF CAVITATING FLOWS

BRIEFINTRODUCTIONOFTHEPAPER :　Thisthesismainlyresearchesonthehydrodynamiccharacteristicsandmechanismfor 3Dcavitatingflowsaroundaxisymmetricbodies ,aswellasthe2Dcavitatingflowsaroundhydrofoils .Anewcavi tatingflowmodel,whichinvolvesviscousandmul ti phaseeffects,isestablishedintwo phaseflowcategory .Accordingtothelocalizedvariationofdensitywithinpredominantlyincompressiblewatermediumandthecharacteristicsofsoundspeedinwater vapormixture ,arelationbetweendensityandpressureisassumed ,in…     

Large eddy simulation of turbulent attached cavitating flow with special emphasis on large scale structures of the hydrofoil wake and turbulence-cavitation interactions

In this paper, the turbulent attached cavitating flow around a Clark-Y hydrofoil is investigated by the large eddy simulation(LES) method coupled with a homogeneous cavitation model. The predicted lift coefficient and the cavity volume show a distinctly quasi-periodic process with cavitation shedding and the results agree fairly well with the available experimental data. The present simulation accurately captures the main features of the unsteady cavitation transient behavior including the attached cavity growth, the sheet/cloud cavitation transition and the cloud cavitation collapse. The vortex shedding structure from a hydrofoil cavitating wake is identified by the Q- criterion, which implies that the large scale structures might slide and roll down along the suction side of the hydrofoil while being further developed at the downstream. Further analysis demonstrates that the turbulence level of the flow is clearly related to the cavitation and the turbulence velocity fluctuation is much influenced by the cavity shedding.