翼型空泡周期性流动的数值模拟及机理分析

Numerical simulation and mechanism analysis on the periodic cavitating flow around hydrofoil

采用基于正压关系的均质平衡流空化模型和低雷诺数修正的k-ε湍流模式,自行开发了空泡流数值模拟方法和计算软件,对绕翼型空泡的周期性流动现象进行了数值模拟。计算结果与实验数据的对比表明,空泡的宏观特征、流动特性、周期性脱落的斯坦顿数St等与试验结果接近,验证了计算结果的可靠性。空泡在大约一个周期的2/3时间段内成长,并在大约1/3周期时刻发生断裂脱落。这两个特征时间与高速摄像实验结果一致。所取工况对应的组合参数σ2α=2.865,以翼弦长计算可得St=0.217,与文献的最新试验结果吻合。空泡周期性运动过程中升阻系数也周期性振荡,时均值Cl=0.41,Cd=0.097,振荡频率与空泡脱落频率一致。对空泡运动过程中流场结构的变化进行了分析,结果表明在大攻角条件下,空泡闭合区后的逆压梯度导致涡的形成及回射流的发展,沿壁面逆向流动的混合介质射流是引起空泡断裂的原因,回射流发展、涡结构变化与空泡非稳态演化过程存在密切的联系,探讨了翼型空泡发生周期性脱落的一些机理。

Based on the self-developed numerical simulation methods and computer code for cavitating flows,the periodic shedding phenomenon of the cavitating flow around hydrofoil is simulated.The homogeneous equilibrium model based on barotropy is employed,coupled with the linear turbulence model with correction for low Reynolds number conditions,and the numerical methods suitable for computation of cavitating flows are proposed as well.It is found from comparison between the computational and the experimental results that,the cavity character,flow property and Strouhal number are generally in accordance with experimental results,therefore the reliability of the numerical methods and the code is verified.The simulated cavity keeps growing during the time span of about 2/3 of a period and breaks up at the moment of about 1/3 of a period.These two characteristic times are in correspondence with experimental observations by high-speed photography.The computed Strouhal number based on the chord length of hydrofoil is =0.217,for the working condition of =2.865 in the present work,which is consistent with the experiments reported in recent literatures.The lift and the drag coefficients oscillate periodically with time-averaged values of =0.41 and =0.097,along with the periodic shedding of the cavity.The oscillation frequency accords well with the shedding frequency.The evolution of flow structure during the process of cavity movement is analyzed in detail to find that,there exist close relations between the unsteady evolution of cavity and the development of vortex structure,and between the adverse pressure and the production of reentrant jet flow.Therefore the mechanisms behind the periodic shedding process of cavity are discovered.