非定常空化流动涡旋特性分析

基于试验和数值结果对三维非定常空化流动的涡旋特性进行分析.试验采用高速摄像系统对三维空穴发展过程进行观测.数值计算中采用混合密度分域的湍流模型和基于相间质量传输的空化模型.与试验结果对比,表明该计算方法能够准确捕捉云状空化的非定常过程：附着空穴的初生和发展,附着空穴的断裂,以及大尺度脱落空泡团的形成.基于计算结果,采用涡量分析法并引入复杂流场中的涡旋结构判据研究了空化现象的发生和发展对涡量输运过程的影响.结果表明：反向射流的形成和发展导致速度梯度的变化,从而引起涡量产生项的非定常变化;气液两相的相互转换会导致流场内体积变化率以及密度梯度的变化,同样引起体积变化率项和斜压矩项的非定常变化.采用速度梯度张量不变量Q和R分析空化现象的涡旋特性可以发现：在水翼前端附着空穴区域,旋转效应大于形变效应;脱落空泡位置附近,旋转效应和形变效应共同支配云状空泡运动.

Vortex dynamic analysis of unsteady cavitating flows around a hydrofoil

Vortex dynamics methods were used to analyze the 3-dimensional unsteady cavitating flows around a hydrofoil based on both experimental and numerical results. The experiment was conducted in the cavitation tunnel in Beijing Institute of Technology. High speed video camera technique was used to capture the 3-dimensional cavity evolutions. Filter-based density correction turbulent model and classical interface mass transfer cavitation model were used in the simulations. Comparing with experimental results, the numerical method is able to capture the initiation of the cavity, growth toward the trailing edge, and subsequent shedding. Based on the computations, vorticity transport equations and complex vortex identify methods were conducted to analyze the influence of cavitation on vortieity transport process. Following conclusions can be drawn： re-entrant jet near trailing of attached cavity leads to distinct changes of velocity gradient, which has great influence on production and dissipation of vorticity. Intensive mass transfer between liquid and vapor phase may induce the dilation and baroclinie torque. Two factors mentioned above keep changing and result in unsteady distributions of vorticity. Cavitation may lead to unsteady mass and momentum transfers between liquid and vapor phase, which will change turbulence and vorticity distributions distinctly. With Q and R methods, following conclusions can be drawn： rotation effect is stronger in the area near the leading edge of attached cavity, while rotation and deformation effect work together in area of detached cloud cavity.