绕圆盘空化流动的拉格朗日拟序结构分析

The Lagrangian Coherent Structure-based Investigation of Cavitating Flows around a Disc

基于有限时间Lyapunov指数(FTLE)和拉格朗日拟序结构(LCS)对绕圆盘空化器非定常空化流动机理进行了分析。数值计算中采用了能准确反映气液两相相间作用的非均相流模型,通过与实验结果的对比,验证了计算结果的可靠性。分析结果表明:FTLE场中出现两种LCS——平直LCS和环状LCS,平直LCS反映了圆盘对流体运动的阻碍作用,环状LCS准确捕捉了圆盘后部大尺度涡旋结构的边界及涡心的位置;随着空穴尺度增大,涡旋结构得到加强,分布更为集中;反向射流强度的周期性变化是造成空穴和涡旋结构周期性变化和运动的原因,反向射流强度增大使得圆盘后端面出现低FTLE值区域,该区域与轴截面上的流线起始点存在对应关系。

The cavitating flows around a disc is analyzed based on finite-time Lyapunov exponents （FTLE） and Lagrangian coherent structure （LCS）. The inhomogeneous model is utilized to predict the interaction between vapour and water flows in the simulation. The simulation results are confirmed by the experimental data. The analytical result shows that LCS defined by the ridges of FTLE can be divided into straight and circular LCSs. The straight LCS reflects that the disc hinders the motion of fluid, and the cir- cular LCS distinguishes the large-scale vortex and identifies the center of vortex accurately. The vortex is intensified and the distribution is concentrated with the development of cavity. It is the periodic change in the intensity of re-entrant flow that causes the periodic expansion and contraction of cavity. A low FTLE region emerges near the support-rod with the increase in re-entrant flow, and there exists a corresponding relationship between the region and the start point of streamline.