绕平头回转体的空化流场研究

Study on the Cavitating Flow around the Blunt Axisymmetric Body

采用实验与数值模拟相结合的方法对绕平头回转体的空化流场进行了研究。数值模拟中,为了精确捕捉由于分离流动而产生的漩涡结构和空泡团的脱落现象,湍流模型采用了一种基于空间尺度修正的滤波器模型(FBM)。实验中,采用高速录像技术观察了在不同空化数下,绕回转体的空泡形态,并应用二维和三维粒子测速系统(DPIV)测量了相应工况下,空化流场的速度及涡量分布。研究结果表明:对于平头回转体,其肩部的高剪切流动区出现了不规则的漩涡分离结构,初生空泡首先在该分离区域内产生。随着空化数的降低,空化区域会有明显的增大,空泡呈椭球状,不规则的小尺度空泡团的脉动转变为周期型的大尺度空泡团的脱落现象。在不同的发展周期内,绕平头回转体的空化存在强烈的三维流动特性,回转体尾部的高压区域造成了反向射流的产生,反向射流以逆时针的螺旋状逐渐向回转体头部推进,致使空泡周向上的断裂,以及空泡脱落的起始位置有较大的区别。流场中的低速区域对应于空化核心区,其值远小于主流速度。高涡量主要亦集中在该区域内,空化区域内部汽液频繁的水汽交换是造成高涡量区域的主要原因。

The structures of unsteady cavitating flows around a blunt axisymmetric body were investigated by the experimental and numerical methods.In order to accurately capture the vortex separation and cloud shedding,a filter-based k-εturbulence model was applied in numerical simulations.In experiment,the high-speed video camera was used to visualize the unsteady flow structure,and 2D and 3D particle image velocimetry（PIV）were used to measure flow field,including the velocity and vorticity fields.The experimental and numerical results show that the high shear vortex structures locate in the shoulder area of the blunt object,and the inception cavity appears in the boundary layer separation zone near the wall,but not the adherent area.With the decrease of cavitation number,cavitation region will significantly increase,and the profile of the cavity is ellipsoidal shape.Irregular pulse of small-scale bubble group changes into larger-scale cloud shedding.As in the experiment,the modeled reentrant flow has been observed to follow a helical pattern.This helical flow revolves around the circumference of the cylinder.The flow enables from the high pressure region downstream of the cavity.The high pressure region is situated at the aft end of the azimuthal section of the cavity of greatest axial extent.During its initial formation,due perhaps to turbulence fluctuations,the reentrant flow was initially driven and then moved permanently in a helical path.The velocity of this region is lower that of the main flow area,but the vorticity is larger than other regions,indicating that obvious vortex structures exist in the part,due to the frequently mass and momentum transformation.