NACA 66_2-415水翼空化梢涡几何特征及涡唱频率分析

Geometry and singing frequency characteristics of cavitating tip vortex around NACA 66_2-415 hydrofoil

在一定的流动条件下,螺旋桨和水翼梢涡空化会伴随着涡唱现象。为了探究涡唱现象的产生机理与梢涡几何特征的关系,该文通过对NACA 66_2-415水翼空化梢涡高速摄像图像的定量处理分析,获得了涡唱发生前后空化梢涡的几何形态特征。发现未发生涡唱梢涡空化区域的形态不随时间而发生变化,呈现稳定的双螺旋形态;而涡唱产生时,梢涡空化区域在双螺旋结构基础上伴随一定频率的径向膨胀和收缩。通过高速摄像图像处理得到的空化梢涡振动频率、梢涡空化色散关系得到的理论拍振频率和圆柱泡本征频率与涡唱噪声频率之间的对比分析,发现梢涡径向振动频率与涡唱频率接近,表明空化梢涡径向膨胀收缩是涡唱产生的直接原因。

In specific flow conditions,singing of tip-vortex occurs for cavitating flow around propellers and hydrofoils.To explore the relationship between the singing and the topology of tip-vortex cavitation,the topological information of tip-vortex cavitation was investigated from high-speed camera images before and after the singing in cavitation tunnel experiments.For non-singing cases,the shape of tip-vortex cavitation appears a stable double-helix structure.While during the singing,this double-helix structure shrinks and expands at a specific frequency.Four frequencies relevant to tip-vortex cavitation,including the oscillation frequency of the topological structure obtained by image processing,theoretical beat frequency obtained from dispersion relation,the natural frequency of cylindrical bubble,and the acoustic singing frequency in experiment,are compared in this paper.It is found that the oscillation frequency of the topological structure is close to the singing frequency,which indicates that the spatial oscillation of the cavitation region is responsible for the singing.