绕单自由度涡激振动圆柱的速度环量的时空分布特征

Spatiotemporal distribution characteristics of velocity circulation around a vortex-induced vibration cylinder with single degree of freedom

对低质量比(质量比为8.63)的单自由度圆柱的涡激振动进行了数值模拟,模拟中选取了四组折合速度(第一组对应位移响应的初始分支,另三组对应下端分支)。模拟所得的圆柱运动响应和升力系数与Williamson的相应实验结果相符。根据模拟所得的流场对各组案例中绕圆柱的速度环量进行了计算和分析,结果表明:①无量纲化速度环量在时空上总体呈交替的条带分布,条带的斜率和宽度分别对应于环量向外传播的速度与环量的变化周期,条带斜率随来流速度增大而增大;②经验正交函数(EOF)分解得到的第1和第2空间模态包含了环量时空分布的主要信息,且其均对应于第一主频;③环量均方根值沿径向的分布在初始分支段与下端分支段之间有显著区别,导致这一差别的主要原因是其对应的尾流脱涡模式不同--分别对应于2S和2P模式。

The vortex-induced vibration(VIV) of a single-degree-of-freedom cylinder with a low mass ratio(mass ratio of 8.63) was simulated numerically. Four sets of reduced velocity were selected in the simulations, with the first group corresponding to the initial branch of the wake patterns and the other threes corresponding to the lower branch. The simulated cylinder’s motion response and lift coefficient are consistent with the experimental results from Williamson. In term of the simulated flow field, the velocity circulations of four sets around the cylinder were calculated and analyzed. The results show that:1) In general, the dimensionless velocity circulations are spatiotemporal distributed in arranged stripes. The slope of the strips corresponds to the spread velocity of the circulation and the width of the strips to the period of the circulation. As the reduced velocity increases, the slope of the strips inclines increases. 2) The first and the second spatial modes obtained by empirical orthogonal function(EOF) contain the main information of the spatiotemporal distribution of circulation, and they all correspond to the first dominant frequency. 3) The distribution of the root mean square value of circulation in the radial direction of the initial branch is significantly different from that of the lower branch, the vortex shedding patterns are the main reason for the difference, corresponding to 2 S and 2 P modes respectively.