圆盘近尾迹流场相关谱分析

CORRELATION SPECTRUM ANALYSIS IN THE NEAR WAKE BEHIND A DISK

对Re=22000时径厚比D/h=5圆盘近尾迹开展大涡模拟数值研究.通过对x/D=1,2和8处脉动速度进行快速傅里叶变换(fast Fourier transformation,FFT),发现3个特征频率:斯特劳哈尔数St2=0.123为自然脱落频率,与文献结果相符;较小频率St1=0.035,与回流区伸缩和剪切层附近涡旋脱落点的旋转相关;高频率St3=1.3～1.7则与剪切层湍流结构相关.通过分析截面r/D=2.8圆周上两点间流向速度相关系数、相干谱和相位谱,发现相关系数受涡旋脱落影响出现以30?或45?为周期的正负波动,表明轴面上涡旋脱落点具有随机性.

A large eddy simulation （LES） is conducted with the diameter-thickness ratio D/h = 5. Through in the near wake behind a circular disk at Re = 22 000 fast Fourier transformation （FFT） of the fluctuating velocities at x/D = 1, 2 and 8, three characteristic frequencies are identified, with corresponding Strouhal numbers being 0.035, 0.1.23 and 1.3-1.7. It is shown that St2 =0.123, is the natural vortex shedding frequency, as in agreement with previous research results; St1 = 0.035, the smaller one, may be related to the combined action of the recirculation zone＇s ＂pump＂ motion and the azimuthal rotation of the shedding location near the shear layer; St3=1.3 - 1.7, might result from the instability of the turbulent shear layer. The correlation coefficients, the coherent spectrum and the phase spectrum are calculated between two points with different separation angles in the same axial section of r/D = 2.8. Due to the vortex shedding, the correlation coefficients fluctuate between positive and negative values in the intervals of 30 or 45 degrees. This indicates that the position of the vortex shedding is random in the azimuthal direction.