Identification and clarification of the spiral vortex structure in the turbulent boundary layer on a rotating cylinder

本研究的目的是通过实验研究轴向流动中旋转圆柱上具有倾斜速度分布的湍流边界层的相干结构，特别是识别和澄清该边界层中的螺旋涡结构。这种流动是三维紊流壁面流动的基本模型之一，应视为受体力和额外应变率影响的复杂紊流。利用VITA技术和四象限方法，对静止圆柱体的瞬时时间序列数据进行了爆破现象和湍流相干结构的研究。这些量与平板湍流边界层没有明显的差别。为了研究螺旋涡结构，构建了利用热线耙的测量系统，并首次尝试提取平板边界层的相干结构。对多点瞬时时间序列数据进行了适当正交分解作为相干结构提取工具之一的分析。结果表明，第二模态特征函数和第一模态特征函数对相干结构的识别至关重要。利用第一模态和第二模态区分了作为外层结构之一的湍流边界层外区域的湍流和非湍流特征。用热线测速法测量了圆柱旋转时的偏斜平均速度分布和波动速度分布，并对目前提出的几种普遍速度分布规律相对于测量的平均速度分布的有效性进行了检验。尝试利用平面度因子和间断度因子来查询外区域的相干结构，但利用这些参数很难捕捉到任何适合旋转圆柱体的结构。然后研究了脉动速度的功率谱。旋转圆柱体的各功率谱在中频范围内均有凹凸，而静止圆柱体和平板的各功率谱随频率的增加而单调减小，且随着转速的增加，凹凸频率逐渐增大。这一事实清楚地表明，旋转圆柱体上湍流边界层的相干结构是相当规则的，足以在光谱中显示出来，并表明存在缠绕圆柱体的螺旋涡结构。少

The objective of the present study is to investigate experimentally the coherent structure of the turbulent boundary layer, which has skewed velocity profiles, on a rotating cylinder in an axial flow, and, in particular, to identify and clarify the spiral vortex structure in this boundary layer. This flow is one of the basic models of three-dimensional turbulent wall flows, which should be treated as the complex turbulent flows affected by body forces and extra rates of strain.For a stationary cylinder, the bursting phenomena and the turbulent coherent structure were investigated using VITA technique and four-quadrant method from the instantaneous time-series data obtained. Any critical differences in these quantities from the flat-plate turbulent boundary layer were not found.In order to investigate the spiral vortex structure, a measuring system utilizing hot-wire rake was constructed, and firstly an attempt was made to extract the coherent structure of the flat-plate boundary layer. … More The analysis of proper orthogonal decomposition as one of the tool to extract the coherent structure was undertaken for the multi-point instantaneous time-series data. The result showed that the eigenfunction of the second mode as well as the first mode was crucial to identify the coherent structure. The turbulent and non-turbulent character in outer region of the turbulent boundary layer that is one of the structures of the outer layer was discerned by using the first and the second modes.Skewed mean velocity profiles and fluctuating velocity distribution in case of rotating cylinder were measured using the hot-wire anemometry, and the validity of several laws of the universal velocity distribution proposed so far with respect to the measured mean velocity profiles were examined.An attempt was made by using the flatness factor and intermittency factor to inquire the coherent structure in the outer region, but it was difficult to capture any structure proper to the rotating cylinder with these parameters. Then the power spectra of the fluctuating velocities were investigated. Each of the power spectra for the rotating cylinder exhibited a bump in the middle frequency range, whereas for the stationary cylinder as well as for the flat plate they decreased monotonically with increase in the frequency, and the bump frequency became higher as the rotation speed increased. This fact showed clearly that the coherent structure of the turbulent boundary layer on a rotating cylinder was fairly regular enough to reveal itself in the spectrum, and it was suggested that there existed a spiral vortex structure enwinding the cylinder. Less