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流中旋转圆柱体上具有倾斜速度分布的湍流边界层的拟序结构，特别是识别和澄清该边界层中的螺旋涡结构。这种流动是三维湍流壁面流动的基本模型之一，应将其视为受体力和附加应变率影响的复杂湍流流动。对于固定的圆柱体，利用VITA技术和四象限方法，根据获得的瞬时时间序列数据，研究了其破裂现象和湍流相干结构。为了研究螺旋涡旋的结构，搭建了一套利用热线前角的测量系统，首先尝试提取平板边界层的相干结构。…在此基础上，对多点瞬时时间序列数据进行了本征正交分解分析，作为提取相干结构的工具之一。结果表明，第二模和第一模的本征函数是识别相干结构的关键。利用第一和第二模式识别了作为外层结构之一的湍流边界层外部区域的湍流和非湍流特性，利用热线风速仪测量了旋转圆柱情况下的倾斜平均速度分布和脉动速度分布，并检验了迄今提出的几个普适速度分布定律相对于测量的平均速度分布的有效性。尝试用平坦度因子和间歇因子来查询外部区域的相干结构，但利用这些参数很难捕捉到适合旋转圆柱的任何结构。然后对脉动速度的功率谱进行了研究。旋转圆柱体的功率谱在中频范围内均出现一个凸起，而静止圆柱体和平板的功率谱随频率的增加而单调减小，且凸起频率随转速的增加而增大。这一事实清楚地表明，旋转圆柱体上的湍流边界层的拟序结构非常规则，足以在谱中显示其自身，并表明圆柱体周围存在螺旋涡结构。较少

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