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

本研究的目的是实验研究湍流边界层的相干结构，它具有倾斜的速度分布，在轴向流中的旋转圆柱，特别是，以确定和澄清在这个边界层的螺旋涡结构。该流动是三维湍流壁面流动的基本模型之一，应将其视为受体积力和附加应变率影响的复杂湍流流动。针对静止圆柱，利用VITA技术和四象限法，从瞬时时间序列数据中研究了其猝发现象和湍流拟序结构。这些量与平板湍流边界层没有发现任何关键的差异。为了研究螺旋涡结构，构造了一个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。 ...更多信息 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，第二模式和第一模式的本征函数对相干结构的识别至关重要。利用第一、二阶模态识别了作为湍流边界层结构之一的湍流边界层外区的湍流与非湍流特性，并利用热线风速仪测量了旋转圆柱情况下的斜平均速度分布和脉动速度分布，并根据实测的平均速度剖面检验了目前提出的几个普适速度分布定律的有效性，尝试用平坦因子和但是用这些参数很难捕捉到旋转圆柱体的相干结构。然后研究了脉动速度的功率谱。旋转圆柱体的每个功率谱在中间频率范围内表现出凸点，而对于固定圆柱体以及平板，它们随着频率的增加而单调下降，并且凸点频率随着旋转速度的增加而变高。这一事实清楚地表明，旋转圆柱湍流边界层的相干结构是相当规则的，足以在频谱中显示出来，并表明存在着一种螺旋涡结构缠绕着圆柱。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