湍流边界层大尺度相干结构空间相位平均拓扑1)
SPATIAL PHASE MEAN TOPOLOGY OF LARGE-SCALE COHERENT STRUCTURES IN TURBULENT BOUNDARY LAYER1)
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摘要: 湍流边界层大尺度相干结构(large-scale coherent structure,LSCS)是目前湍流边界层研究的热点之一。湍流边界层中存在大尺度相干结构均是通过各种谱的低波数峰值证实的,而对大尺度相干结构各种物理量的空间拓扑形态及其发展演化规律还缺乏研究。应用高时间分辨粒子图像测速多相机阵列系统,测量湍流边界层大尺度相干结构各种状态的空间拓扑。实验雷诺数Re_\tau =422,采用沿流向方向按顺序排列四个高速相机同步拍摄,得到约6.7\delta\times 1.2\delta的大视场。在所有法向层沿流向方向进行空间多尺度连续小波变换,根据分尺度小波系数得到湍动能随流向尺度和法向位置的分布,根据各尺度小波系数的正负极值点检测各尺度相干结构喷射和扫掠的拓扑中心,使用条件采样和空间相位平均的方法测得各尺度相干结构喷射和扫掠阶段的空间拓扑形态,分别得到喷射与扫掠事件的脉动速度矢量、展向涡量、流线等物理量的空间相位平均拓扑,发现大尺度喷射和扫掠事件的结构都是多个小涡组成的大尺度涡包结构,每个小涡的流线形成鞍点和焦点组成的局部不稳定动力系统。Abstract: Large-scale coherent structure (LSCS) is one of the hot topics in turbulent boundary layer research. The existence of large-scale coherent structure in turbulent boundary layer is confirmed by the low wave number peak of various spectrum, but the spatial topology of various physical quantities of large-scale coherent structure and its development and evolution law are lack of research. A multi-camera array system with time-resolved particle image velocimetry is employed to measure the spatial topology of various states of large-scale coherent structures in turbulent boundary layer. Four high-speed cameras arranged sequentially along the streamwise to obtain a field of view of about 6.7\delta \times 1.2 \delta sufficient to investigate the large-scale structures, where the experimental Reynolds number is Re_\tau=422. The spatial multi-scale continuous wavelet transform is carried out in all normal-wall layers along the longitudinal direction of flow, and the energy distribution along the scale and normal position is obtained according to the wavelet coefficients. The topological centers of the coherent structures are detected according to the positive maximum points and negative minimum points of the wavelet coefficients at each scale, the spatial topological morphology of the coherent structures at different scales during the eject and sweep are measured by means of conditional sampling and spatial phase averaging methods, then the spatial-phased average topologies of the fluctuating velocity vector, the spread vorticity and the streamline of the eject and sweep events are obtained respectively. It is found that the vortex structure of the large-scale eject and sweep events is a large-scale vortex packet structure composed of several small vortices, and the streamline of each small vortex forms a local unstable dynamic system composed of saddle points and focal points.