SPATIAL PHASE MEAN TOPOLOGY OF LARGE-SCALE COHERENT STRUCTURES IN TURBULENT BOUNDARY LAYER¹⁾

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.