摘要：

Since the 60s of the last century, the low-speed streaks, high-speed streaks, burst, ejections, sweeps and various vortical structures in turbulent boundary layers were discovered using flow visualization techniques. These coherent structures have significant influences on the skin friction and heat transfer of turbulent boundary layers, and also on the generation of turbulence kinetic energy. The induced velocities of vortical structures influence the evolution of other coherent structures, thus they are the kernels of the coherent structures in a turbulent boundary layers. Hairpin vortex is generally accepted as the basic structure in a turbulent boundary layer. There were many papers on hairpin vortex after the 80s. The generation and re-generation of hairpins and other vortices are the essential factor for the self-sustaining of coherent structures in turbulent boundary layers. It becomes one of the hot topics in studies of turbulent boundary layers in the 90s. Burst created by the low speed fluid ejected from wall region is the most important source for the generation of turbulence energy. Conditional sampling is an important technique for measuring the frequency of burst and the frequency of occurrence of other coherent structures. Observations using flow visualizations revealed the basic qualitative characteristics of coherent structures, on which many ideas were developed for drag reduction and enhancing heat transfer. After the 80s, multi-hot wire and PIV techniques were developed, which were applied for measuring the instantaneous flow velocity vector field. The development of three-dimensional PIV technique may be a break-through in the experimental research on coherent structures. In the present paper, discussions are focused on flow visualization, multi-hot wire and PIV techniques, their advantages and defects, also their contributions to their investigations of coherent structures.

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