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An experimental investigation was performed on the wake flow field of an isolated roughness element of a flat plate at Mach 6 by employing the nanoparticle-based planar laser scattering (NPLS) approach. The three-dimensional features and causes of the flow field structure were scrutinized by transient flow field images of roughness elements on various planes. The time-resolved NPLS technique was implemented to examine the time evolution characteristics of the wake flow field of roughness elements. In the following, the process of dynamic evolution of large-scale vortex structures in the wake flow field was methodically assessed. Additionally, the influences of roughness element heights on the wake vortex structure were evaluated and the obtained results were compared.

High-speed and low-speed streak structures in the near wall region of a turbulent boundary layer with a Mach number of 3 are experimentally examined by employing the spatiotemporally resolved nanoparticle plane laser scattering technique. The time evolution characteristics of the high-speed and low-speed streaks in the supersonic turbulent boundary layer are systematically investigated through the speed field sequence results at various time intervals. The obtained results reveal that the dynamic behavior of the bands is chiefly represented by the translation along the flow direction. The process of dissipation of the existing streaks and the formation of a new streak is also observed and analyzed. The duration values of the high-speed and low-speed streak structures are assessed by utilizing the time-resolved characteristics of the speed field, and the predicted duration of the streak structure and the maximum flow length exceed 306 µs and 23.6 times the thickness of the boundary layers, respectively. Finally, the merging phenomenon of medium and low-speed streaks in the turbulent boundary layer is carefully scrutinized. The merging of low-speed streaks observed under supersonic conditions is consistent with the vortex packet merging model proposed by Tomkins and Adrian.