This paper presents an experimental investigation of the spatial-temporal evolution of Lagrangian structures in the transition process of a boundary layer with a unit Reynolds number of 9.7×106/m and Mach number of 6.The development of unstable waves and the associated wall temperature distribution are measured by an infrared camera;the turbulence generation process is observed by utilizing state-of-the-art ultrafast flow visualization methods and particle image velocimetry.The results show that the second mode waves are modulated by long-wavelength and low-frequency waves,forming wave packets at the late transition stage.The evolution of Lagrangian material lines,based on the velocity fields derived from particle image velocimetry,demonstrates that the wave packets undergo three steps.Firstly,near-wall fluids are lifted upward to be soliton-like waves;secondly,the fluids away from the wall are swept down over the bulge,yielding a localized strong shear layer;finally,the shear layer eventually evolves into vortical structures.
维普
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