The commonly accepted concept that “Lambda vortex self-deforms to hairpin vortex” does not exist. The flow transition is mainly an inherent property of fluid flow, which shows that fluids cannot tolerate shear and shear must transfer to rotation when Reynolds number is large. They cannot form the vortex by themselves and cannot cause the flow transition directly.

Second, the linearly unstable modes are important, but the role of these modes is only to trigger the vorticity rollup. Turbulence is built up by organized “vortex packets” which can be accepted by Navier-Stokes equations. First, it is not appropriate to call the turbulent flow as a “random” motion since the conservation law of mass, momentum and energy must be satisfied. This includes mechanism of the large coherent vortex structure formation, small length scale generation and flow chaos. In this paper, a systematic report of our recent DNS study on physics of late boundary layer transition is presented. It is believed that the small vortices are generated by the interaction of higher level vortices with the solid wall, but not by the “vortex breakdown”. The small vortices can be found on the bottom of the boundary layer near the wall surface. The “hairpin vortex breakdown” does not happen.

The multiple ring-like vortex structure is found quite stable and can travel for a long distance. Actually, the U-shaped vortex, which is a third level vortex, serves as a second neck to supply vorticity to the multiple rings. The U-shaped vortices are part of existing coherent large vortex structure. This process keeps going onto form a multiple ring structure.

The bridge finally develops as a new ring. A bridge must be formed to link two Λ-vortex legs. The formation of the multiple ring structure follows the first Helmholtz vortex conservation law. The ring-like vortex formation from the Λ-vortex is the result of the interaction of two pairs of counter-rotating primary and secondary streamwise vortices.

The coherent vortex structure of the late flow transition stages and the mechanism of formation of single vortex ring, multiple vortex rings, and small length scales are discussed. A new DNS using compact high order scheme and MPI parallel computation has been conducted with 1920×241×128 grid points for non-linear stages of flow transition.

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