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This paper is devoted to the investigation of the origin and mechanism ofrandomization in late boundary layer transition over a flat plate withoutpressure gradient. The flow randomization is a crucial phase before flowtransition to the turbulent state. According to existing literatures, therandomization was caused by the big background noises and non-periodic spanwiseboundary conditions. It was assumed that the large ring structure is affectedby background noises first, and then the change of large ring structure affectsthe small length scales quickly, which directly leads to randomization andformation of turbulence. However, by careful analysis of our high order DNSresults, we believe that the internal instability of multiple ring cyclesstructure is the main reason. What we observed is that randomization beginswhen the third cycle overlaps the first and second cycles. A significantasymmetric phenomenon is originated from the second cycle in the middle of bothstreamwise and spanwise directions. More technically, a visible asymmetricphenomenon in the middle vortex ring cycle starts at time step t=16.25T andx=838.9{\delta}in where the top and bottom level rings are still completelysymmetric. The non-symmetric structure of middle level ring affects the smalllength scale in boundary layer bottom quickly. The randomization phenomenonspreads to top level through ejections. Finally, the whole flow domain becomesrandomized. A hypothesis of C- and K-types shift is given as a possiblemechanism of flow randomization.

DNS Study for the origin of the flow Randomization in Late Boundary Layer Transition