  The mechanism of randomization in late boundary layer transition is a key
issue of late boundary layer transition and turbulence theory. We studied the
mechanism carefully by high order DNS. The randomization was originally
considered as a result of large background noise and non-periodic spanwise
boundary conditions. It was addressed that the large ring structure is affected
by background noises first and then the change of large ring structure affects
the small length scale quickly, which directly leads to randomization and
formation of turbulence. However, what we observed is that the loss of symmetry
starts from the middle level rings while the top and bottom rings are still
symmetric. The nonsymmetric structure of second level rings will influence the
small length scales at the boundary layer bottom quickly. The symmetry loss at
the bottom of the boundary layer is quickly spread to up level through
ejections. This will lead to randomization of the whole flow field. Therefore,
the internal instability of multiple level ring structure, especially the
middle ring cycles, is the main reason for flow randomization, but not the
background noise. A hypothesis is given that the loss of symmetry may be caused
by the shift from C-type transition to K-type transition or reverses.
