Numerical Study on Randomization in Late Boundary Layer Transition

The mechanism of randomization in late boundary layer transition is studied carefully by high order DNS. The randomization was originally considered as a result of background noise. 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, our new DNS results show that all small length scales are generated by high shear layers without exception. The asymmetric structure is firstly happened at the middle level of streamwise and spanwise direction, which eventually deforms the shape of shear layer. The consequence is that some of the small length scales around high shear disappear and the small vortex legs detached from the wall through separation, so that the flow loses symmetry and becomes randomized.