  The small vortex generation is a key issue of the mechanism for late flow
transition and turbulence generation. It was widely accepted that small length
vortices were generated by large vortex breakdown. According to our recent DNS,
we find that the hairpin vortex structure is very stable and never breaks down
to small pieces. On the other hand, we recognize that there are strong positive
spikes besides the ring neck in the spanwise direction. The strongly positive
spikes are caused by second sweeps which are generated by perfectly circular
and perpendicularly standing vortex rings. The second sweep brings energy from
the invisid region downdraft to the bottom of the boundary layers, which
generates high shear layers around the positive spikes.Since the high shear
layer is not stable, all small length scales (turbulence) are generated around
high shear layers especially near the wall surface (bottom of boundary layers).
This happens near the ring neck in the streamwise direction and besides the
original vortex legs in the spanwise direction.The small length scales then
rise up from the wall surface and travel to the upper boundary layer.
