DNS on the Evolution of Vortices in the Upper Boundary Layer in Transition

The ring-like vortices (or hairpin vortices) are universal vortical structures in late boundary layer transition.They play a very important role in the boundary layer transition process.In this paper, the development of ring-like vortices and the mechanism of sustainability of these vortices in the upper boundary layer are investigated numerically by direct numerical simulation (DNS).Intense interaction among ring-like vortices and the other vortical structures in different vortical packets which generated at different time are observed.It shows that steamwise counterrotating cores will generate new ring-like vortices and the ring-like vortices become more stronger and raise to the upper boundary layer after mutual interaction and merging.In the meanwhile, the boundary layer of the late transition becomes thicker.