| Abstract: | Previous experimental and numerical studies have revealed that the hairpin vortex is a basic flow element of transitional boundary layer. The hairpin vortex is believed to have legs, necks and a ring head. Based on our DNS study, the legs and the ring head are generated separately by different mechanisms. The legs function like an engine to generate low speed zones by rotation, create shear layers with surrounding high speed neighbor fluids, and further cause vortex ring formation through shear layer instability. In addition, the ring head is ?-shaped and separated from quasi-streamwise legs from the beginning. Contrary to the classical concept of "vortex breakdown", we believe transition from laminar flow to turbulence is a "buildup" process of multiple level vortical structures. The vortex rings of first level hairpins are mostly responsible for positive spikes, which cause new vorticity rollup, second level vortex leg formation and finally smaller second level vortex ring generation. The third and lower level vortices are generated following the same mechanism. In this paper, the physical process from ?-vortex to multi-level hairpin vortices is described in detail. |
