Numerical Study on Mechanism of Small Vortex Generation in Boundary Layer Transition

TL;DR

This study uses DNS to investigate the generation of small vortices in boundary layer transition, revealing that high shear layers caused by second sweeps around vortex rings are key to turbulence initiation.

Contribution

It uncovers a new mechanism where small vortices originate from high shear layers induced by second sweeps, challenging the idea of vortex breakdown as the primary source.

Findings

Small vortices are generated by high shear layers near vortex rings.

Second sweeps transfer energy from the inviscid region to the boundary layer.

Small scales originate near the wall and ascend through the boundary layer.

Abstract

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.