DNS study on mechanism of small length scale generation in late boundary layer transition

It has been widely accepted that small length vortices, a key mechanism for late flow transition and turbulence generation, are generated by “large vortex breakdown”. However, according to our recent DNS results, the hairpin vortex structure remains very stable and never breaks down. We also recognize strong multiple level negative and positive spikes and consequent multiple level high shear (HS) layers near the bottom of the boundary layer. From our DNS results, we found a second sweep of the first level rings gathering energy from the inviscid region downdraft and generating positive spikes and consequent HS layers. With the instability of the HS layer, new vortex rings and consequent new sweeps are generated. Then, the second level negative and positive spikes and high shear layers are formed near the laminar sub-layer. According to our observation, all small length scales (turbulence) are found around high shear layers generated by multilevel positive spikes and the wall surface, especially near the bottom of the boundary layer, but no small length vortices are found generated by “large vortex breakdown”. This paper provides detailed description and analysis for the energy transfer paths and small length scale (turbulence) generation and sustenance observed by our DNS.

► All small length vortices (turbulence) are generated by high shear layers not by “vortex breakdown”.
► Multiple level negative and positive spikes can generate multiple level high shear layers.
► Positive spikes are generated by multiple level sweeps and negative spikes by ejections.
► Energy is transferred from inviscid area to the BL bottom through multiple level sweeps.
► When the ring is deformed and inclined, the sweeps and positive spikes will be weakened.