Professor Chaoqun Liu
Department of Mathematics, University of Texas at Arlington, USA
This presentation serves as a review of our recent new DNS study on physics of late boundary layer transition. This includes mechanism of the large coherent vortex structure formation, small length scale generation and flow randomization. The widely spread concept “vortex breakdown to turbulence”, which was considered as the last stage of flow transition, is not observed and is found theoretically incorrect. The classical theory on boundary layer transition is challenged and we proposed a new theory with five steps, i.e. receptivity, linear instability, large vortex formation, small length scale generation, loss of symmetry and randomization to turbulence. We have also proposed a new theory about turbulence generation. The new theory shows that all small length scales (turbulence) are generated by shear layer instability which is produced by large vortex structure with multiple level vortex rings, multiple level sweeps and ejections, and multiple level negative and positive spikes near the laminar sub-layers. Therefore, “turbulence” is not generated by “vortex breakdown” but rather positive and negative spikes and consequent high shear layers. “Shear layer instability” is considered as the “mother of turbulence”. This new theory may give a universal mechanism for turbulence generation and sustenance – the energy is brought by large vortex structure through multiple level sweeps not by “vortex breakdown”. The ring-like vortex with or without legs is found the only form existing inside the flow field. The first ring-like vortex shape, which is perfectly circular and perpendicularly standing, is the result of the interaction between two pairs of counter-rotating primary and secondary streamwise vortices. Multiple rings are consequences of shear layer instability produced by momentum deficit which is formed by vortex ejections. The Helmoholtz vorticity conservation law must be satisfied during the multiple vortex ring formation process as increasing in spanwise vorticity and decreasing in streamwise vorticity. The U-shaped vortices are not new but existing coherent large vortex structure. Actually, the U-shaped vortex, which is not a secondary but tertiary vortex, serves as an additional channel to support the multiple ring structure. The loss of symmetry and randomization are caused by internal property of the boundary layer. The loss of symmetry starts from the second level ring cycle in the middle of the flow field and spreads to the bottom of the boundary layer and then the whole field. More other new physics have also been discussed. Finally, classical theory such as “Richardson energy cascade” and “Kolmogorov small length scale” is revisited.
Date:15:00,May 21st,2012
Venue:1512#,YiFu Technology and Science Building
