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Short Course: New Theory on Turbulence Generation and Sustenance, Feb 2014, United States

Posted By: Xiaoqing Zheng
Date:Fri, 21 Mar 2014, 2:24 p.m.

This short course serves as a systematic report on 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 disordering. The widely spread concept "vortex breakdown to turbulence", which was considered as the last stage of flow transition, is not observed. We proposed a new theory on boundary layer transition with five steps, i.e. receptivity, linear instability, large vortex formation, small length scale generation, loss of symmetry and being chaotic to turbulence. We have also proposed a new theory about turbulence generation and sustenance. 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 sublayers. 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". When the Reynolds number is large enough fluid shear, which is unstable, is dominant in laminar boundary layer and fluid rotation, which is stable, is dominant in turbulent boundary layer. In other words, laminar boundary layer is an unstable state, but turbulent is a stable state, and thus transition from laminar to turbulent is unavoidable. More other new physics have also been discussed

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