题目：湍流产生和维持机制

时间：6月12日（周五）上午9：30

地点：新主楼C706

报告人：刘超群教授

Abstract

Classical and current turbulence theories are fully filled with self-contradictions and have no way to be correct. No one was able to observe Richardson’s eddy cascade and Kolmogorov’s hypothesis that the larger vortices give energy to smaller vortices through “vortex breakdown” Turbulence cannot be random since the conservation law of mass, momentum and energy must be satisfied. The commonly accepted concept that “Lambda vortex self-deforms to hairpin vortex” does not exist since vortex ring is not part of a “vortex tube”. Turbulence cannot be generated by vortex “breakdown” since turbulence has structure. Flow transition is a vorticity redistribution through “rollup” and the vorticity comes from the base flow not from disturbance. Linear unstable modes only play a role to trigger the vorticity rollup. The nature of turbulence generation is that after vorticity rollup, fluid cannot tolerate shear which is unstable and shear must transfer to fast rotation cores which are stable with minimized energy consumption. We proposed a new theory that all small length scales (turbulence) are generated by shear layer instability 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 “vortex buildup” through positive and negative spikes and consequent high-shear layers with low speed zone in the center of vortex rings. The energy is brought by large vortex structure through multiple level sweeps. By the way, turbulence bursting and intermittence are misunderstandings of vortex package motion. It is a serious mistake to consider vortex as vortex tube.

Author’s Profile

Dr. Chaoqun Liu received his BS in 1968 and MS in 1981 from Department of Engineering Mechanics and Mathematics, Tsinghua University, Beijing, China and PhD in 1989 fromDepartment of Mathematics, University of Colorado at Denver, Colorado, USA. He was Adjunct Professor in University of Colorado for 1990-1995, Associate and Full Professor of Louisiana Tech University for 1995-2000. He is currently the Distinguished Professor and Director of Center for Numerical Simulation and Modeling at University of Texas at Arlington, Arlington, Texas, USA. He has worked on high order direct numerical simulation (DNS) and large eddy simulation (LES) for flow transition and turbulence for 25 years since 1990. As PI, he has been awarded with 49 federal research grants of over 5 million US dollars from NASA, AFOSR, ONR etc. since 1990 in the United States. He has published 7 books, 82 journal papers and 119 AIAA and other conference papers. He also chaired first and third AFOSRinternational conference on DNS/LES. He is a well established expert in high order DNS/LES.His research interests include high order numerical scheme for small length scale and shock/discontinuity capturing, direct numerical simulation for flow transition and turbulence, large eddy simulation for shock boundary layer interaction and flow control, multi-grid and multiple level adaptive methods. He is the AIAA Associate Fellow since 1998, Recipient of UTA Distinguished Record of Research or Creative Activity Award in 2008 and UTA Academy Member of Distinguished Scholars.