报告题目：Third Generation of Vortex Identification Methods - Omega, Liutex/Rortex, Omega-Liutex
Vortex is intuitively recognized as the rotational/swirling motion of the fluids, but a universally accepted definition of vortex is still not available. In thousands of research papers and almost all textbooks, vorticity tube/filament is regarded equivalent to vortex and the magnitude of vorticity is deemed the strength of vortex, which is a misunderstanding of the vortex nature since Helmholtz (1858). These vorticity-based methods can be considered as the first generation of vortex identification methods. During the last three decades, a lot of vortex identification methods, including Q-,-,λ2-,λci- criteria, have been developed. Most of these criteria are based on Cauchy-Stokes decomposition and/or eigenvalues of the velocity gradient tensor, which can be considered as the second generation of vortex identification methods. Starting from 2014, the Vortex and Turbulence Research Team at University of Texas at Arlington (UTA Team) focus on the development of a new generation of vortex identification methods. A new Omega vortex identification method, which defined the vortex as a connected region where vorticity overtakes deformation, was published in 2016. A Liutex (previously called Rortex) vector was proposed by UTA Team to represent the local rigid rotational part of fluid motion, which is a mathematical definition with its direction as the local rotational axis and its magnitude as the rigid rotation strength. Liutex is a new physical quantity with scalar, vector and tensor forms exactly representing the local rigid rotation of fluids. Meanwhile, a decomposition of vorticity to a rotational part namely Liutex and an anti-symmetric shear part (RS decomposition) was introduced in 2018 and a velocity gradient tensor decomposition to a rotation part (R) and a non-rotation part (NR) was also given in 2018 as a counterpart of Cauchy-Stokes decomposition. Later in early 2019, a Liutex based Omega method called Omega-Liutex was developed, which combines the advantages of both Liutex and Omega methods. These breakthroughs in the development of vortex science by UTA Team are classified as the third generation of vortex identification methods. The critical problems for vortex identification include the absolute vortex strength, relative vortex strength, the local rotation axis, the vortex core center location, vortex core size, and vortex boundary. Only the new third generation of vortex identification methods can answer these questions while all the other vortex identification methods fail to answer all questions except for approximation of the vortex mathematical boundary. A new vortex core definition and identification method and a new modified Omega-Liutex method are also presented.
Dr. Chaoqun Liu received his BS in 1968 and MS in 1981 from Tsinghua University, Beijing, China and PhD degree from University of Colorado at Denver, USA. He is currently the Tenured and Distinguished Professor, a highly honored academic title, and the 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 almost 30 years since 1989. As PI, he has been awarded by NASA, US Air Force and US Navy with 50 federal research grants of over 5.7'106 US dollars since 1990 in the United States. As Co-PI, he received 7.3'106 US Dollars from AFOSR in 2017. He has published 11 professional books, 107 journal papers and 145 conference papers. He chaired first and third US AFOSR International Conference on DNS/LES in 1997 and 2001. As a principal lecturer, he held TGS2015 Workshop, New Theory of Turbulence Generation and Sustenance, Tsinghua University, Beijing, China, June 4-6, 2015, sponsored by 20 research institutions with over 220 audiences. He also organized many other conferences or workshops as the chairman or principal lecturer. He is a well-established expert in high order DNS/LES for flow transition, turbulence, and shock and boundary layer interaction. Since 2014, he has focused on research of the vortex definition and identification. He is the founder and major contributor of the third generation of vortex identification methods including the Omega method, Liutex/Rortex method, Omega-Liutex method, RS vorticity decomposition and R-NR velocity gradient decomposition.