题目：DNS Study on Relation between Vorticity and Vortex
报告人：Prof. Chaoqun Liu（美国德克萨斯大学阿灵顿分校）
时间：2017年3月8日15:00
地点：C12-619
主办单位：国际交流合作处、航空宇航学院、科协
报告内容摘要：
Vorticity, vorticity line, vorticity tube have rigorous mathematical definition, but the rigorous definition of vortex is still an open question. However, for long time until now, in many research papers and text books, vortex is defined as vorticity tube confined by vorticity lines without vorticity line leakage. Vortex is also considered as congregation of vorticity lines with larger vorticity. Our DNS study shows that vortex is not a vortex tube but open for vorticity line penetration, is not a congregation of vorticity lines but a dispersion of vorticity lines, is not a concentration of vorticity with larger vorticity but with smaller vorticity in most 3-D cases. In general, vortex is an intuitive concept of rotation core with weak dissipation, e.g. zero dissipation when it becomes rigid rotation. In addition, vortex is not a vorticity tube, being different from what suggested by many research papers and textbooks. At the laminar boundary layer, vorticity is large but we have no vortex. A core with 2 nearly pure rotations per second, where vorticity is small, is a vortex, but a core with 10000 strongly sheared rotation, where vorticity is very large but deformation is very large as well, may not be a vortex. Therefore, vorticity magnitude and vortex are irrelevant. Unlike solid body, vortex is always a mixture of vorticity and deformation and vortex is really defined as an open area (not tube) where deformation is weak and vorticity is dominant. A function so-called “Omega” is defined to identify the vortex and Omega=0.52 well represents the vortex area boundary. Vortex is mathematically defined as Omega >0.5 which means a place where vorticity overtakes deformation. The minimum gradient of “Omega” is well representing the vortex axis. Several examples including DNS for late flow transition and LES for shock vortex interaction have been tested and the outcome is promising.
报告人简介：
Dr. Chaoqun Liu received his BS in 1968 and MS in 1981 from Tsinghua University, Beijing, China and PhD in 1989 from University of Colorado at Denver, Colorado, USA. He is currently the Distinguished Professor, a highest honoured academic title, and Director of Center for Numerical Simulation and Modelling 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 since 1990. As PI, he has been awarded with 47 federal research grants of over 5.7 million US dollars since 1990 in the United States. As Co-PI, he just received an AFOSR MURI grant of $7.3 millions for next 5 years. He has published 7 books, 93 journal papers and 134 conference papers. As a principal lecturer, he just 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 chaired first and third AFOSR international conference on DNS/LES. He is a well- established expert in high order DNS/LES for flow transition, turbulence, and shock and boundary layer interaction.
