Seminar - Energy gradient theory for flow instability and turbulent transition by Prof. Hua-Shu Dou
日期：2017 年 01 月 17 日 ( 星期二)
Time：11:00 am – 12:00 pm
The physics of flow instability and turbulent transition in shear flows is studied by analyzing the energy variation of fluid particles under the interaction of base flow with a disturbance. For the first time, a simple model derived strictly from physics is proposed to show that the flow instability under finite amplitude disturbance leads to turbulent transition. The proposed model is named as “energy gradient method”. It is demonstrated that it is the transverse energy gradient that leads to the disturbance amplification while the disturbance is damped by the energy loss due to viscosity along the streamline. It is also shown that there is a threshold of disturbance amplitude which interacts with the base flow to result in transition to turbulence. The threshold of disturbance amplitude obtained is scaled with the Reynolds number by an exponent of -1, which is in excellent agreement with experiments for pipe flow with injection disturbance. The mechanism for velocity inflection and hairpin vortex formation are explained with reference to analytical results. Following from this analysis, it can be demonstrated that the critical value of the so called energy gradient parameter Kmax is a constant about 380 for turbulent transition in any parallel flows, and this is confirmed by experiments for pipe Poiseuille flow, plane Poiseuille flow, and plane Couette flow. The location of instability initiation in the flow field predicted with the theory accords well with the experiments for both pipe Poiseuille flow (r/R=0.58) and plane Poiseuille flow (y/h=0.58). It is also inferred from the proposed method that the transverse energy gradient can serve as the power for the self-sustaining process of boundary layer turbulence. The theory also agrees well with all the available experimental data for Taylor-Couette flows between concentric cylinders. Therefore, the theory is a universal theory for flow instability and turbulent transition. As last, criteria of turbulent transition are proposed: (1) For pressure driven flow, the necessary condition and sufficient condition for turbulent transition is the presence of velocity inflection on the averaged flow profile. (2) For shear driven flow, the necessary condition and sufficient condition for turbulent transition is the existence of zero velocity gradient on the velocity profile of the averaged flow.
Prof. Hua-Shu Dou received his PhD degree in Fluid Mechanics from Beijing University of Aeronautics and Astronautics, in 1991. Then, he joined Tsinghua University as a Postdoctoral Fellow and Lecturer, and was subsequently promoted to Associate Professor in 1993. From 1994 to 1996, he held positions as Visiting Research Fellow at Tohoku University (Japan) and Guest Professor at Hosei University (Japan). From 1996 to 2002, he was an ARC Research Associate/Research Scientist at The University of Sydney, Australia. He worked in the National University of Singapore from 2002 to 2011, in Mechanical Engineering Department (Research Fellow) and Temasek Laboratories (Research Scientist). From 2011, Dr. Dou is a Chair Professor in Faculty of Mechanical Engineering and Automation at Zhejiang Sci-Tech University in China; He is also the Director of the High Performance Computing Centre; Head of Fluid Science and Engineering in the university. He holds the title “QianRenJiHua” Professor in Zhejiang Province; Head and Chief Scientist of Zhejiang Key Innovation Team on Fluid Machinery. He is also an adjunct professor in Zhejiang University, Chongqing University and Northeastern University, etc. Prof. Hua-Shu Dou’s researches focused on computational fluid dynamics and fluid mechanics including aerodynamics and hydrodynamics, combustion and detonation, flow instability and turbulent transition, multi-phase flows, Non-Newtonian flows as well as turbomachinery. He has published more than 150 papers in various refereed international journal and conferences, and one co-authored book published by Springer. He is also active in academic activities and was in the committees of several international conferences. He now services as Editorial Board Member of seven international journals in mechanical and aerospace engineering. Prof. Dou has been invited to give lectures in several international conferences, universities and organizations. He was selected as AIAA Associate Fellow in 2011 and is a member of APS and ASME.