威尼斯娱人城官网

Title: Mechanisms of flow laminarisation due to buoyancy

Time: 14:00 to 15:30, Oct.30, 2016

Place: F203, School of Mechanical Engineering

Speaker: Professor Shuisheng He, Department of Mechanical Engineering, University of Sheffield

Host: Zhao Changying, Professor

Abstract:

Direct numerical simulation of a turbulent flow subjected to various distributions of buoyant forces has been carried out to gain new insights into the mechanisms of flow laminarisation. It is well established that when a turbulent flow is subjected to a non-uniform body force, the turbulence may be significantly suppressed when compared with that of the flow of the same flow rate and hence the flow is said to be laminarised. This is the situation in buoyancy-aided mixed convection when severe heat transfer deterioration may occur. It is shown in this paper however that it causes little changes to the key characteristics of the turbulence when a buoyant force is added to a turbulent flow while keeping the initial pressure force unchanged. In particular, the mixing characteristics of the turbulence represented by the turbulent viscosity, and the wall-normal and the spanwise turbulent stresses, remain largely unaffected. In terms of the near-wall turbulence structure, the numbers of ejections and sweeps are little influenced by the imposition of the body force, whereas the strength of each event may/may not be stronger dependent on the distribution of the body force. The former is true when the coverage of the buoyant force extends significantly away from the wall, and under such a condition, the body-force induced flow perturbation results in a greater turbulent shear stress. The streamwise turbulent stress may also be increased, which is associated with the observation of more and stronger elongated streaks. In light with these new insights, the so-called flow laminarisation due to a buoyant force is in effect a reduction in the flow's apparent Reynolds number, based on an apparent friction velocity associated with only the pressure force of the flow (i.e., excluding the component due to the body force). Within this new framework, the level of the flow `laminarisation' and when the full laminarisation occurs are readily predictable.

Biography:

Professor Shuisheng He is a Chair in Thermofluids and Head of the Thermofluids Subject Group of the Department of Mechanical Engineering at the University of Sheffield. He received his BSc and MSc from Huazhong University of Science and Technology of China and his PhD from the University of Manchester of the UK. He has spent over three years with British Energy (now EDF Energy) as a reactor thermal hydraulics analyst, before starting an academic career with Robert Gordon University, later the University of Aberdeen and since 2011, the University of Sheffield. Professor His research focuses on non-equilibrium turbulence, including unsteady and buoyancy-influence flows, nuclear thermal hydraulics, and computational fluids dynamics using RANS and DNS. He is a Member of UK EPSRC Review College, member of UK Turbulence Consortium and Fellow of the Institute of Mechanical Engineers.