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Observation of laminar to turbulent transition in an already turbulent flow

Event Type: 
Date and Time: 
Friday, July 7, 2017 - 16:15
Location: 
CTR Conference Room 103
Event Sponsor: 
Parviz Moin, Director of Center for Turbulence Research
Speaker(s): 
Professor Shuisheng He, Department of Mechanical Engineering, University of Sheffield, United Kingdom

Professor He will present a new perspective of transient turbulent flow and show that in such flows, turbulence does not progressively evolve from one state to another. Instead, the flow is characterised by the development of a laminar boundary layer followed by transition to turbulence. The talk will begin with a review of DNS results, followed by discussing recent laboratory experiments of a flow accelerating from an initially turbulent state following the opening of a valve, together with LES of the experiments and extended Stokes first problem solutions for the early stages of the flow. As previously predicted by DNS simulations of transient flow following a near-step increase in flow rate, the flow has been found to respond to the acceleration in a manner that is closely analogous to a laminar flow accelerating from rest. In both cases, the primary consequence of the acceleration is the temporal growth of a boundary layer from the wall, gradually leading to a strong instability causing transition. In this interpretation, the initial turbulent fluctuations can be regarded as noise in an otherwise well-defined flow behaviour. We observe the spontaneous appearance of turbulent spots and discontinuities in the velocity signals in time and space, revealing rich detail of the transition process, including a striking contrast between streamwise and wall-normal fluctuating velocities. 

Bio: 
Professor Shuisheng He is the Chair in Thermofluids in Department of Mechanical Engineering at the University of Sheffield in the United Kingdom (UK). He received his BSc and MSc from Huazhong University of Science and Technology (HUST) in China and his PhD from the University of Manchester in 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, and later the University of Aberdeen, and since 2011, the University of Sheffield. Professor He’s research focuses on non-equilibrium turbulence, including unsteady and buoyancy-influenced flows, nuclear thermal hydraulics, and computational fluid dynamics using RANS, LES and DNS. He is a Member of UK EPSRC Review College, Member of UK Turbulence Consortium, and Fellow of the Institution of Mechanical Engineers (IMechE).