Our turbulence research focuses on non-equilibrium problems, including, for example, unsteady turbulent flow and buoyancy-influenced turbulent flow. Complementary experimental and computational methods are used. Experimental studies are carried out using advanced flow measurement techniques, including Laser Doppler Anemometry (LDA), Particle Image Velocimetry (PIV) and hot-film anemometry. Computational studies are carried out using DNS, LES and RANS approaches with computer codes developed and maintained by the group.
DNS, as a major tool used in our recent studies, provides detailed faithful information on the flow which enhances our understanding of the physics as well as providing detailed benchmark data for validation of engineering modelling methodologies. Nevertheless, DNS (and LES) are currently limited to simple geometry flows at relatively low Reynolds numbers. RANS approaches on the other hand are used to extend the flow conditions studied after carefully designed validation exercises. Experimental studies are used to back-up these numerical simulations.
Why unsteady flow?
Unsteady flow is encountered in many engineering applications as well as the natural environment, including for example, power plant start-up or shut down, the sudden closure of valves in a pipeline system, passenger trains going through a railway tunnel, the blood flow in the human body and waves over the beach. A better understanding and improved prediction of unsteady flows have remained a strong drive for investigations into various forms of unsteady flows to improve the safety, economy and comfort of engineering systems and for the preservation of the environment. In addition to practical applications, unsteady turbulent flow has also been an intriguing topic of fundamental turbulence research since it reveals flow physics that is not always apparent in steady flows.
Why buoyancy-influenced flows?
Buoyancy plays an important role in many heat transfer systems where there is a large variation of temperature and hence non-uniform distribution of buoyancy. A very important example is flow in nuclear reactors under certain conditions. Buoyancy could cause turbulence to be enhanced or retarded and hence causing heat transfer enhancement or deterioration which in turn has safety and/or economical implications.
- Fundamental Study of Turbulence in unsteady flows, PhD project, Benjamin Oluwadare, 2015-2018
- Study of flow and heat transfer of fluids at supercritical pressure using DNS, EPSRC through EP/K007777/1, 2012-2015
- Effect of body force on turbulent pipe flow, PhD project, Kui He (Tom), 2011-2015
- Studies of turbulence in unsteady flow, PhD project, Akshat Mathur, 2012-2015.
- Turbulence and wall shear stress in unsteady internal flows with rough surfaces EPSRC (EP/G068925/1), 2010-2013.
- Unsteady friction in pipes and ducts (Hydralab), 2007-2008
- Theoretical and experimental study of the dependence of transient pipe friction on turbulence dynamics, EPSRC (EP/C015177/1), 2006-2008.
- Modelling of turbulent heat transfer to fluids at supercritical pressure EPSRC (GR/S19424/01), 2003-2006.
Key research topics
1. Laminar-turbulent Transition of transient turbulent flow
Ever since the publication of the pioneering work of Osborne Reynolds in 1883 defining the concept of laminar and turbulent flows, the subject has remained a central theme in fluid mechanics due to its fundamental importance to the subject and its relevance to engineering applications and the natural world. We know, for example, that the flow in a pipe is likely to be laminar when the Reynolds number is below around 2300 and turbulent when the Re is higher.
Recently, we made some ‘surprising’ observations which lead to a radically new interpretation of transient turbulent flows, seemingly deviating from the conventional turbulence theory. The new findings lead to the following statement:
The transient flow following a rapid increase of flow rate from an initially turbulent flow is characterised by a time-developing laminar boundary layer followed by laminar-turbulent transition even though the initial flow is turbulent.
An alternative statement of the finding is that:
The transient turbulent channel flow represents a new category of bypass transition, which develops from an established turbulent wall shear flow. This contrasts the conventional boundary layer bypass transition induced by free stream turbulence.
These findings are reported in the following publications:
- S. He, M Seddighi (2013), Turbulence in transient channel flow, Journal of Fluid Mechanics, 715, 60-102.
- Seddighi M, He S, Vardy AE & Orlandi P (2014) Direct numerical simulation of an accelerating channel flow. Flow, Turbulence and Combustion, 92(1-2), 473-502.
- S. He & M. Seddighi (2015) Transition of transient channel flow after a change in Reynolds number, Journal of Fluid Mechanics, 764, 395-427.
- Seddighi M, He S, Pokrajac D, O’Donoghue T & Vardy AE (2015) Turbulence in a transient channel flow with a wall of pyramid roughness. Journal of Fluid Mechanics, 781, 226-260.
- He K, Seddighi M & He S (2016) DNS study of a pipe flow following a step increase in flow rate. International Journal of Heat and Fluid Flow, 57, 130-141.
Figure (left) shows generation of turbulent spots; Figure (right) shows that the streamwise and spanwise turbulence exhibiting 'discontinuities' at the time of transition in a transient turbulent flow.
2. 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 and the flow is laminarised. This is the situation in buoyancy-aided mixed convection when severe heat transfer deterioration may occur.
In contrast to this conventional view, we show that the essential turbulence characteristics including mixing characteristics of the turbulence represented by the turbulent viscosity remain largely unaffected while applying a body force to a turbulent flow while keeping the pressure force unchanged. The so-called flow laminarisation can be viewed as a reduction in the apparent Reynolds number of the flow, value of which can be readily established. Please read:
He S, He K & Seddighi M (2016) Laminarisation of flow at low Reynolds number due to streamwise body force. Journal of Fluid Mechanics, 809, 31-71.
Figure (left) shows instantaneous velocities in various 'laminarised' flows; Figure (right) shows that turbulent shear stresses of DNS results are well predicted by simply theory assuming turbulent viscosity is not influenced by non-uniform body force.
3. Unsteady flow modelling and experiments
For unsteady turbulent flows, experimental and computational studies have been carried out on accelerating, decelerating and sinusoidal flows over many years. Both smooth and rough walls with 2D and 3D roughness configurations are considered. A selection of publications are listed below:
- Vardy AE, Brown JMB, He S, Ariyaratne C & Gorji S (2015) Applicability of Frozen-Viscosity Models of Unsteady Wall Shear Stress. Journal of Hydraulic Engineering, 141(1), 04014064-04014064.
- Gorji S, Seddighi M, Ariyaratne C, Vardy AE, O'Donoghue T, Pokrajac D & He S (2014) A comparative study of turbulence models in a transient channel flow. Computers and Fluids, 89, 111-123.
- Seddighi M, He S, Vardy AE & Orlandi P (2013) Direct Numerical Simulation of an Accelerating Channel Flow. Flow, Turbulence and Combustion, 1-30.
- Mathur A & He S (2013) Performance and implementation of the Launder–Sharma low-Reynolds number turbulence model. Computers & Fluids, 79(0), 134-139.
- S. He, C. Ariyaratne and A.E. Vardy, Wall shear stress in accelerating turbulent pipe flow, Journal of Fluid Mechanics, 685, pp. 440-460, 2011.
- M. Seddighi, S. He, P. Orlandi & A. Vardy, A comparative study of turbulence in a ramp-up and a ramp-down flow, Flow Turbulence and combustion, 86 (3-4), pp. 439-454, 2011.
- S. He, C. Ariyaratne, Wall shear stress in the early stage of unsteady turbulent pipe flow, J. Hydraul. Eng., 137 (5), pp. 606-610, 2011.
- C. Ariyaratne, S. He and A.E. Vardy, Wall Friction and Turbulence Dynamics in Decelerating Pipe Flows, Journal of Hydraulic Research, 48 (6), pp. 810-821, 2010.
- S. He and J.D. Jackson , ‘An experimental study of pulsating turbulent flow in a pipe’, European Journal of Mechanics – B/Fluids, Vol. 28, pp309-320, 2009.
- S. He, W.S. Kim and J.H., Bae, Assessment of performance of turbulence models in predicting supercritical pressure heat transfer in a vertical tube, International Journal of Heat and Mass Transfer, Vol. 5, pp4659-4675, 2008.
- S. He, C. Ariyaratne and A.E. Vardy, ‘A computational study of wall friction and turbulence dynamics in accelerating pipe flows’, Computers and Fluids, Vol. 37, pp674-689, 2008.
- W.S. Kim, S. He, J.D. Jackson ‘Assessment by comparison with DNS data of turbulence models used in simulations of mixed convection’, International Journal of Heat and Mass Transfer, Vol. 51, pp 1293-1312, 2008
- S. He, Z. Xu and J.D. Jackson ‘An experimental study on buoyancy-opposed wall jet in a confined space’, Int. J. of Heat and Fluid Flow, v23, pp487-496, 2002.
- S. He and J.D. Jackson 'A study of turbulence under conditions of transient flow in a pipe’, Journal of Fluid Mechanics, vol. 408, pp1-38, 2000.
4. Heat transfer to fluids at supercritical pressure
Flow of water at a supercritical pressure between parallel plates with heating/cooling on the two walls (DNS) - The project aims at developing a better understanding of the ‘direct effect of buoyancy’ and strong variations of thermal properties. DNS of flows in stably and unstably stratified flows and those in a vertical channel with fluids at supercritical are performed.
Figure shows instantaneous iso-surface of λ_2 criterion, coloured by the streamwise velocity. The left surface is cooling wall and the right surface is the heating wall. Cases from left to right: Isothermal flow, forced convection cases 1 & 2 and mixed convection cases 1 & 2.