Nuclear Thermal Hydraulics

We perform thermal hydraulic analysis to support safety cases, design modifications & operations; develop and improve computational tools, methods and correlations; and conduct fundamental research to better understand active and passive cooling phenomena in general.

Our key research areas are:

  • Advanced gas-cooled reactors (AGR) - those currently in operation in the UK.
  • Future reactor designs - the so-called Generation IV reactors, including SCWRs and SFRs. 
  • Fundamental and generic research on phenomena commonly encountered in many reactor designs.


Our turbulence research focuses on non-equilibrium problems mostly based on computational simulations using DNS, LES and RANS. We also operate an unsteady flow facility equipped with LDA and PIV optical measurements.

Our key research topics are:

  • Bypass transition of transient turbulent flow
  • Flow laminarisation
  • Unsteady flow - experiments and modelling
  • Heat transfer to fluids at supercritical pressure

Methodology & resources

We develop and maintain a number of in-house computer codes and use various local and national high performance computing (HPC) resources. We operate an unsteady flow facility. 

  • CHAPSim - a direct numerical simulation code 
  • SWIRL - a conventional CFD (RANS) code
  • LBMCODE - Lattice Boltzmann Method (LBM)
  • SUFF - Sheffield Unsteady Flow Facility


Turbulent flow may undergo laminar-to-turbulent transition

Recent DNS and experimental results have led us to established a radically  new interpretation of transient turbulent flows: such already turbulent flow is characterized by laminar-to-turbulent transition. Consider a pipe or channel flow initially at a stationary, turbulent state. The flow rate of the flow is rapidly increased to a new level. The initial turbulence does not gradually evolve to reach a new state; instead, the transient flow is characterized by a laminar boundary initially formed on the wall, which grows in thickness with time, becoming unstable later. This is followed by the generation of turbulent spots and subsequently transition to turbulence. Such transient flows may consequently be considered to represent a new category of bypass transition that starts from a well established turbulent wall shear flow, in contrast to those induced by free-stream turbulence. This new finding is published in a series of publications. Please read:

lambda2 transition showing discontinuity

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.

Is the flow laminarised?

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 when a body force is applied 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, the value of which can be readily determined. Please read:

Laminarisation_contours Laminarisation_uv

Figure (left) shows instantaneous velocities in various 'laminarised' flows; Figure (right) shows that turbulent shear stresses of DNS results are well predicted with a simple theory based on apparent Reynolds number.  

Heat transfer to fluids at supercritical pressure in support of SCWR designs

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.

SupercriticalFigure: 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.

Fuel pin-brace interactions in an Advanced Gas-cooled Reactor (AGR)

Fuel pins rubbing braces that hold them in place due to flow turbulence is a current issue in some AGRs which causes fuel failures. This project aims at studying the vortex shedding generated at the end of the fuel rods and its impingement onto the downstream rods. Large Eddy Simulation (LES) has been carried out to investigate turbulent flow in an annular channel with streamwise inter-rod gapping. The spacing between the rods is varied to study the effect of the mean flow as well as the turbulence structure revealing two distinct flow patterns analogous to the flow over the k-type and d-type roughness.
Reference: Kristin N. (2014), Numerical investigation of the flow structure in an annular channel with inter-rod spacing, PhD Thesis, University of Aberdeen.