T2T-VHF

Transition to Turbulence of Volumetrically Heated Flows

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 Obiettivo del progetto (Objective)

'Numerical techniques are proposed that can capture the transition to turbulence of shear flow and in the process they offer the capability of state of the art control of such transitions. The methods can enhance the calculation of fluid flow by identifying the hierarchical bifurcation of the evolving states. Thus, the predictive power of the underlying mathematical models is strengthened. Concurrently they offer the unique possibility of unifying the results with those obtained by techniques that are developed with the sole aim of capturing the fully developed turbulent state. The novel methods can be used to pinpoint the transition of the flow from its laminar (basic) state to its fully developed (turbulent) state. Meanwhile programmes that are designed to capture the nature of the flow at its final (turbulent) state will be benchmarked against the programmes that pinpoint its transition. Software that unifies the programmes will be able to oversee the development of the fluid flow throughout its evolution. The emerging software will run on single or shared memory (parallel) hardware, thus reducing dramatically the computational costs. It is the ultimate aim of this set of programmes to apply the resulting software to complex configurations applicable to a variety of configurations. Simple geometries will be considered at first to act as benchmarks and common ground for the two different state of the art software avenues at our disposal: the proprietary code developed at Aston University and a commercially available CFD code. We intend to use the results of our studies to be applicable to the Nuclear Industry to model regime transition in complex systems where molten metals, molten salts and water are used as the coolant. Operating regimes of interest include the thermo-hydraulic behaviour of the coolant in reactors undergoing passive decay heat removal. Meteorological and geological applications will also be considered as a by-product of our studies.'

Introduzione (Teaser)

Complex fluid flows when fluids are heated from within occur in many natural and engineered processes, from cloud convection to cooling in nuclear reactors. New models will enhance prediction of transition to turbulence for better control of flow.

Descrizione progetto (Article)

In volumetrically heated fluids, the heat can come from chemical or biochemical reactions, a phase transition or radioactive decay. Volumetric heating is behind convection in clouds and effects on weather patterns. It is also responsible for the transport of humidity in greenhouses and its effects on weather and plant growth. Industrially, it is involved in a number of processes in nuclear reactors, including decay heat removal and convection of molten reactor cores.

Many computational fluid dynamics (CFD) models are focused on the fully developed turbulent state. The EU-funded project T2T-VHF (Transition to turbulence of volumetrically heated flows) sought to develop mathematical models that capture the transition from uniform laminar flow to the chaotic flow of turbulence.

In particular, scientists studied the pre-chaotic bifurcation behaviour of strongly non-linear equilibrium solutions for incompressible volumetrically heated shear flows (VHSFs) in a long channel. The models will enable design for control of that transition and, when unified with turbulent flow models, provide a holistic picture of fluid flow through its entire evolution.

The transition to turbulence of VHSFs was modelled using both spectral stability analysis and CFD (finite volume methods). The former easily and quickly identifies stable and unstable flow states, but requires careful representation of the geometries considered, which can be quite complicated. The goal was to create a toolbox facilitating amendments to the finite element code such that it can model flow states in more complex geometries currently not possible with the spectral method.

Scientists focused on the complex state space found using the spectral stability analysis for conditions of constant pressure gradient. This allowed them to clearly understand the state space at the transition from laminar conduction to laminar convection to unsteady coherent wavy flow. Model results were largely consistent with experimental data in the literature.

Work was presented at several conferences and meetings as well as in peer-reviewed scientific journals. The advanced models describing the transition from laminar to turbulent flow of VHSFs in a long channel are expected to extend the performance capability of existing descriptions. They could find application in numerous fields, from weather patterns to nuclear energy.