Broader Approach
The aim of the Broader Approach agreement, signed in Feb. 2007 between Euratom and Japan, is "complement the ITER Project and to accelerate the realisation of fusion energy by carrying out R&D and developing some advanced technologies for future demonstration fusion power reactors (DEMO)" [1]. As for ITER, F4E is the Implementing Agency of Euratom for the Broader Approach. The resources for the implementation of the Broader Approach are largely provided on a voluntary basis from several participating European countries (Belgium, France, Germany, Italy, Spain and Switzerland).
Among the three main projects being implemented, our research group is involved in the design of the International Fusion Materials Irradiation Facility (IFMIF). The purpose of IFMIF is test and qualify materials that can be operated at high temperatures and under intense (neutron) radiation, conditions typical of nuclear fusion devices. In IFMIF, the material samples will be exposed to an intense neutron flux created by bombarding a liquid lithium flow by a dual deuterium beam (see adjacent figure)
. A high speed liquid lithium flow is required to rapidly and continuously evacuate the heat deposited by the two deuterium beams.
At Université Libre de Bruxelles, we focus our attention on the evaluation of the stability of the free surface lithium flow by means of Computational Fluid Dynamics (CFD). Indeed, a safe and reproducible operation of the 25mm thick lithium target requires that its surface should not present irregularities larger than +/- 1mm in the range of parameters envisaged for nominal flow conditions. Our simulations are performed using two solvers. The first one is a finite volume code called YALES2 [2] capable of achieving high resolution computations on massively parallel clusters. This solver is based on the large-eddy simulation technique and makes used of the volume of fluid (VOF) method coupled to the level-set method. The second solver used is Ansys Fluent [3] which allows the computation of free surface flows using a variety of methods including large-eddy simulations or Reynolds Average Navier-Stokes simulations.
[1] http://fusionforenergy.europa.eu/understandingfusion/broaderapproach.aspx
[2] http://www.coria.fr/
[3] http://www.ansys.com/
Dynamics of conductive flows
Some fluids, like liquid-metals or electrolytes, are able to conduct electricity. As a consequence, their motion can be influenced by their interactions with electromagnetic fields. For example, induced electrical currents can arise when a flow evolves in the presence of a magnetic field. These currents will dissipate energy by Joule heating and therefore reduce the kinetic energy of the flow. One can also show that all the flow structures will have a tendancy to elongate in the direction of the magnetic field.
This interaction between conductive fluids with electromagnetic fields is important for numerous industrial processes, like the continuous casting of steel, the growth of semi-conductors, the production of aluminium etc. Another important context in which they are relevant is the design of coolant blankets for nuclear fusion reactors.
Our objective is to further understand the dynamics of conductive fluids, especially in the turbulent regime, and to provide efficient numerical tools and models to predict their evolution in real-life applications.
Fusion Plasma
Our group has a long research history on the application of numerical, modelling and theoretical approaches to the study of transport processes in fusion plasmas. Fusion is the main energy production mechanism in the universe. In particular, the basic energy source for the Sun is the nuclear fusion of hydrogen which produces helium and energy. On earth, fusion devices are using two hydrogen isotopes (deuterium and tritium) to form helium nuclei and produce energy. The challenge for fusion researchers is to compensate by heating a lower-density plasma to a higher temperature (about 100 million K) to initiate fusion reaction.
The fusion plasma can be described using either kinetic approaches such as the neo-classical transport theory of magnetised plasmas or fluid type equations such as magneto-hydrodynamics. Our group is actively working on these two levels of descriptions.
This research is developed in the framework of the EURATOM Framework Programme and our research unit is an active member of the EURATOM-Belgian state association. Our activities are conducted in close collaboration with other EURATOM association form The Netherlands, Germany, France, Romania, Greece, UK, ... .
Numerical simulation of turbulence
The complete description of hydrodynamic turbulence requires the resolution of a range of scales that is known to increase very rapidly with the turbulent intensity. Direct numerical simulations (DNS) are thus restricted to moderately turbulent flows. However, the detailed characterisation of every excited mode is unnecessary to describe the large scale turbulence. This has prompted the derivation of large eddy simulations (LES). This numerical technique is based on the application of a spatial filter to the Navier-Stokes equation. The resulting equation can then be simulated using a coarser grid since the fluctuations with small characteristic scales are filtered out. However, the LES equation contains an unknown subgrid scale stress tensor that needs to be modelled. This term accounts for the effects of the unresolved small scales on the resolved scales.
In this framework, our activities consists in developing codes for DNS and LES. In particular, we are activley participating in the development of a solver for arbitrary geometries with one direction of periodicity (SFELES). This code combines a finite element representation in a two-dimensional plane together with a Fourier decomposition along the periodic direction. Another objective is the development of accurate subgrid scale models for LES for both Navier-Stokes and magneto-hydrodynamic turbulence.
Contemporary physical challenges for Heliospheric and AstRophysical Models
CHARM is one of 47 selected IAP phase VII networks. CHARM focuses on high performance computing approaches, on the challenges to confront models with observations and in the most enigmatic aspects of the physical processes at work:
- Turbulence and particle acceleration aspects
- Improved treatments of the radiative-dynamical feedback loop
- Challenges in space-plasmas, where the presence of charged constituents requires to recognize fully the interplay between global magnetohydrodynamic and phase-space based kinetic physics
- Understanding of reconnection events, at play in solar atmospheric, interplanetary to Earth near space environment.
- Galaxy evolution questions, that are on the horizon thanks to simulation and software engineering efforts like the EAGLE cosmological hydro challenge or the AMUSE code coupling framework
CHARM network will make use of the latest opportunities brought about by the unique armada of space instruments monitoring our Sun and the heliosphere. It will likewise hook into state-of-the-art international consortium efforts to unravel galaxy evolution questions, that are on the horizon thanks to simulation and software engineering efforts like the EAGLE cosmological hydro challenge or the AMUSE code coupling framework.
CHARM objectives can be summarized as in the following:
- Couple existing tools and models towards more realistic multi-physics descriptions
- Confront model predictions with measurements
- Gas and plasma dynamics with turbulent fluctuations (leader of the work package: ULB)
- MHD to kinetic treatments in the heliosphere
- Dynamics with radiatively controlled phenomena