5 research outputs found
Self-organization and symmetry-breaking in two-dimensional plasma turbulence
The spontaneous self-organization of two-dimensional magnetized plasma is
investigated within the framework of magnetohydrodynamics with a particular
emphasis on the symmetry-breaking induced by the shape of the confining
boundaries. This symmetry-breaking is quantified by the angular momentum, which
is shown to be generated rapidly and spontaneously from initial conditions free
from angular momentum as soon as the geometry lacks axisymmetry. This effect is
illustrated by considering circular, square, and elliptical boundaries. It is
shown that the generation of angular momentum in nonaxisymmetric geometries can
be enhanced by increasing the magnetic pressure. The effect becomes stronger at
higher Reynolds numbers. The generation of magnetic angular momentum (or
angular field), previously observed at low Reynolds numbers, becomes weaker at
larger Reynolds numbers
Rapid generation of angular momentum in bounded magnetized plasma
Direct numerical simulations of two-dimensional decaying MHD turbulence in
bounded domains show the rapid generation of angular momentum in
nonaxisymmetric geometries. It is found that magnetic fluctuations enhance this
mechanism. On a larger time scale, the generation of a magnetic angular
momentum, or angular field, is observed. For axisymmetric geometries, the
generation of angular momentum is absent; nevertheless, a weak magnetic field
can be observed. The derived evolution equations for both the angular momentum
and angular field yield possible explanations for the observed behavior
The decay of magnetohydrodynamic turbulence in a confined domain
The effect of non periodic boundary conditions on decaying two-dimensional
magnetohydrodynamic turbulence is investigated. We consider a circular domain
with no-slip boundary conditions for the velocity and where the normal
component of the magnetic field vanishes at the wall. Different flow regimes
are obtained by starting from random initial velocity and magnetic fields with
varying integral quantities. These regimes, equivalent to the ones observed by
Ting, Matthaeus and Montgomery [Phys. Fluids {\bf 29}, 3261, (1986)] in
periodic domains, are found to subsist in confined domains. We examine the
effect of solid boundaries on the energy decay and alignment properties. The
final states are characterized by functional relationships between velocity and
magnetic field
A pseudo-spectral method with volume penalisation for magnetohydrodynamic turbulence in confined domains
International audienceWe present a Fourier pseudo-spectral method for solving the resistive magnetohydrodynamic equations of incompressible flow in confined domains. A volume penalisation method allows to take into account boundary conditions and the geometry of the domain. A code validation is presented for the z-pinch test case. Numerical simulations of decaying MHD turbulence in two space dimensions show spontaneous spin-up of the flow in non-axisymmetric geometries, which is reflected by the generation of angular momentum. First results of decaying MHD turbulence in a cylinder illustrate the potential of the new method for three-dimensional simulations
Lagrangian dynamics of drift-wave turbulence
International audienceThe statistical properties of Lagrangian particle transport are investigated in dissipative drift-wave turbulence modelled by the Hasegawa–Wakatani system. By varying the adiabaticity parameter c, the flow regime can be modified from a hydrodynamic limit for c=0 to a geostrophic limit for c→∞. For c of order unity the quasi-adiabatic regime is obtained, which might be relevant to describe the edge turbulence of fusion plasmas in tokamaks. This particularity of the model allows one to study the change in dynamics when varying from one turbulent flow regime to another. By means of direct numerical simulation we consider four values for c and show that the Lagrangian dynamics is most intermittent in the hydrodynamic regime, while the other regimes are not or only weakly intermittent. In both quasi-adiabatic and quasi-geostrophic regimes the PDFs of acceleration exhibit exponential tails. This behaviour is due to the pressure term in the acceleration and not a signature of intermittency