The transformation of initially isotropic turbulent flow of electrically
conducting
incompressible viscous fluid under the influence of an imposed homogeneous
magnetic
field is investigated using direct numerical simulation. Under the assumption
of large
kinetic and small magnetic Reynolds numbers (magnetic Prandtl number
Pm[Lt ]1) the quasi-static
approximation is applied for the computation of the magnetic
field fluctuations. The flow is assumed to be homogeneous and contained
in a
three-dimensional cubic box with periodic boundary conditions. Large-scale
forcing
is applied to maintain a statistically steady level of the flow energy.
It is found
that the pathway traversed by the flow transformation depends decisively
on the
magnetic interaction parameter (Stuart number). If the magnetic interaction
number
is small the flow remains three-dimensional and turbulent and no detectable
deviation
from isotropy is observed. In the case of a strong magnetic field (large
magnetic
interaction parameter) a rapid transformation to a purely two-dimensional
steady state
is obtained in agreement with earlier analytical and numerical results
for decaying
MHD turbulence. At intermediate values of the magnetic interaction parameter
the system exhibits intermittent behaviour, characterized by organized
quasi-two-dimensional
evolution lasting several eddy-turnover times, which is interrupted by
strong three-dimensional turbulent bursts. This result implies that the
conventional
picture of steady angular energy transfer in MHD turbulence must be refined.
The
spatial structure of the steady two-dimensional final flow obtained in
the case of
large magnetic interaction parameter is examined. It is found that due
to the type of
forcing and boundary conditions applied, this state always occurs in the
form of a
square periodic lattice of alternating vortices occupying the largest possible
scale. The
stability of this flow to three-dimensional perturbations is analysed using
the energy
stability method.
356 Flow Analysis on a Two Dimensional Flip-flop Jet due to Direct Numerical Simulation
