29 research outputs found
The decay of wall-bounded MHD turbulence at low Rm
We present Direct Numerical Simulations of decaying Magnetohydrodynamic (MHD)
turbulence at low magnetic Reynolds number. The domain considered is bounded by
periodic boundary conditions in the two directions perpendicular to the
magnetic field and by two plane Hartmann walls in the third direction. High
magnetic fields (Hartmann number of up to 896) are considered thanks to a
numerical method based on a spectral code using the eigenvectors of the
dissipation operator. It is found that the decay proceeds through two phases:
first, energy and integral lengthscales vary rapidly during a
two-dimensionalisation phase extending over about one Hartmann friction time.
During this phase, the evolution of the former appears significantly more
impeded by the presence of walls than that of the latter. Once the large scales
are close to quasi-two dimensional, the decay results from the competition of a
two-dimensional dynamics driven by dissipation in the Hartmann boundary layers
and the three-dimensional dynamics of smaller scales. In the later stages of
the decay, three-dimensionality subsists under the form of barrel-shaped
structures. A purely quasi-two dimensional decay dominated by friction in the
Hartmann layers is not reached, because of residual dissipation in the bulk.
However, this dissipation is not generated by the three-dimensionality that
subsists, but by residual viscous friction due to horizontal velocity
gradients. Also, the energy in the velocity component aligned with the magnetic
field is found to be strongly suppressed, as is transport in this direction.
This results reproduces the experimental findings of Kolesnikov & Tsinober
(1974).Comment: submitted to JF
Formation mechanism of hairpin vortices in the wake of a truncated square cylinder in a duct
We investigate the laminar shedding of hairpin vortices in the wake of a
truncated square cylinder placed in a duct, for Reynolds numbers around the
critical threshold of the onset of vortex shedding. We single out the formation
mechanism of the hairpin vortices by means of a detailed analysis of the flow
patterns in the steady regime. We show that unlike in previous studies of
similar structures, the dynamics of the hairpin vortices is entwined with that
of the counter-rotating pair of streamwise vortices, which we found to be
generated in the bottom part of the near wake (these are usually referred to as
base vortices). In particular, once the hairpin structure is released, the base
vortices attach to it, forming its legs, so these are streamwise, and not
spanwise as previously observed in unconfined wakes or behind cylinders of
lower aspect ratios. We also single out a trail of Omega-shaped vortices,
generated between successive hairpin vortices through a mechanism that is
analogous to that active in near-wall turbulence. Finally, we show how the
dynamics of the structures we identified determine the evolution of the drag
coefficients and Strouhal numbers when the Reynolds number varies.Comment: 17 pages, accepted for publication in the Journal of Fluid Mechanic