4 research outputs found
The MHD Kelvin-Helmholtz Instability II: The Roles of Weak and Oblique Fields in Planar Flows
We have carried out high resolution MHD simulations of the nonlinear
evolution of Kelvin-Helmholtz unstable flows in 2 1/2 dimensions. The modeled
flows and fields were initially uniform except for a thin shear layer with a
hyperbolic tangent velocity profile and a small, normal mode perturbation. The
calculations consider periodic sections of flows containing magnetic fields
parallel to the shear layer, but projecting over a full range of angles with
respect to the flow vectors. They are intended as preparation for fully 3D
calculations and to address two specific questions raised in earlier work: 1)
What role, if any, does the orientation of the field play in nonlinear
evolution of the MHD Kelvin-Helmholtz instability in 2 1/2 D. 2) Given that the
field is too weak to stabilize against a linear perturbation of the flow, how
does the nonlinear evolution of the instability depend on strength of the
field. The magnetic field component in the third direction contributes only
through minor pressure contributions, so the flows are essentially 2D. Even a
very weak field can significantly enhance the rate of energy dissipation. In
all of the cases we studied magnetic field amplification by stretching in the
vortex is limited by tearing mode, ``fast'' reconnection events that isolate
and then destroy magnetic flux islands within the vortex and relax the fields
outside the vortex. If the magnetic tension developed prior to reconnection is
comparable to Reynolds stresses in the flow, that flow is reorganized during
reconnection. Otherwise, the primary influence on the plasma is generation of
entropy. The effective expulsion of flux from the vortex is very similar to
that shown by Weiss for passive fields in idealized vortices with large
magnetic Reynolds numbers. We demonstrated that thisComment: 23 pages of ApJ Latex (aaspp4.sty) with 10 figures, high resolution
postscript images for figs 4-9 available through anonymous at
ftp://ftp.msi.umn.edu/pub/twj To appear in the June 10, 1997 Ap