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**Session 7 - Gas and Dust in the ISM.**

*Display session, Monday, June 10*

*Great Hall, *

## [7.16] The MHD Kelvin-Helmholtz Instability: Oblique Field 2\frac12D Simulations

*J. B. Gaalaas, T. W. Jones, D. Ryu, A. Frank (U. Minnesota)*

We extend recently published numerical simulations of the nonlinear
evolution of the MHD Kelvin-Helmholtz instability (Frank etal, ApJ, 460, 777 (1996)).
The earlier work considered the behavior of a K-H unstable shear layer
in which the initial magnetic and velocity fields were aligned; that is
if |\hat B \cdot \hat u| = \cos\theta, then \theta = 0 .
Those simulations were carried out with a new multi-dimensional MHD TVD
code. The new work examines the evolution of similar MHD flows,
but for which the magnetic field is oblique to the the computational
plane; that is, \theta > 0, so that the flows are 2\frac12 dimensional. For comparison
we also computed flows for which the initial fields are aligned, but
that have commensurate field strengths as the plane-projected component
of the oblique field cases. We have followed the evolution for times
between 20 and 30 linear growth times, beginning with a linear, normal
mode perturbation in a periodic box. All of the runs extend well beyond the initial
saturation of the instability.

To a good approximation for the cases we have considered
there is no difference between the
influence of an oblique field of a strength, B with \theta > 0 and an aligned field of
strength B\cos\theta. We identify four distinctive roles for the
field, which can be characterized in terms of the strength of the field
in the computational plane: 1) B_x = B\cos\theta = 0, no influence, a classic vortex is formed;
2) very weak B_x, reconnection enhances dissipation of the vortex, but
little other influence;
3) moderately weak B_x, the magnetic field plays the role of a
catalyst to help reorganize the flow. Reconnection leads to a
simple field and a stable, laminar flow;
4) strong B_x, field tension stabilizes the initial flow.

This work
is supported by the NSF, by NASA and by the Univesity of Minnesota
Supercomputer Institute.

**Program
listing for Monday**