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.