A NUMERICAL STUDY OF RAYLEIGH-TAYLOR INSTABILITY IN MAGNETIC FLUIDS

Mixing by the Rayleigh-Taylor (R-T) instability in incompressible magn etic fluids is studied with two- and three-dimensional ideal MHD simul ations. Evolution and amplification of magnetic fields in the Rayleigh -Taylor instability are explored in the linear and nonlinear regime fo r various perturbations. In a single-mode perturbation, both tangentia l and normal magnetic fields to the density interface decrease the gro wth of instabilities, while a tangential field is more efficient in st abilizing the flow, as predicted by linear theory. However, the growth of a multiple-mode perturbation in the nonlinear regime tends to be e nhanced by a normal magnetic field because the flow is collimated alon g the field lines. A strong tangential field suppresses the growth of short-wavelength modes, as expected from linear theory. Three-dimensio nal numerical simulations are carried out to study the amplification o f magnetic fields in the turbulent mixing layer developed by the insta bility in the nonlinear regime. We find the following results. (1) The turbulent flow amplifies the magnetic held more efficiently in three- dimensions than in two-dimensions. (2) The peak of the magnetic energy spectrum occurs at high wavenumbers, which means that the field is am plified preferentially on small scales. (3) The peak of the kinetic en ergy spectrum is at scales significantly longer than the grid scale, i s independent of grid resolution, and reflects the characteristic size of the R-T fingers at a given time. (4) Secondary Kelvin-Helmholtz in stabilities generate vortex rings and ring-structured magnetic fields. However, the stretching of field lines by R-T fingers is the dominant amplification mechanism. (5) The final structures are very sensitive to the initial magnetic field and numerical resolution. (6) The magnet ic field component along the gravity vector dominates as the instabili ties grow. This dominance is stronger in three dimensions than in two dimensions, but it becomes less dominant as the resolution increases. (7) higher resolution produces a greater total magnetic energy and a s maller total kinetic energy.