Numerical Studies of the Rayleigh-Taylor Instability

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The Rayleigh-Taylor (RT) instability occurs frequently in astrophysical hydrodynamic flows (e.g., the interaction of interstellar clouds with quasar winds; supernovae explosions; the interaction of supernova ejecta with the ISM, etc.). Since many of these astrophysical flows are being studied using numerical hydrodynamics (HD), we have completed a detailed investigation of the RT instability, to better understand the instability itself and the effects of numerical methods on the simulation of the instability. We present a series of 2D numerical simulations of the RT instability. The main objective of this work is to understand the effects of numerical resolution on the growth of the simulated instability. Furthermore, we have made a comparison of two popular numerical HD techniques. To this end, we have run simulations at four different grid resolutions (64(2) , 128(2) , 256(2) , 512(2) ). The two codes we have tested are VH-1, which uses the Lagrangian remap version of the piecewise parabolic method (PPM), and HYDI, an explicit Eulerian code which employs van Leer's (VL) monotonicity constraints and uses artificial viscosity. By examining the amount of power contained at different scales (i.e., power vs. wavenumber), it is easily seen that the PPM code better resolves small scale features. However, for this problem (one containing subsonic flow and no shocks), we have found the VL code requires only about a factor of sqrt 2 more zones per dimension to achieve the same accuracy as the PPM code. Furthermore, the VL code runs roughly 4 times faster (i.e., zones updated per second) than the PPM code. Therefore, our results show the VL code, although requiring more zones to achieve comparable accuracy, still requires less total CPU time. Other diagnostics will also be presented, such as comparisons with analytic expressions for instability growth rates and the growth of the mixing region versus time.

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