SIMULATION OF THE INSTABILITY OF MACH 2-AND MACH-4 GASEOUS JETS IN 2-DIMENSIONS AND 3-DIMENSIONS

Instabilities affecting the propagation of supersonic gaseous jets have been studied using high-resolution computer simulations with the piecewise-parabolic method. These results are discussed in relation to jets from galactic nuclei. These studies involve a detailed treatment of a single section of a very long jet, approximating the dynamics by using periodic boundary conditions. Convergence of the numerical approximations has been tested by comparing jet simulations with different grid resolutions. The effects of initial conditions and geometry on the dominant disruptive instabilities have also been explored. For simulations of periodic jets, the initial velocity perturbations set up zig-zag shock patterns inside the jet. In each case a single zig-zag shock pattern (an odd mode) or a double zig-zag shock pattern (an even mode) grows to dominate the flow. The dominant kink instability responsible for these shock patterns moves approximately at the linear resonance velocity, upsilon(mode) = c(ext) upsilon(rel)/(c(jet) + c(ext)). For high-resolution simulations (those with 150 or more computational zones across the jet width), the even mode dominates if the even perturbation is higher in amplitude initially than the odd perturbation. For low-resolution simulations, the odd mode dominates even for a stronger even mode perturbation. In high-resolution simulations, the jet boundary rolls up and large amounts of external gas are entrained into the jet. In low-resolution simulations, this entrainment process is impeded. Large-amplitude modes take much longer to develop in jets perturbed at one point in space (long, nonperiodic jets) than those with spatially periodic perturbations, but once a dominant mode is established, disruption occurs at essentially the same rate. Except for the presence of small-scale turbulence at low resolution, the three-dimensional jet simulations behave similarly to two-dimensional jet runs.