NONLINEAR DYNAMICS OF A 3-DIMENSIONAL JET

A three-dimensional hydrodynamical simulation of a cylindrical jet initialized across a Cartesian grid has been performed. The jet is driven at the origin by a small-amplitude precession to break the symmetry and excite helical modes of the Kelvin-Helmholtz instability. The simulation reveals a wealth of structures including filaments twisted around the circumference of the jet. Rapid spatial growth of relatively small amplitude filaments that can be attributed to high-order Kelvin-Helmholtz fluting modes is overwhelmed at larger distance by a slower spatially growing helically twisted elliptical distortion of the jet cross section. At slightly larger spatial scales, the jet is helically twisted and the flow bifurcates (trifurcates) as a further development of the elliptical (triangular) Kelvin-Helmholtz fluting modes. The observed behavior of this simulation is in good agreement with linear stability theory. The observed large-amplitude bifurcation of the flow into a helically twisted filament pair overlaying a helical twist of the whole jet appears similar to the observed oscillation and twisted filament pair seen in the Cygnus A jet. The small-amplitude multiple filamentation observed appears similar to the filamentation seen in the M87 jet inside knot A.