THE STABILITY AND COLLIMATION OF 3-DIMENSIONAL JETS

A three-dimensional hydrodynamical simulation of a Mach 5 cylindrical jet established in equilibrium with a surrounding uniform atmosphere of 10 times the jet density has been performed and is compared with a previous Mach 3 simulation. The grid resolution used is significantly higher than was used in the previous simulation of a Mach 3 equilibrium jet. The higher resolution has not significantly changed the development of large-scale structures such as the helical twisting of the jet, the elliptical distortion and bifurcation of the jet, or the triangular distortion and trifurcation of the jet seen in the lower resolution simulation. These structures arise from the Kelvin-Helmholtz unstable surface modes predicted by the linear theory, and the elliptical mode appears coupled to the precessionally driven helical mode. Newly discovered is a short-wavelength, relatively large-amplitude transverse velocity oscillation on the jet axis. The effect of the unstable body modes on jet structure is analyzed for the first time, and a helical internal body mode is revealed to be responsible for the observed short-wavelength velocity oscillation on the jet axis. This body mode helically twists the inner 20%-25% of the jet, and the transverse velocity oscillation occurs at the fastest spatially growing wavelength predicted by the theory. While the higher resolution of the new simulation has allowed the development of structures on smaller scales and reveals more detail, large-scale surface distortion and accompanying surface filamentation still appear to dominate jet dynamics, and decollimation occurs as the helically distorted jet bifurcates or trifurcates. This newer simulation suggests that mass entrainment, and significant shock heating and dissipation, occur as the jet breaks up into multiple streams, and qualitative comparison between the two simulations suggests that more shock heating and decollimation occur in the Mach 5 simulation.