Consideration of gusts, blasts and prolonged velocity bursts in supersonic overpressured jets

ABSTRACT The origin, motion and brightness changes of travelling discrete knots in astrophysical jets remain open to interpretation. Such observations in active galactic nuclei jets have inspired our generic study of velocity bursts propagating along overpressured supersonic jets. As a first step we employ fundamental two-dimensional axisymmetric hydrodynamic simulations. We focus on the region immediately following jet launch in order to understand the basic physics of flow patterns. We systematically explore parameter space for various velocity burst interactions in adiabatic jets, and target jet configurations of interest. A very short-duration burst (gust) coherently creates interacting shock patterns as it passes successive recollimation shocks. A longer burst (blast) also temporarily reconfigures the channel. Successive shock-shock interactions increase jet pressure, and temporarily establish high pressure concentric reverse cone shells, increase jet diameter, and cause delayed after-shocks. In contrast, a prolonged-burst alters jet shape and affects multiple standing-shocks simultaneously. A burst causes a temporary translation of a standing-shock along the jet axis, more so in higher overpressure jets, as well as transmitting a sound wave into the surrounding ambient medium. There is potential to infer some jet physics using observational diagnostics, but more complex simulation physics would be required.