The influence of a surrounding wind on overpressured jets – I. Subsonic winds

There is evidence that supersonic jet flows in astrophysics are accompanied by surrounding slow winds. This is analogous to the flow past jet engines when an aircraft is in flight. If the jet itself enters the medium underexpanded, there will be a distinct flow pattern as the wind is deflected by the diverging jet. Here, we explore the general problem of such an overpressured jet flow encompassed by a wind which is subsonic. We simulate the long-term adjustment by performing a series of hydrodynamic adiabatic gas experiments in two dimensions with cylindrical symmetry. The major result is that the oscillations in the jet induced by feedback are suppressed once the wind has swept clean the initial jet overspill. Therefore, a slow ambient wind reduces the noise caused by the exhaust and may fully stabilize jet propagation. The differences between the shock diamonds of regular reflection and Mach discs are elucidated. In particular, with a wind, a distinctive pattern of repeated Mach shock discs occurs at high overpressures that replaces the turbulent plume. Secondly, the presence of a faster subsonic wind generates central enhanced pressure arcs and knots within each diamond. These features could provide indirect evidence for winds and are briefly discussed in terms of potential consequences for star and galaxy formation.