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Colliding supersonic bulk flows shape observable properties and internalphysics of various astrophysical objects, like O-star winds, molecular clouds,galactic sheets, binaries, or gamma-ray bursts. Using numerical simulations, weshow that the bulk flows leave a clear imprint on the collision zone, its meanproperties and the turbulence it naturally develops. Our model setup consistsof 3D head-on colliding isothermal hydrodynamical flows with Mach numbersbetween 2 and 43. Simulation results are in line with expectations fromself-similarity: root mean square Mach numbers (Mrms) scale linearly withupstream Mach numbers, mean densities remain limited to a few times theupstream density. The density PDF is not log-normal. The turbulence isinhomogeneous: weaker in the zone center than close to the confining shocks. Itis anisotropic: while Mrms is generally supersonic, Mrms transverse to theupstream flow is always subsonic. We argue that uniform, isothermal, head-oncolliding flows generally disfavor isotropic, supersonic turbulence. Theanisotropy carries over to other quantities like the density variance - Machnumber relation. Structure functions differ depending on whether they arecomputed along a line-of-sight perpendicular or parallel to the upstream flow.We suggest that such line-of-sight effects should be kept in mind wheninterpreting turbulence characteristics derived from observations.

Supersonic turbulence in 3D isothermal flow collision