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Supersonic turbulence in 3D isothermal flow collision

Colliding supersonic bulk flows shape observable properties and internal physics of various astrophysical objects, like O-star winds, molecular clouds, galactic sheets, binaries, or gamma-ray bursts. Using numerical simulations, we show that the bulk flows leave a clear imprint on the collision zone, its mean properties and the turbulence it naturally develops. Our model setup consists of 3D head-on colliding isothermal hydrodynamical flows with Mach numbers between 2 and 43. Simulation results are in line with expectations from self-similarity: root mean square Mach numbers (Mrms) scale linearly with upstream Mach numbers, mean densities remain limited to a few times the upstream density. The density PDF is not log-normal. The turbulence is inhomogeneous: weaker in the zone center than close to the confining shocks. It is anisotropic: while Mrms is generally supersonic, Mrms transverse to the upstream flow is always subsonic. We argue that uniform, isothermal, head-on colliding flows generally disfavor isotropic, supersonic turbulence. The anisotropy carries over to other quantities like the density variance - Mach number relation. Structure functions differ depending on whether they are computed 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 when interpreting turbulence characteristics derived from observations.