Supersonic turbulence in 3D isothermal flow collision

Abstract

Large scale supersonic bulk flows are present in a wide range of astrophysical objects, from O-star winds to molecular clouds, galactic sheets, accretion, or γ-ray bursts. Associated flow collisions shape observable properties and internal physics alike. Our goal is to shed light on the interplay between large scale aspects of such collision zones and the characteristics of the compressible turbulence they harbor. Our model setup is as simple as can be: 3D hydrodynamical simulations of two head-on colliding, isothermal, and homogeneous flows with identical upstream (subscript u) flow parameters and Mach numbers 2 < M u < 43. The turbulence in the collision zone is driven by the upstream flows, whose kinetic energy is partly dissipated and spatially modulated by the shocks confining the zone. Numerical results are in line with expectations from self-similarity arguments. The spatial scale of modulation grows with the collision zone. The fraction of energy dissipated at the confining shocks decreases with increasing M u . The mean density is ρ m ≈ 20ρ u , independent of M u . The root mean square Mach number is M rms ≈ 0.25M u . Deviations toward weaker turbulence are found as the collision zone thickens and for small M u . The density probability function is not log-normal. The turbulence is inhomogeneous, weaker in the center of the zone than close to the confining shocks. It is also anisotropic: transverse to the upstream flows M rms is always subsonic. We argue that uniform, head-on colliding flows generally disfavor turbulence that is at the same time isothermal, supersonic, and isotropic. The anisotropy carries over to other quantities like the density variance -Mach number relation. Line-of-sight effects thus exist. Structure functions differ depending on whether they are computed along a line-of-sight perpendicular or parallel to the upstream flow. Turbulence characteristics generally deviate markedly from those found for uniformly driven, supersonic, isothermal turbulence in 3D periodic box simulations. We suggest that this should be kept in mind when interpreting turbulence characteristics derived from observations. Our simulations show that even a simple model setup results in a richly structured interaction zone. The robustness of our findings toward more realistic setups remains to be tested.

Effects of radiative losses on the relativistic jets of high-mass microquasars

“…It is thus not clear whether we under-or over-estimate this particular effect. Secondly, radiative multi-dimensional shocks can generate and drive turbulence (Walder & Folini 1998) and turbulent thin-shells (Folini & Walder 2006;Folini et al 2014).…”

Experimental Observation of Thin-shell Instability in a Collisionless Plasma

“…This thin shell is stable against linear instabilities but not necessarily against the nonlinear thinshell instability (NTSI; Vishniac 1994). The NTSI has been studied with hydrodynamic (Folini et al 2014) and magnetohydrodynamic (Heitsch et al 2007;McLeod & Whitworth 2013) simulations. However, unlike Rayleigh-Taylor or linear thin-shell instabilities (Grun et al 1991;Edens et al 2005;Ditmire & Edens 2008), the NTSI in a fluid or plasma has not yet been detected experimentally.…”

The onset of large-scale turbulence in the interstellar medium of spiral galaxies

“…This phenomenon has been identified in numerical simulations (e.g. Hunter et al 1986;Walder & Folini 2000;Audit & Hennebelle 2005;Heitsch et al 2005;Vázquez-Semadeni et al 2006Heitsch, Naab & Walch 2011;Inoue & Fukui 2013;Folini et al 2014, and others), being promptly related to the collapse of dense strucutures in the ISM and star formation, and analytically described by Vishniac (1994) as the nonlinear evolution of perturbations in shock-bounded slabs.…”

“…As pointed before, radiative shocks in converging flows (see Heitsch et al 2005;Audit & Hennebelle 2005;Heitsch et al 2006;Bonnell et al 2006Bonnell et al , 2013 have been shown to be very efficient in forming dense and cold structures in the ISM, but not very efficient in driving supersonic turbulence though. Theoretical studies of these systems reveal that the main process that lead to the structuring of clumps is the NTLI, which is also known to result in small scale subsonic/transonic turbulent flows (Vishniac 1994;Folini et al 2014). …”