The supernova-regulated ISM

III. Generation of vorticity, helicity, and mean flows

¹ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: kapyla@mps.mpg.de
² ReSoLVE Centre of Excellence, Department of Computer Science, PO Box 15400, 00076 Aalto, Finland
³ Department of Physics, PO Box 64, University of Helsinki, 00014 Helsinki, Finland
⁴ School of Mathematics and Statistics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK

Received: 23 May 2017
Accepted: 9 October 2017

Abstract

Context. The forcing of interstellar turbulence, driven mainly by supernova (SN) explosions, is irrotational in nature, but the development of significant amounts of vorticity and helicity, accompanied by large-scale dynamo action, has been reported.

Aim. Several earlier investigations examined vorticity production in simpler systems; here all the relevant processes can be considered simultaneously. We also investigate the mechanisms for the generation of net helicity and large-scale flow in the system.

Methods. We use a three-dimensional, stratified, rotating and shearing local simulation domain of the size 1 × 1 × 2 kpc³, forced with SN explosions occurring at a rate typical of the solar neighbourhood in the Milky Way. In addition to the nominal simulation run with realistic Milky Way parameters, we vary the rotation and shear rates, but keep the absolute value of their ratio fixed. Reversing the sign of shear vs. rotation allows us to separate the rotation- and shear-generated contributions.

Results. As in earlier studies, we find the generation of significant amounts of vorticity, the rotational flow comprising on average 65% of the total flow. The vorticity production can be related to the baroclinicity of the flow, especially in the regions of hot, dilute clustered supernova bubbles. In these regions, the vortex stretching acts as a sink of vorticity. In denser, compressed regions, the vortex stretching amplifies vorticity, but remains sub-dominant to baroclinicity. The net helicities produced by rotation and shear are of opposite signs for physically motivated rotation laws, with the solar neighbourhood parameters resulting in the near cancellation of the total net helicity. We also find the excitation of oscillatory mean flows, the strength and oscillation period of which depend on the Coriolis and shear parameters; we interpret these as signatures of the anisotropic-kinetic-α (AKA) effect. We use the method of moments to fit for the turbulent transport coefficients, and find α_AKA values of the order 3–5 km s⁻¹.

Conclusions. Even in a weakly rotationally and shear-influenced system, small-scale anisotropies can lead to significant effects at large scales. Here we report on two consequences of such effects, namely on the generation of net helicity and on the emergence of large-scale flows by the AKA effect, the latter detected for the first time in a direct numerical simulation of a realistic astrophysical system.