Vorticity production through rotation, shear and baroclinicity

TL;DR

This study investigates how rotation, shear, and baroclinicity contribute to vorticity generation in the interstellar medium using 3D simulations, revealing that baroclinicity is a key factor in supersonic turbulence.

Contribution

The paper systematically analyzes vorticity production mechanisms in idealized conditions, highlighting the dominant role of baroclinicity in supersonic turbulence.

Findings

Vorticity production is small with slow rotation in isothermal gas.

Vorticity saturates at higher Coriolis numbers.

Baroclinic effects significantly produce vorticity in supersonic turbulence.

Abstract

In the absence of rotation and shear, and under the assumption of constant temperature or specific entropy, purely potential forcing by localized expansion waves is known to produce irrotational flows that have no vorticity. Here we study the production of vorticity under idealized conditions when there is rotation, shear, or baroclinicity, to address the problem of vorticity generation in the interstellar medium in a systematic fashion. We use three-dimensional periodic box numerical simulations to investigate the various effects in isolation. We find that for slow rotation, vorticity production in an isothermal gas is small in the sense that the ratio of the root-mean-square values of vorticity and velocity is small compared with the wavenumber of the energy-carrying motions. For Coriolis numbers above a certain level, vorticity production saturates at a value where the aforementioned ratio becomes comparable with the wavenumber of the energy-carrying motions. Shear also raises the vorticity production, but no saturation is found. When the assumption of isothermality is dropped, there is significant vorticity production by the baroclinic term once the turbulence becomes supersonic. In galaxies, shear and rotation are estimated to be insufficient to produce significant amounts of vorticity, leaving therefore only the baroclinic term as the most favorable candidate. We also demonstrate vorticity production visually as a result of colliding shock fronts.