Volume 528, April 2011
|Number of page(s)||8|
|Section||Interstellar and circumstellar matter|
|Published online||17 March 2011|
Vorticity production through rotation, shear, and baroclinicity
1 Nordita, AlbaNova University
23, SE-10691 Stockholm, Sweden
2 Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
Received: 31 August 2010
Accepted: 14 February 2011
Context. 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.
Aims. 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.
Methods. We use three-dimensional periodic box numerical simulations to investigate the various effects in isolation.
Results. 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.
Key words: turbulence / dynamo / magnetohydrodynamics (MHD) / ISM: bubbles / galaxies: magnetic fields / galaxies: ISM
© ESO, 2011
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