Mean fields and fluctuations in compressible magnetohydrodynamic flows

James F. Hollins, Graeme R. Sarson*, Cetin Can Evirgen, Anvar Shukurov, Andrew Fletcher, Frederick A. Gent

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

2 Citations (Scopus)
57 Downloads (Pure)

Abstract

We apply Gaussian smoothing to obtain mean magnetic field, density, velocity, and magnetic and kinetic energy densities from our numerical model of the interstellar medium, based on three-dimensional magnetohydrodynamic equations in a shearing box 1 x 1 x 2 kpc in size. The interstellar medium is highly compressible, as the turbulence is transonic or supersonic; it is thus an excellent context in which to explore the use of smoothing to represent physical variables in a compressible medium in terms of their mean and fluctuating parts. Unlike alternative averaging procedures, such as horizontal averaging, Gaussian smoothing retains the three-dimensional structure of the mean fields. Although Gaussian smoothing does not obey the Reynolds rules of averaging, physically meaningful and mathematically sound central statistical moments are defined as suggested by Germano [Turbulence - The filtering approach. J. Fluid Mech. 1992, 238, 325-336]. We discuss methods to identify an optimal smoothing scale l and the effects of this choice on the results. From spectral analysis of the magnetic, density and velocity fields, we find a suitable smoothing length for all three fields, of l approximate to 75 pc. Such a smoothing scale is likely to be sensitive to the choice of simulation parameters; this may be considered in future work, but here we just explore the methodology. We discuss the properties of third-order statistical moments in fluctuations of kinetic energy density in compressible flows, and suggest their physical interpretation. The mean magnetic field, amplified by a mean-field dynamo, significantly alters the distribution of kinetic energy in space and between scales, reducing the magnitude of kinetic energy at intermediate scales. This intermediate-scale kinetic energy is a useful diagnostic of the importance of SN-driven outflows.

Original languageEnglish
Pages (from-to)261-289
Number of pages29
JournalGeophysical and Astrophysical Fluid Dynamics
Volume116
Issue number4
Early online date5 Mar 2022
DOIs
Publication statusPublished - 2022
MoE publication typeA1 Journal article-refereed

Keywords

  • galaxies: ISM
  • ISM: kinematics and dynamics
  • Magnetohydrodynamics
  • methods: statistical
  • turbulence

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  • Newcastle University

    Gent, F. (Visiting researcher), Qazi, Y. (Student), Shukurov, A. (Visiting researcher) & Tharakkal, D. (Student)

    1 Jan 202331 Aug 2023

    Activity: Visiting an external institution typesVisiting an external academic institution

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