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Mean fields and fluctuations in compressible magnetohydrodynamic flows

Lookup NU author(s): James Hollins, Dr Graeme Sarson, Can Evirgen, Professor Anvar ShukurovORCiD, Dr Andrew Fletcher, Dr Fred Gent



This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).


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 1×1×2 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 (1992). We discuss methods to identify an optimal smoothing scale ` 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 ≈ 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.

Publication metadata

Author(s): Hollins JF, Sarson GR, Evirgen CC, Shukurov A, Fletcher A, Gent FA

Publication type: Article

Publication status: Published

Journal: Geophysical and Astrophysical Fluid Dynamics

Year: 2022

Volume: 116

Issue: 4

Pages: 261-289

Online publication date: 07/03/2022

Acceptance date: 18/01/2022

Date deposited: 20/01/2022

ISSN (print): 0309-1929

ISSN (electronic): 1029-0419

Publisher: Taylor and Francis


DOI: 10.1080/03091929.2022.2032022


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