Turbulent viscosity and magnetic Prandtl number from simulations of isotropically forced turbulence

P. J. Käpylä*, M. Rheinhardt, A. Brandenburg, M. J. Käpylä

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

1 Downloads (Pure)

Abstract

Context. Turbulent diffusion of large-scale flows and magnetic fields plays a major role in many astrophysical systems, such as stellar convection zones and accretion discs. Aims. Our goal is to compute turbulent viscosity and magnetic diffusivity which are relevant for diffusing large-scale flows and magnetic fields, respectively. We also aim to compute their ratio, which is the turbulent magnetic Prandtl number, Pmt, for isotropically forced homogeneous turbulence. Methods. We used simulations of forced turbulence in fully periodic cubes composed of isothermal gas with an imposed large-scale sinusoidal shear flow. Turbulent viscosity was computed either from the resulting Reynolds stress or from the decay rate of the large-scale flow. Turbulent magnetic diffusivity was computed using the test-field method for a microphysical magnetic Prandtl number of unity. The scale dependence of the coefficients was studied by varying the wavenumber of the imposed sinusoidal shear and test fields. Results. We find that turbulent viscosity and magnetic diffusivity are in general of the same order of magnitude. Furthermore, the turbulent viscosity depends on the fluid Reynolds number (Re) and scale separation ratio of turbulence. The scale dependence of the turbulent viscosity is found to be well approximated by a Lorentzian. These results are similar to those obtained earlier for the turbulent magnetic diffusivity. The results for the turbulent transport coefficients appear to converge at sufficiently high values of Re and the scale separation ratio. However, a weak trend is found even at the largest values of Re, suggesting that the turbulence is not in the fully developed regime. The turbulent magnetic Prandtl number converges to a value that is slightly below unity for large Re. For small Re we find values between 0.5 and 0.6 but the data are insufficient to draw conclusions regarding asymptotics. We demonstrate that our results are independent of the correlation time of the forcing function. Conclusions. The turbulent magnetic diffusivity is, in general, consistently higher than the turbulent viscosity, which is in qualitative agreement with analytic theories. However, the actual value of Pmt found from the simulations (≈0.9-0.95) at large Re and large scale separation ratio is higher than any of the analytic predictions (0.4-0.8).

Original languageEnglish
Article numberA93
Number of pages12
JournalAstronomy and Astrophysics
Volume636
DOIs
Publication statusPublished - 1 Apr 2020
MoE publication typeA1 Journal article-refereed

Keywords

  • Stars: rotation
  • Sun: rotation
  • Turbulence

Fingerprint Dive into the research topics of 'Turbulent viscosity and magnetic Prandtl number from simulations of isotropically forced turbulence'. Together they form a unique fingerprint.

  • Projects

    UniSDyn: Building up a Unified Theory of Stellar Dynamos

    Käpylä, M., Rheinhardt, M. & Pekkilä, J.

    01/01/202031/12/2024

    Project: EU: ERC grants

    Cite this