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Energy pathways for large- and small-scale magnetic field generation in convection-driven plane layer dynamos

Published online by Cambridge University Press:  18 March 2025

Souvik Naskar Open the ORCID record for Souvik Naskar [Opens in a new window]  and
Anikesh Pal Open the ORCID record for Anikesh Pal [Opens in a new window]
Souvik Naskar*
Affiliation:
Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208016, India School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
Anikesh Pal
Affiliation:
Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208016, India
*
Corresponding author: Souvik Naskar, [email protected]
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Abstract

We use direct numerical simulations to investigate the energy pathways between the velocity and the magnetic fields in a rotating plane layer dynamo driven by Rayleigh–Bénard convection. The kinetic and magnetic energies are divided into mean and turbulent components to study the production, transport and dissipation in large- and small-scale dynamos. This energy balance-based characterisation reveals distinct mechanisms for large- and small-scale magnetic field generation in dynamos, depending on the nature of the velocity field and the conditions imposed at the boundaries. The efficiency of a dynamo in converting the kinetic energy to magnetic energy, apart from the energy redistribution inside the domain, is found to depend on the kinematic and magnetic boundary conditions. In a small-scale dynamo with a turbulent velocity field, the turbulent kinetic energy converts to turbulent magnetic energy via small-scale magnetic field stretching. This term also represents the amplification of the turbulent magnetic energy due to work done by stretching the small-scale magnetic field lines owing to fluctuating velocity gradients. The stretching of the large-scale magnetic field plays a significant role in this energy conversion in a large-scale turbulent dynamo, leading to a broad range of energetic scales in the magnetic field compared with a small-scale dynamo. This large-scale magnetic field stretching becomes the dominant mechanism of magnetic energy generation in a weakly nonlinear dynamo. We also find that, in the weakly nonlinear dynamo, an upscale energy transfer from the small-scale magnetic field to the large-scale magnetic field occurs owing to the presence of a gradient of the mean magnetic field.

JFM classification

Geophysical and Geological Flows: Geostrophic turbulence MHD and Electrohydrodynamics: Dynamo theory MHD and Electrohydrodynamics: MHD turbulence
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 1007 , 25 March 2025 , A74
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Copyright
© The Author(s), 2025. Published by Cambridge University Press

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