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- Title
Mean-field dynamo due to spatio-temporal fluctuations of the turbulent kinetic energy.
- Authors
Gopalakrishnan, Kishore; Singh, Nishant K.
- Abstract
In systems where the standard a effect is inoperative, one often explains the existence of mean magnetic fields by invoking the 'incoherent a effect', which appeals to fluctuations of the mean kinetic helicity at a mesoscale. Most previous studies, while considering fluctuations in the mean kinetic helicity, treated the mean turbulent kinetic energy at the mesoscale as a constant, despite the fact that both these quantities involve second-order velocity correlations. The mean turbulent kinetic energy affects the mean magnetic field through both turbulent diffusion and turbulent diamagnetism. In this work, we use a double-averaging procedure to analytically show that fluctuations of the mean turbulent kinetic energy at the mesoscale (giving rise to ?-fluctuations at the mesoscale, where the scalar = is the turbulent diffusivity) can lead to the growth of a large-scale magnetic field even when the kinetic helicity is zero pointwise. Constraints on the operation of such a dynamo are expressed in terms of dynamo numbers that depend on the correlation length, correlation time and strength of these fluctuations. In the white-noise limit, we find that these fluctuations reduce the overall turbulent diffusion, while also contributing a drift term which does not affect the growth of the field. We also study the effects of non-zero correlation time and anisotropy. Turbulent diamagnetism, which arises due to inhomogeneities in the turbulent kinetic energy, leads to growing mean-field solutions even when the ?-fluctuations are statistically isotropic.
- Subjects
KINETIC energy; ELECTRIC generators; WHITE noise; MAGNETIC fields; DIAMAGNETISM
- Publication
Journal of Fluid Mechanics, 2023, Vol 973, pA29-1
- ISSN
0022-1120
- Publication type
Article
- DOI
10.1017/jfm.2023.765