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| The '''Magnetic Reynolds number''' ('''R<sub>m</sub>''') is a [[dimensionless quantity|dimensionless group]] that
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| occurs in [[magnetohydrodynamics]]. It gives an estimate of the effects of magnetic [[advection]] to magnetic [[diffusion]], and is typically defined by:
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| : <math>\mathrm{R}_\mathrm{m} = \frac{U L}{\eta}</math>
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| where
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| * <math>U</math> is a typical velocity scale of the flow
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| * <math>L</math> is a typical length scale of the flow
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| * <math>\eta</math> is the [[magnetic diffusivity]]
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| == General Characteristics for Large and Small R<sub>m</sub> ==
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| For <math>\mathrm{R}_\mathrm{m} \ll 1</math>, advection is relatively unimportant, and so
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| the magnetic field will tend to relax towards a purely diffusive state, determined by the boundary conditions rather than the flow.
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| For <math>\mathrm{R}_\mathrm{m} \gg 1</math>, diffusion is relatively unimportant on the length scale ''L''. Flux lines of the magnetic field are then advected with the fluid flow, until such time as gradients are concentrated into regions of short enough length scale that diffusion can balance advection.
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| == Relationship to the Reynolds Number and Péclet Number ==
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| The Magnetic Reynolds number has a similar form to both the [[Péclet number]] and the [[Reynolds number]]. All three can be regarded as giving the ratio of advective to diffusive effects for a particular physical field, and have a similar form of a velocity times a length divided by a diffusivity. The magnetic Reynolds number is related to the magnetic field in an MHD flow, while the Reynolds number is related to the fluid velocity itself, and the Péclet number a related to heat. The dimensionless groups arise in the non-dimensionalization of the respective governing equations, the [[Faraday's law of induction#The Maxwell-Faraday_equation|induction equation]], the [[Navier–Stokes equations|momentum equation]], and the [[heat equation]].
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| ==See also==
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| * [[Lundquist number]]
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| * [[Magnetohydrodynamics]]
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| * [[Reynolds number]]
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| * [[Péclet number]]
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| {{NonDimFluMech}}
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| == References ==
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| * {{ScienceWorld|title=Magnetic Reynolds Number|urlname=physics/MagneticReynoldsNumber}}
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| * Moffatt, H. Keith, 2000, [http://www.igf.fuw.edu.pl/KB/HKM/PDF/HKM_122_s.pdf Reflections on Magnetohydrodynamics]. In: ''Perspectives in Fluid Dynamics'' (ISBN 0-521-53169-1) (Ed. G.K. Batchelor, H.K. Moffatt & M.G. Worster) [[Cambridge University Press]], p347-391.
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| * P. A. Davidson, 2001, "''An Introduction to Magnetohydrodynamics''" (ISBN 0-521-79487-0), [[Cambridge University Press]].
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| [[Category:Dimensionless numbers of fluid mechanics]]
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| [[Category:Fluid dynamics]]
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| [[Category:Magnetohydrodynamics]]
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