dorsal/arxiv
View SchemaParadox of inductionless magnetorotational instability in a Taylor-Couette flow with a helical magnetic field
| Authors | J. Priede, I. Grants, G. Gerbeth |
|---|---|
| Categories | |
| ArXiv ID | physics/0606122 |
| URL | https://arxiv.org/abs/physics/0606122 |
| DOI | 10.1088/1742-6596/64/1/012011 |
| Journal | J.Phys.Conf.Ser.64:012011,2007 |
Abstract
We consider the magnetorotational instability (MRI) of a hydrodynamically stable Taylor-Couette flow with a helical external magnetic field in the inductionless approximation defined by a zero magnetic Prandtl number ($\Pm=0)$. This leads to a considerable simplification of the problem eventually containing only hydrodynamic variables. First, we point out that the energy of any perturbation growing in the presence of magnetic field has to grow faster without the field. This is a paradox because the base flow is stable without the magnetic while it is unstable in the presence of a helical magnetic field without being modified by the latter as it has been found recently by Hollerbach and Rudiger [Phys. Rev. Lett. 95, 124501 (2005)]. We revisit this problem by using a Chebyshev collocation method to calculate the eigenvalue spectrum of the linearized problem. In this way, we confirm that MRI with helical magnetic field indeed works in the inductionless limit where the destabilization effect appears as an effective shift of the Rayleigh line. Second, we integrate the linearized equations in time to study the transient behavior of small amplitude perturbations, thus showing that the energy arguments are correct as well. However, there is no real contradiction between both facts. The linear stability theory predicts the asymptotic development of an arbitrary small-amplitude perturbation, while the energy stability theory yields the instant growth rate of any particular perturbation, but it does not account for the evolution of this perturbation.
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"abstract": "We consider the magnetorotational instability (MRI) of a hydrodynamically\nstable Taylor-Couette flow with a helical external magnetic field in the\ninductionless approximation defined by a zero magnetic Prandtl number\n($\\Pm=0)$. This leads to a considerable simplification of the problem\neventually containing only hydrodynamic variables. First, we point out that the\nenergy of any perturbation growing in the presence of magnetic field has to\ngrow faster without the field. This is a paradox because the base flow is\nstable without the magnetic while it is unstable in the presence of a helical\nmagnetic field without being modified by the latter as it has been found\nrecently by Hollerbach and Rudiger [Phys. Rev. Lett. 95, 124501 (2005)]. We\nrevisit this problem by using a Chebyshev collocation method to calculate the\neigenvalue spectrum of the linearized problem. In this way, we confirm that MRI\nwith helical magnetic field indeed works in the inductionless limit where the\ndestabilization effect appears as an effective shift of the Rayleigh line.\nSecond, we integrate the linearized equations in time to study the transient\nbehavior of small amplitude perturbations, thus showing that the energy\narguments are correct as well. However, there is no real contradiction between\nboth facts. The linear stability theory predicts the asymptotic development of\nan arbitrary small-amplitude perturbation, while the energy stability theory\nyields the instant growth rate of any particular perturbation, but it does not\naccount for the evolution of this perturbation.",
"arxiv_id": "physics/0606122",
"authors": [
"J. Priede",
"I. Grants",
"G. Gerbeth"
],
"categories": [
"physics.flu-dyn",
"astro-ph"
],
"doi": "10.1088/1742-6596/64/1/012011",
"journal_ref": "J.Phys.Conf.Ser.64:012011,2007",
"title": "Paradox of inductionless magnetorotational instability in a Taylor-Couette flow with a helical magnetic field",
"url": "https://arxiv.org/abs/physics/0606122"
},
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