Magnetohydrodynamic experiments on cosmic magnetic fields

Stefani, Frank; Gailitis, A.; Gerbeth, Günter

doi:10.1002/zamm.200800102

Cited by 57 publications

(46 citation statements)

References 248 publications

(269 reference statements)

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“…A review on dynamo experiments is given in Ref. [8]. Detrimental effects of turbulence on the excitation threshold of large-scale dynamos were confirmed by numerical simulations of MHD systems with well-defined mean-flows in periodic-box geometry [9,10,11,12] as well as in spherically bounded geometry [13,14,15].…”

Section: Introductionmentioning

confidence: 88%

“…Turbulence develops which manifests itself in an increase of the critical magnetic Reynolds number Rm c (Re), as it was shown numerically for s2t2 systems [13,14,15]. This increase in the stability curve due to turbulence is a big challenge for laboratory dynamo experiments which use simply connected flows [3,4,5,6,7,8]. Liquid sodium which is preferred as a working fluid has a magnetic Prandtl number Pm = Rm/Re of the order of 10 −5 [19].…”

Section: L(t)mentioning

confidence: 94%

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Wave-driven dynamo action in spherical magnetohydrodynamic systems

et al. 2009

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Hydrodynamic and magnetohydrodynamic numerical studies of a mechanically forced two-vortex flow inside a sphere are reported. The simulations are performed in the intermediate regime between the laminar flow and developed turbulence where a hydrodynamic instability is found to generate internal waves with a characteristic m = 2 zonal wave number. It is shown that this time-periodic flow acts as a dynamo although snapshots of the flow as well as the mean flow are not dynamos. The magnetic fields' growth rate exhibits resonance effects depending on the wave frequency. Furthermore, a cyclic self-killing and self-recovering dynamo based on the relative alignment of the velocity and magnetic fields is presented. The phenomena are explained in terms of a mixing of non-orthogonal eigenstates of the time dependent linear operator of the magnetic induction equation. The potential relevance of this mechanism to dynamo experiments is discussed.

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Section: Introductionmentioning

confidence: 88%

Section: L(t)mentioning

confidence: 94%

Wave-driven dynamo action in spherical magnetohydrodynamic systems

et al. 2009

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“…Besides the temporal decay and growth, the (constant) azimuthal field drift determines the only time scale in the resonant regime, whereas outside of the resonant window two different time scales appear: an average azimuthal field drift that is roughly determined by the equatorial symmetrybreaking parameter a [solid curves in Fig. 11(a) 1 and the aforementioned modulation of the field amplitude which scales ∼2ω v [ Fig. 11(c)].…”

Section: B Impact Of Nonaxisymmetric Velocity Perturbationsmentioning

confidence: 99%

“…Meanwhile, fluid-flow-driven generation of magnetic fields has also been obtained in laboratory experiments providing a complementary tool to astronomical observations or numerical simulations. However, whereas astrophysical dynamo action is comparably easy because of the large dimensions of the involved flows, its experimental realization requires considerable technical efforts [1]. An important obstacle for the occurrence of laboratory dynamo action arises from the scaling behavior of the power that is required to drive a flow with a requested magnetic Reynolds number, Rm.…”

Section: Introductionmentioning

confidence: 99%

Impact of time-dependent nonaxisymmetric velocity perturbations on dynamo action of von Kármán-like flows

2012

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We present numerical simulations of the kinematic induction equation in order to examine the dynamo efficiency of an axisymmetric von Kármán-like flow subject to time-dependent nonaxisymmetric velocity perturbations. The numerical model is based on the setup of the French von Kármán-sodium dynamo (VKS) and on the flow measurements from a water experiment conducted at the University of Navarra in Pamplona, Spain. The principal experimental observations that are modeled in our simulations are nonaxisymmetric vortexlike structures which perform an azimuthal drift motion in the equatorial plane. Our simulations show that the interactions of these periodic flow perturbations with the fundamental drift of the magnetic eigenmode (including the special case of nondrifting fields) essentially determine the temporal behavior of the dynamo state. We find two distinct regimes of dynamo action that depend on the (prescribed) drift frequency of an (m = 2) vortexlike flow perturbation. For comparatively slowly drifting vortices we observe a narrow window with enhanced growth rates and a drift of the magnetic eigenmode that is synchronized with the perturbation drift. The resonance-like enhancement of the growth rates takes place when the vortex drift frequency roughly equals the drift frequency of the magnetic eigenmode in the unperturbed system. Outside of this small window, the field generation is hampered compared to the unperturbed case, and the field amplitude of the magnetic eigenmode is modulated with approximately twice the vortex drift frequency. The abrupt transition between the resonant regime and the modulated regime is identified as a spectral exceptional point where eigenvalues (growth rates and frequencies) and eigenfunctions of two previously independent modes collapse. In the actual configuration the drift frequencies of the velocity perturbations that are observed in the water experiment are much larger than the fundamental drift frequency of the magnetic eigenmode that is obtained from our numerical simulations. Hence, we conclude that the fulfillment of the resonance condition might be unlikely in present day dynamo experiments. However, a possibility to increase the dynamo efficiency in the VKS experiment might be realized by an application of holes or fingers on the outer boundary in the equatorial plane. These mechanical distortions provoke an anchorage of the vortices at fixed positions thus allowing an adjustment of the temporal behavior of the nonaxisymmetric flow perturbations.

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“…As for the dynamo problem, quite a number of experiments have been carried out (Stefani, Gailitis & Gerbeth 2008). Up to present, magnetic field self-excitation has been attained in the liquid sodium experiments in Riga (Gailitis et al 2000), Karlsruhe (Müller & Stieglitz 2000), and Cadarache (Monchaux et al 2007).…”

Section: Introductionmentioning

confidence: 99%

Local instabilities in magnetized rotational flows: a short-wavelength approach

Kirillov

Stefani

Fukumoto³

2014

J. Fluid Mech.

Self Cite

189

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We perform a local stability analysis of rotational flows in the presence of a constant vertical magnetic field and an azimuthal magnetic field with a general radial dependence. Employing the short-wavelength approximation we develop a unified framework for the investigation of the standard, the helical, and the azimuthal version of the magnetorotational instability, as well as of current-driven kink-type instabilities. Considering the viscous and resistive setup, our main focus is on the case of small magnetic Prandtl numbers which applies, e.g., to liquid metal experiments but also to the colder parts of accretion disks. We show that the inductionless versions of MRI that were previously thought to be restricted to comparably steep rotation profiles extend well to the Keplerian case if only the azimuthal field slightly deviates from its current-free (in the fluid) profile. We find an explicit criterion separating the pure azimuthal inductionless magnetorotational instability from the regime where this instability is mixed with the Tayler instability. We further demonstrate that for particular parameter configurations the azimuthal MRI originates as a result of a dissipation-induced instability of the Chandrasekhar's equipartition solution of ideal magnetohydrodynamics.

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Magnetohydrodynamic experiments on cosmic magnetic fields

Cited by 57 publications

References 248 publications

Wave-driven dynamo action in spherical magnetohydrodynamic systems

Wave-driven dynamo action in spherical magnetohydrodynamic systems

Impact of time-dependent nonaxisymmetric velocity perturbations on dynamo action of von Kármán-like flows

Local instabilities in magnetized rotational flows: a short-wavelength approach

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