Riga dynamo experiment and its theoretical background

Gailitis, A.; Lielausis, O.; Platacis, E.; Gerbeth, Günter; Stefani, Frank

doi:10.1063/1.1666361

Cited by 55 publications

(74 citation statements)

References 12 publications

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“…104501-2 flow) it was proved that the vertical magnetic field condition ( [6,26]) provided reasonable predictions of the theoretically estimated (based on the convective instabilities of the Ponomarenko dynamo, [4,6] …”

Section: 104501 (2007) P H Y S I C a L R E V I E W L E T T E R S mentioning

confidence: 99%

“…Because of the highly turbulent flow regime, the wall functions are used for providing the wall boundary conditions for hydrodynamical variables. The vertical magnetic field boundary condition is imposed for the magnetic field components at the outer side of the surrounding ring of the sodium initially at rest, [6,26]. The very first step in numerical simulation was to obtain the fully developed (statistically steady) RANS solutions for the fluid flow and turbulence field (without electromagnetic effects).…”

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confidence: 99%

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Numerical Simulation of a Turbulent Magnetic Dynamo

Kenjereš

Hanjalić

2007

Phys. Rev. Lett.

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We present numerical simulations of a turbulent magnetic dynamo mimicking closely the Riga-dynamo experiment at Re 3:5 10 6 and 15 Re m 20. The Reynolds-averaged Navier-Stokes equations for the fluid flow and turbulence field are solved simultaneously with the direct numerical solution of the magnetic field equations. The fully integrated two-way-coupled simulations reproduced all features of the magnetic self-excitation detected by the Riga experiment, with frequencies and amplitudes of the selfgenerated magnetic field in good agreement with the experimental records, and provided full insight into the unsteady magnetic and velocity fields and the mechanisms of the dynamo action. DOI: 10.1103/PhysRevLett.98.104501 PACS numbers: 47.11.ÿj, 47.27.Eÿ, 47.65.ÿd, 91.25.Cw Complex interactions between turbulent flow of electrically conductive fluids and electromagnetic fields play the key role in many physical phenomena in nature and technology. Of particular interest is the conversion of the mechanical energy of a moving electrically conductive medium into the magnetic energy, known as the magnetic dynamo. It is believed that the magnetic dynamo effects are responsible for the creation of magnetic fields in spiral galaxies, stars, and planets (including Earth's magnetic field), e.g., [1]. In order to create the favorable conditions for possible self-excitation of a magnetic field, it is necessary to achieve regimes where stretching of the magnetic field will overcome its resistive damping. This condition is defined by the critical magnetic Reynolds number Re m UL= > 1 (where U and L are typical velocity and length scale, respectively, and is magnetic diffusivity). In order to reach such conditions, one needs relatively large length scales and high velocities -even for the best electrically conductive fluids (e.g., liquid sodium, 1= 0 0:1 m 2 =s), where 0 and are the magnetic permeability and electric conductivity. For such configurations, the hydrodynamical Reynolds number (Re UL= ) will be also very high (Re 10 5 -10 6 ), making the flow highly turbulent. Experimental studies of magnetic fluid dynamos face many practical problems associated with large dimensions of setups and potentially hazardous working fluids (sodium). This explains why it was not until late 1999 when two experimental groups in Riga ([2 -6]) and Karlsruhe ([7-9]) finally and independently succeeded in detecting the self-excitation and subsequent sustenance of a magnetic field for the very first time. Although this is an important step towards understanding and explaining the Earth's magnetic field from the magnetic dynamo effect, both experiments were not designed to actually mimic the Geo-Dynamo (Earth-like) conditions, but rather to provide experimental proof of the magnetic field self-amplification.Despite their remarkable success, the experimental studies provided only the time records of the magnetic field components at particular locations -so information addressing the detailed spatial distribution of the magnetic field and its dynamics...

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Section: 104501 (2007) P H Y S I C a L R E V I E W L E T T E R S mentioning

confidence: 99%

mentioning

confidence: 99%

Numerical Simulation of a Turbulent Magnetic Dynamo

Kenjereš

Hanjalić

2007

Phys. Rev. Lett.

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“…In the employed finitedifference solver this problem was overcome by solving the Laplace equation in the exterior and using matching conditions at the interface to the dynamo domain. 14,28 A similar approach, although based on the finite element method, was presented by Guermond et al 30 Other methods of handling this boundary condition problem are the integral equation approach [31][32][33] and a hybrid boundary element/finite volume method. 34 For the Riga experiment, we also checked the use of simplified boundary conditions ͑so-called vertical field conditions 35 ͒ which led, however, to a significant 20% error in the determination of the critical Re m .…”

Section: Introductionmentioning

confidence: 99%

“…As a follow up of a preceding paper, 14 we aim at an improved numerical simulation of the interaction of velocity and magnetic field. In contrast to Ref.…”

Section: Introductionmentioning

confidence: 99%

“…5,6 Since then, a number of experiments have been carried out at both places providing a wealth of data on the dynamo behavior in the kinematic and the saturation regime. [7][8][9][10][11][12][13][14][15][16][17][18] Further dynamorelated experiments are being prepared or carried out at various places in the world. [19][20][21][22][23] Although, up to present, they have not yet reached the self-excitation condition, they have brought about many interesting results, in particular concerning the role of turbulence in flows with large Re m .…”

Section: Introductionmentioning

confidence: 99%

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Coupled fluid-flow and magnetic-field simulation of the Riga dynamo experiment

et al. 2006

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Magnetic fields of planets, stars, and galaxies result from self-excitation in moving electroconducting fluids, also known as the dynamo effect. This phenomenon was recently experimentally confirmed in the Riga dynamo experiment ͓A. Gailitis et al., Phys. Rev. Lett. 84, 4365 ͑2000͒; A. Gailitis et al., Physics of Plasmas 11, 2838 ͑2004͔͒, consisting of a helical motion of sodium in a long pipe followed by a straight backflow in a surrounding annular passage, which provided adequate conditions for magnetic-field self-excitation. In this paper, a first attempt to simulate computationally the Riga experiment is reported. The velocity and turbulence fields are modeled by a finite-volume Navier-Stokes solver using a Reynolds-averaged-Navier-Stokes turbulence model. The magnetic field is computed by an Adams-Bashforth finite-difference solver. The coupling of the two computational codes, although performed sequentially, provides an improved understanding of the interaction between the fluid velocity and magnetic fields in the saturation regime of the Riga dynamo experiment under realistic working conditions.

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Energy oscillations and a possible route to chaos in a modified Riga dynamo

Stefani¹,

Gailitis²,

Gerbeth³

2011

Astron Nachr

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Starting from the present version of the Riga dynamo experiment with its rotating magnetic eigenfield dominated by a single frequency we ask for those modifications of this set-up that would allow for a non-trivial magnetic field behaviour in the saturation regime. Assuming an increased ratio of azimuthal to axial flow velocity, we obtain energy oscillations with a frequency below the eigenfrequency of the magnetic field. These new oscillations are identified as magneto-inertial waves that result from a slight imbalance of Lorentz and inertial forces. Increasing the azimuthal velocity further, or increasing the total magnetic Reynolds number, we find transitions to a chaotic behaviour of the dynamo.

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Riga dynamo experiment and its theoretical background

Cited by 55 publications

References 12 publications

Numerical Simulation of a Turbulent Magnetic Dynamo

Numerical Simulation of a Turbulent Magnetic Dynamo

Coupled fluid-flow and magnetic-field simulation of the Riga dynamo experiment

Energy oscillations and a possible route to chaos in a modified Riga dynamo

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