Dynamos in precessing cubes

Goepfert, O; Tilgner, A.

doi:10.1088/1367-2630/18/10/103019

Cited by 22 publications

(15 citation statements)

References 31 publications

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“…Ekman pumping at the boundaries and triad resonances were the first to be observed (Tilgner 2005). Dynamos in long slender vortices which form at low Ekman numbers were found later (Goepfert and Tilgner 2016). Recently (Giesecke et al 2018), it was shown that for certain parameters, the flow in precessing cylinders resembles the s 2 t 1 flow studied by Dudley and James (1989) as kinematic dynamo in a sphere.…”

Section: Resultsmentioning

confidence: 91%

“…There are several reasonable options for removing dimensions from the governing equations. Here, we adopt the choice already made in Goepfert and Tilgner (2016) and base the unit of time on the total angular frequency of rotation about the container axis, denoted as x-axis, to which bothω D andΩ P contribute. The unit of time is then 1/(ω D +Ω P cos α) and the nondimensional rotation rates ω D and Ω P derived formω D andΩ P are…”

Section: The Mathematical Model Of a Precessing Cubementioning

confidence: 99%

“…The decrease in L D correlates with the increase of Rm c,rot in figure 9. To make this clear, figure 11 transitions between different states as Ek is varied (Goepfert and Tilgner 2016), for example between different triad resonances, or a triad resonance and a single vortex state, or a significant axisymmetric component may come and go. This is particularly true of the points with Ω = −0.02 which are therefore not shown in figure 11.…”

Section: Kinematic Dynamosmentioning

confidence: 99%

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Mechanisms for magnetic field generation in precessing cubes

Goepfert

Tilgner

2018

Geophysical & Astrophysical Fluid Dynamics

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It is shown that flows in precessing cubes develop at certain parameters large axisymmetric components in the velocity field which are large enough to either generate magnetic fields by themselves, or to contribute to the dynamo effect if inertial modes are already excited and acting as a dynamo. This effect disappears at small Ekman numbers. The critical magnetic Reynolds number also increases at low Ekman numbers because of turbulence and small scale structures.

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Section: Resultsmentioning

confidence: 91%

Section: The Mathematical Model Of a Precessing Cubementioning

confidence: 99%

Section: Kinematic Dynamosmentioning

confidence: 99%

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Mechanisms for magnetic field generation in precessing cubes

Goepfert

Tilgner

2018

Geophysical & Astrophysical Fluid Dynamics

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“…Defining Ω o = Ω s + Ω p , we choose Ω −1 o as the unit of time such that it remains relevant in both limits of large Ω s or large Ω p (see also Goepfert & Tilgner, 2016). We choose R and RΩ o √ µρ as the respective units of length and magnetic field.…”

Section: Mathematical Formulationmentioning

confidence: 99%

Precessing spherical shells: flows, dissipation, dynamo and the lunar core

Cébron

Laguerre

Noir

et al. 2019

Geophysical Journal International

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Precession of planets or moons affects internal liquid layers by driving flows, instabilities and possibly dynamos. The energy dissipated by these phenomena can influence orbital parameters such as the planet's spin rate. However, there is no systematic study of these flows in the spherical shell geometry relevant for planets, and the lack of scaling law prevents convincing extrapolation to celestial bodies.We have run more than 900 simulations of fluid spherical shells affected by precession, to systematically study basic flows, instabilities, turbulence, and magnetic field generation. We observe no significant effects of the inner core on the onset of the instabilities. We obtain an analytical estimate of the viscous dissipation, mostly due to boundary layer friction in our simulations. We propose theoretical onsets for hydrodynamic instabilities, and document the intensity of turbulent fluctuations.We extend previous precession dynamo studies towards lower viscosities, at the limits of today's computers. In the low viscosity regime, precession dynamos rely on the presence of large-scale vortices, and the surface magnetic fields are dominated by small scales. Interestingly, intermittent and self-killing dynamos are observed. Our results suggest that large-scale planetary magnetic fields are unlikely to be produced by a precession-driven dynamo in a spherical core. But this question remains open as planetary cores are not exactly spherical, and thus the coupling between the fluid and the boundary does not vanish in the relevant limit of small viscosity. Moreover, the fully turbulent dissipation regime has not yet been reached in simulations.Our results suggest that the melted lunar core has been in a turbulent state throughout its history. Furthermore, in the view of recent experimental results, we propose updated formulas predicting the fluid mean rotation vector and the associated dissipation in both the laminar and the turbulent regimes.

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“…Gans 1970;Meunier et al 2008;Herault et al 2015), the cube (e.g. Goepfert & Tilgner 2016), the sphere (e.g. Kida 2011;Boisson et al 2012;Goto et al 2014;Lin et al 2015), the spherical shell (e.g.…”

Section: Introductionmentioning

confidence: 99%