Abstract
We study different types of stationary dynamos observed in the Von Kármán sodium (VKS) experiment when varying the electromagnetic boundary conditions on (and in) the impellers. The flow is driven with two impellers made of soft iron (Monchaux et al 2007 Phys Rev. Lett.98 044502) or using one soft-iron impeller and one stainless steel impeller. The magnetic field is mapped using 40 three-dimensional probes distributed within the flow and its surroundings. Symmetry and coupling properties are then retrieved from direct probe measurements and/or from the field structure as reconstructed using the inversion procedure described by Boisson and Dubrulle (2011 New J. Phys.13 023037). Several salient results are obtained: (i) dynamo action is not achieved unless at least one iron impeller is rotating, at a frequency larger than 15 Hz; (ii) the resulting dynamo is a dipolar, mostly axisymmetric structure; and (iii) the self-sustained magnetic field properties depend on the sodium flow structure between the two impellers. We propose to interpret the stationary dynamos generation as the (constructive or destructive) superposition of two one-impeller fluid dynamos generated close to the soft-iron impellers, nonlinearly coupled through the turbulent flow, as suggested by Verhille et al (2010 New J. Phys.12 033006). The normal form equation describing this coupling is similar to the one obtained in a theoretical model (Pétrélis et al 2009 Phys. Rev. Lett.102 144503).
Highlights
Impellers), the fluid in between the two impellers being only a current-carrying medium
Because of (iii), we can exclude a fluid dynamo solely based on the mean velocity, which would produce an equatorial dipole [9, 23]
This is either a semi-fluid dynamo (e.g. α–ω dynamo, with an α process generated by vortices in between the blades [12] or by a turbulence gradient in the impellers vicinity [25, 26] and an ω process generated by the shear close to the impeller and amplified by the high magnetic permeability of the impellers [7]), or a fully fluid dynamo (e.g. α–ω dynamo, with an α process generated by vortices in-between the blades and an ω process generated at the blade interface, where rotation gradients are the largest)
Summary
The experimental setup of the configurations is schematized in figure 1. F is the mean forcing frequency F = (F1 + F2)/2; μ0 is the magnetic permeability of vacuum; σ is the electrical conductivity of sodium; and K = A±(1 + |θ |) is a non-dimensional factor describing the efficiency of the impellers as measured in a water experiment through maximal velocities achieved in the vessel [9]. Note that the conductivity of sodium is quite sensitive to temperature variations These variations are taken into account in the computation of Rm. The magnetic field is measured with the Hall probe arrays partly inserted inside the fluid, as shown in figure 1. For configurations where all probes are available, we have applied the Galerkin reconstruction method introduced in [21] to estimate the global magnetic field properties
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
