M. Bourgoin, L. Marié, F. Pétrélis, C, Gasquet, A. Guigon, J.B. Luciani , M. Moulin, F. Namer, J. Burguete, A. Chiffaudel, F. Daviaud, S. Fauve, P. Odier, and J.F. Pinton “MHD measurements in the von Kármán sodium experiment” Physics of Fluids 14 pp 3046-3058, (2002).
We study the magnetic induction in a confined swirling flow of liquid sodium, at integral magnetic
Reynolds numbers up to 50. More precisely, we measure in situ the magnetic field induced by the
flow motion in the presence of a weak external field. Because of the very small value of the
magnetic Prandtl number of all liquid metals, flows with even modest Rm are strongly turbulent.
Large mean induction effects are observed over a fluctuating background. As expected from the von
Kármán flow geometry, the induction is strongly anisotropic. The main contributions are the
generation of an azimuthal induced field when the applied field is in the axial direction ~an V effect!
and the generation of axial induced field when the applied field is the transverse direction ~as in a
large scale a effect!. Strong fluctuations of the induced field, due to the flow nonstationarity, occur
over time scales slower than the flow forcing frequency. In the spectral domain, they display a f21
spectral slope. At smaller scales ~and larger frequencies! the turbulent fluctuations are in agreement
with a Kolmogorov modeling of passive vector dynamics.