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 Ω effect) and the generation of axial induced field when the applied field is the transverse direction (as in a large scale α 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 f−1 spectral slope. At smaller scales (and larger frequencies) the turbulent fluctuations are in agreement with a Kolmogorov modeling of passive vector dynamics.

1.
H. K. Moffatt, Magnetic Field Generation in Electrically Conducting Fluids (Cambridge University Press, Cambridge, 1978).
2.
R. Moreau, Magnetohydrodynamics (Kluwer, Dordrecht, 1990).
3.
A.
Alemany
,
R.
Moreau
,
L.
Sulem
, and
U.
Frisch
, “
Influence of an external magnetic field on homogeneous MHD turbulence
,”
J. Mec.
18
,
278
(
1979
).
4.
S.
Fauve
,
C.
Laroche
, and
A.
Libchaber
, “
Effect of a horizontal magnetic field on convective instabilities in mercury
,”
J. Phys. (France) Lett.
42
,
L455
(
1981
);
S.
Fauve
,
C.
Laroche
, and
A.
Libchaber
,
J. Phys. (France) Lett.
“Horizontal magnetic and the oscillatory instability onset,”
45
,
L101
(
1984
).
5.
R.
Moreau
and
J.
Sommeria
, “
Why, how and when MHD turbulence becomes two-dimensional
,”
J. Fluid Mech.
118
,
507
(
1982
).
6.
J. Larmor, “How could a rotating body such as the sun become a magnet?” Rep. Brit. Assoc. Adv. Sci., 159 (1919).
7.
A.
Gailitis
,
O.
Lielausis
,
S.
Dement’ev
,
E.
Placatis
,
A.
Cifersons
,
G.
Gerbeth
,
T.
Gundrum
,
F.
Stefani
,
M.
Chrsiten
,
H.
Hänel
, and
G.
Will
, “
Detection of a flow induced magnetic field eigenmode in the Riga dynamo facility
,”
Phys. Rev. Lett.
84
,
4365
(
2000
).
8.
A.
Gailitis
,
O.
Lielausis
,
S.
Dement’ev
,
E.
Placatis
,
A.
Cifersons
,
G.
Gerbeth
,
T.
Gundrum
,
F.
Stefani
,
M.
Chrsiten
,
H.
Hänel
, and
G.
Will
, “
Magnetic field saturation in the Riga dynamo experiment
,”
Phys. Rev. Lett.
86
,
3024
(
2001
).
9.
Yu. B.
Ponomarenko
, “
Theory of the hydromagnetic generator
,”
J. Appl. Mech. Tech. Phys.
6
,
755
(
1973
).
10.
R.
Stieglitz
and
U.
Müller
, “
Can the Earth’s magnetic field be simulated in the laboratory?
Naturwissenschaften
87
,
381
(
2000
).
11.
R.
Stieglitz
and
U.
Müller
, “
Experimental demonstration of a homogeneous two-scale dynamo
,”
Phys. Fluids
13
,
561
(
2001
).
12.
G. O.
Roberts
, “
Kinematic dynamo models
,”
Philos. Trans. R. Soc. London, Ser. A
271
,
411
(
1972
).
13.
N. L.
Dudley
and
R. W.
James
, “
Time-dependent kinematic dynamos with stationary flows
,”
Proc. R. Soc. London, Ser. A
425
,
407
(
1989
).
14.
J. Léorat, private communication at the 2nd Pamir Conference, Aussois (France), September 1998.
15.
L. Marié, J. Burguete, A. Chiffaudel, F. Daviaud, D. Ericher, C. Gasquet, F. Pétrélis, S. Fauve, M. Bourgoin, M. Moulin, P. Odier, J.-F. Pinton, A. Guigon, J.-B. Luciani, F. Namer, and J. Léorat, “MHD in von Kármán swirling flows,” in Ref. 18.
16.
C.
Nore
,
M.-E.
Brachet
,
H.
Politano
, and
A.
Pouquet
, “
Dynamo action in the Taylor–Green vortex near threshold
,”
Phys. Plasmas
4
,
1
(
1997
).
17.
N. L.
Peffley
,
A. B.
Cawthrone
, and
D. P.
Lathrop
, “
Toward a self-generating magnetic dynamo: The role of turbulence
,”
Phys. Rev. E
61
,
5287
(
2000
).
18.
W. L. Shew, D. R. Sisan, and D. P. Lathrop, “Hunting for dynamos: Eight different liquid sodium flows,” in Dynamo and Dynamics, A Mathematical Challenge, Proceedings of the NATO Advanced Research Workshop, Cargèse, France, 21–26 August 2000. NATO Science Series II, Vol. 26, edited by P. Chossat, D. Armbruster, and I. Oprea (Kluwer Academic, Dordrecht, 2001).
19.
D.
Sweet
,
E.
Ott
,
J. M.
Finn
,
T. M.
Antonsen
,Jr.
, and
D. P.
Lathrop
, “
Blowout bifurcations and the onset of magnetic activity in turbulent dynamos
,”
Phys. Rev. E
63
,
066211
(
2001
);
D.
Sweet
,
E.
Ott
,
T. M.
Antonsen
, Jr.
,
D. P.
Lathrop
, and
J. M.
Finn
, “
Blowout bifurcations and the onset of magnetic dynamo action
,”
Phys. Plasmas
8
,
1944
(
2001
).
20.
P. J.
Zandbergen
and
D.
Dijkstra
, “
von Kármán swirling flows
,”
Annu. Rev. Fluid Mech.
19
,
465
(
1987
).
21.
N.
Mordant
,
J.-F.
Pinton
, and
F.
Chillà
, “
Characterization of turbulence in a closed flow
,”
J. Phys. II
7
,
1
(
1997
).
22.
J.-F.
Pinton
and
R.
Labbé
, “
Correction to Taylor hypothesis in swirling flows
,”
J. Phys. II
4
,
1461
(
1994
).
23.
O.
Cadot
,
S.
Douady
, and
Y.
Couder
, “
Characterization of the low-pressure filaments in a three-dimensional turbulent shear flow
,”
Phys. Fluids
7
,
630
(
1995
).
24.
S.
Fauve
,
C.
Laroche
, and
B.
Castaing
, “
Pressure fluctuations in swirling turbulent flows
,”
J. Phys. II
3
,
271
(
1993
).
25.
S.
Douady
,
Y.
Couder
, and
M.-E.
Brachet
, “
Direct observation of intense vorticity filaments in turbulence
,”
Phys. Rev. Lett.
67
,
983
(
1991
).
26.
B.
Dernoncourt
,
J.-F.
Pinton
, and
S.
Fauve
, “
Study of vorticity filaments using ultrasound scattering in turbulent swirling flows
,”
Physica D
117
,
181
(
1998
).
27.
P.
Odier
,
J.-F.
Pinton
, and
S.
Fauve
, “
Advection of a magnetic field by a turbulent swirling flow
,”
Phys. Rev. E
58
,
7397
(
1998
).
28.
A.
Martin
,
P.
Odier
,
J.-F.
Pinton
, and
S.
Fauve
, “
Magnetic permeability of a diphasic flow, made of liquid gallium and iron beads
,”
Eur. Phys. J. B
18
,
337
(
2000
).
29.
L. Marié et al., Proceedings of the Sixth European Chaos Conference, Potsdam, Germany, 22–26 July 2001, edited by S. Boccaletti, L. Peccora, and J. Kurths (American Institute of Physics, Melville, NY, 2002).
30.
H. K.
Moffatt
, “
The amplification of a weak applied magnetic field by turbulence in fluids of moderate conductivity
,”
J. Fluid Mech.
11
,
625
(
1961
).
31.
A.
Alemany
,
P.
Marty
,
F.
Plunian
, and
J.
Soto
, “
Experimental investigation of dynamo effect in the secondary pumps of the fast breeder reactor Superphenix
,”
J. Fluid Mech.
403
,
263
(
2000
).
32.
U.
Frisch
,
A.
Pouquet
,
J.
Léorat
, and
A.
Mazure
, “
Possibility of an inverse cascade of magnetic helicity in magnetohydrodynamic turbulence
,”
J. Fluid Mech.
68
,
769
(
1975
).
33.
G. S.
Golitsyn
, “
Fluctuation of the magnertic field and current density in a turbulent flow of a weakly conducting fluid
,”
Sov. Phys. Dokl.
5
,
536
(
1960
).
34.
F.
Pétrélis
,
L.
Marié
,
M.
Bourgoin
,
A.
Chiffaudel
,
F.
Daviaud
,
S.
Fauve
,
P.
Odier
, and
J.-F.
Pinton
, “
Large scale alpha-effect in a turbulent swirling flow
,” arXiv:physics/0205019 (
2002
).
This content is only available via PDF.
You do not currently have access to this content.