The generation of a magnetic field by a flow of liquid sodium has been observed in recent experiments (Karlsruhe, Riga). We emphasize two very interesting features displayed by these experiments. - The observed dynamo threshold is in good agreement with the one computed from the mean flow alone, i. e. neglecting turbulent fluctuations although the kinematic Reynolds number is of order 105 to 106. - On the contrary, the mean magnetic field measured above dynamo threshold is 1000 times larger than the one predicted from a laminar weakly nonlinear calculation. We first understand these two observations and give the expected scaling law for the magnetic energy above dynamo threshold.
We then consider magnetic fluctuations above dynamo threshold or in MHD turbulence and report a Kolmogorov type spectrum in the inertial range and 1/f noise at low frequency.
Finally, we discuss the effect of turbulence and rotation on dynamo onset and saturation in flows without strong geometrical constraints.