Reduction of velocity fluctuations in a turbulent flow of gallium by an external magnetic field

The magnetic field of planets or stars is generated by the motion of a conducting fluid through a dynamo instability. The saturation of the magnetic field occurs through the reaction of the Lorentz force on the flow. In relation to this phenomenon, we study the effect of a magnetic field on a turbulent flow of liquid gallium. The measurement of electric potential differences provides a signal related to the local velocity fluctuations. We observe a reduction of velocity fluctuations at all frequencies in the spectrum when the magnetic field is increased.

Figure 1

Cross section of the experimental setup. A propeller is placed off axis in a cylindrical cell filled with liquid gallium. The vertical magnetic field is generated by a cylindrical coil and is monitored with a gaussmeter. An electrical potential difference is measured by a pair of copper electrodes.

Figure 2

(Color online) (a) Temporal signal measured with the potential probe for a rotation frequency of propeller of $7\mathrm{Hz}$ under a magnetic field of $738\mathrm{G}$. (b) Corresponding power spectrum.

Figure 3

(Color online) Standard deviation of the fluctuations of the potential difference divided by the rotation frequency of the propeller versus applied magnetic field.

Figure 4

(Color online) Standard deviation of dimensionless velocity fluctuations versus interaction parameter $N$.

Figure 5

(Color online) Comparison between power spectra of velocity for a rotation frequency of $5\mathrm{Hz}$ for two different values of the external magnetic field. Inset: ratio between the two power spectrum densities.