Fluctuation of magnetic induction in von K\'arm\'an swirling flows

Abstract

Studies of magnetic induction in von K\'arm\'an swirling flows have so far linked the time-averaged induced magnetic field to the structure of the mean flow. They have evidenced the Omega and Parker mechanism generated respectively by the flow differential rotation and helicity, which underly the Duddley and James \cite{DuddleyJames} dynamos. Using an array of Hall probes we study here the dynamical regime. In the experimental flow, turbulence is fully developed and large fluctuations are observed in the magnetic induction processes. We find that the large scale turbulent fluctuations have different characteristics when induction results from the differential rotation or from the dynamics of the stagnation point in the mid plane of the von K\'arm\'an flow. Symmetry considerations indicate that the dynamical flow spends half of its time away from the time-averaged structure. The consequences of these observations for dynamo experiments are discussed.