Abstract : In this work we investigate the induction mechanisms which are at the origin of the dynamo effect in liquid metal flows at high magnetic Reynolds number (Rm). The flows are fully turbulent and exhibit fluctuations on a large range of scales. Measuring the induced magnetic field in gallium (Rm<5) and sodium (Rm<50) flows, we considered the following points : Do small scale fluctuations play any role in the dynamics of the large scale magnetic field? Although mean field theory predicts, in the case of scale separation, that turbulent fluctuations can produce a large scale magnetic field. The measurements performed in two different types of gallium flows show that the small scales contribution is negligible as compared to the mean flow contribution. How to describe the large scale movements contribution ? Measuring magnetic induction profiles in the VKG experiment, we show that the von Karman flow presents large scales turbulent fluctuations. We find that 50% of the time, the flow is away from its average structure, which causes intense fluctuations of induction mechanisms. Do these results have an influence on the realization of a dynamo experiment? These results suggest that the small scale turbulence will not modify the instability threshold while, in case of unconstrained flows, the predictions based on the topology of the mean flow may be wrong due to the large scale fluctuations.
In a second part, we investigate numerically induction mechanisms in the case of an assembly of helical vortices organized on a ring. For such a flow that mimics Busse's columns of rotating convection, we find that both dipoles and quadrupoles can be sustained. The results obtained with this simple flow underline the strong link between the geometry of the Karlsruhe flow and the problem of the earth magnetic field generation in Busse's model of rotating convection.