Ponomarenko dynamo sustained by a free swirling jet

Abstract: We present numerical results on dynamo action in a flow driven by an azimuthal body force localized near the end of an elongated cylindrical container. The analysis focuses on the central region of the cylinder, where axial variations in the flow are relatively weak, allowing the magnetic field to be represented as a helically traveling wave. Four magnetic impeller configurations and multiple forcing intensities are examined. In all cases, the velocity profiles in the central region display a similar \propto r^{-2} dependence across a wide range of Reynolds numbers and forcing region widths. The magnetic field is found to start growing under conditions similar to those of the Riga dynamo. However, the growing modes exhibit a substantial nonzero group velocity, indicating that the associated instability is convective: the flow can amplify an externally applied magnetic field but cannot sustain it autonomously. We outline several approaches for overcoming this limitation in order to realize a working laboratory dynamo based on an internally unconstrained swirling jet-type flow.