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Experimental results are reported on the magnetohydrodynamic interaction between oscillating flows in adjoining channels of the Karlsruhe dynamo test facility. In the experiment, designed to simulate the Roberts–Busse kinematic dynamo model, a time periodic perturbation has been imposed on the axial channel flows, while the flow rates in the other two helical channel systems were supplied at constant power level. It has been observed that the onset of dynamo action is shifted slightly to lower mean flow rates. A temporal analysis shows distinct correlations between the evolving magnetic field and the different kinds of forced flows in the channels. From the beginning the environmental magnetic field adjusts to fluctuations of the axial channel flow. The fluctuations of the magnetic field increase owing to dynamo action in a transition range and induce quasiperiodic fluctuations on the flow in the helical channels by acting Lorentz forces. In the high supercritical range the fluctuations are dampened. [ABSTRACT FROM AUTHOR]

Turbulent fluctuations in the velocity and magnetic fields of electrically conducting fluids have been experimentally shown to be capable of inducing large-scale magnetic fields. Here, simulations of the Madison Dynamo Experiment are used to qualitatively reproduce these experimental results. Due to the high magnetic Prandtl number of the simulations, Pm=0.08 vs Pm∼10-5 for liquid sodium, the simulations do not identically reproduce the fluctuation levels of the experiment’s magnetic and velocity fields. Nonetheless, the simulations reproduce the qualitative behavior of the fluctuation-induced large-scale magnetic field as a function of applied field magnitude and magnetic Reynolds number. The scaling of the induced dipole moment as a function of Reynolds number is also presented, demonstrating that the nature of the fluctuations in the simulations changes after a critical value of the Reynolds number is crossed, resulting in a change in the direction of the induced dipole moment. Experimental conditions using corotating impellers are presented, demonstrating that the induced dipole moment may be dependent on the shear layer present in the counter-rotating case. Measurements of velocity field fluctuations are examined to determine the possibility of an inhomogeneous turbulent resistivity. [ABSTRACT FROM AUTHOR]

It is shown that the α2-dynamo of magnetohydrodynamics, the hydrodynamic Squire equation as well as an interpolation model of PT-symmetric quantum mechanics are closely related as spectral problems in Krein spaces. For the α2-dynamo and the PT-symmetric model the strong similarities are demonstrated with the help of a 2×2 operator matrix representation, whereas the Squire equation is reinterpreted as a rescaled and Wick-rotated PT-symmetric problem. Based on recent results on the Squire equation the spectrum of the PT-symmetric interpolation model is analyzed in detail and the Herbst limit is described as spectral singularity. [ABSTRACT FROM AUTHOR]

Mean field dynamo theory is a leading candidate to explain the large scale magnetic fields of galaxies and stars. However, controversy arises over the extent of quenching by the backreaction of the growing field. Here boundary conditions and magnetic helicity flow are shown to play a role in determining whether the mean field dynamo action is fast, as required by astrophysical systems, or resistively limited (slow). Existing work suggesting that mean field dynamos are resistively limited include restrictive approximations such as stationarity and periodic boundary conditions that suppress magnetic helicity flow. Thus even though the backreaction is present, such studies cannot unambiguously reveal whether real astrophysical mean field dynamos are dynamically suppressed when the helicity flow is allowed. If the dynamo is sustained by an outflow of helicity from the system, then a magnetically active corona is expected. Open boundaries alone may not be sufficient for rapid dynamo action and the additional physics of buoyancy and outflows may be required. Possible simulation approaches to test some of the principles are briefly discussed. Some limitations of the"Zeldovich relation" are also addressed. [ABSTRACT FROM AUTHOR]