Search

Direct measurements of the vector turbulent emf in a driven two-vortex flow of liquid sodium were performed in the Madison Dynamo Experiment [K. Rahbarnia et al., Astrophys. J. 759, 80 (2012)]. The measured turbulent emf is anti-parallel with the mean current and is almost entirely described by an enhanced resistivity, which increases the threshold for a kinematic dynamo. We have demonstrated that this enhanced resistivity can be mitigated by eliminating the largest-scale eddies through the introduction of baffles. By tailoring the flow to reduce large-scale components and control the helical pitch, we have reduced the power required to drive the impellers, doubled the magnetic flux generated by differential rotation, and increased the decay time of externally applied magnetic fields. Despite these improvements, the flows remain sub-critical to the dynamo instability due to the reemergence of turbulent fluctuations at high flow speeds. [ABSTRACT FROM AUTHOR]

The generation of a magnetic field by dynamo action in a Taylor-vortex flow is investigated numerically.We first discuss how the Taylor vortices generate a spatially subharmonic dynamo, for which the axial wavelength of the magnetic field is twice the one of the flow pattern. Then, we investigate the influence of the Reynolds number and the turbulent fluctuations on the structure and the onset of the Taylor-Couette dynamo. Finally, based on the subharmonic nature of this dynamo, we propose new configurations which could be relevant for laboratory experiments. [ABSTRACT FROM AUTHOR]

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]

The turbulent electromotive force in the mean-field equation needs to be modeled to predict a large-scale magnetic field in magnetohydrodynamic turbulence at high Reynolds number. Using a statistical theory for inhomogeneous turbulence, model expressions for transport coefficients appearing in the turbulent electromotive force are derived including the α coefficient and the turbulent diffusivity. In particular, as one of the dynamo effects, the pumping effect is investigated and a model expression for the pumping term is obtained. It is shown that the pumping velocity is closely related to the gradient of the turbulent residual energy, or the difference between the turbulent kinetic and magnetic energies. The production terms in the transport equation for the turbulent electromotive force are also examined and the validity of the model expression is assessed by comparing with earlier results concerning the isotropic α coefficient. The mean magnetic field in a rotating spherical shell is calculated using a turbulence model to demonstrate the pumping effect. [ABSTRACT FROM AUTHOR]

The effect of turbulent fluctuations on the zonal (flux surface averaged) magnetic field in drift-ballooning turbulence is shown to be equivalent to a turbulent anti-resistivity. The flux surface average of the convective nonlinearity in Ohm’s law is proportional to the flux surface average of the current. The coefficient of proportionality can be defined as a turbulent resistivity ηturb. The correlation of the flux surface average of the convective nonlinearity with the negative of the flux surface average of the current is nearly 100%. Because the convective nonlinearity is correlated with the negative of the current, and not the current, the turbulent resistivity is negative. The magnitude of ηturb is virtually identical to the magnitude of the collisional resistivity η, but opposite in sign, so that the total resistivity ηtotal=η+ηturb is nearly zero. The effect of the fluctuations is to balance the effect of collisional resistive diffusion. As a result, while the energy in the zonal flow increases to a large value as the fluctuations grow and saturate, the energy in the zonal magnetic field remains very small. [ABSTRACT FROM AUTHOR]

Two-dimensional Boussinesq magnetoconvection with idealized stress-free boundary conditions is numerically investigated in order to make clear the difference between chaos and turbulence. It is shown that the long-term behavior of magnetoconvection exhibits spatially coherent and temporally chaotic rolls in marked contrast to highly turbulent fluids. It is also shown that heat transport becomes larger anomalously when the polarity reversal of the magnetic field occurs intermittently in the case of temporally chaotic magnetoconvection. It is found that the Poincaré return map of the relative maximum temperature fluctuation of partial differential equations as a function of the preceding maximum resembles the famous Lorenz plot in narrow rolls of magnetoconvection. The chaotic behavior of narrow rolls for individual parameter values robustly persists up to rolls about one fifth as wide as they are high near the codimension-two bifurcation point. [ABSTRACT FROM AUTHOR]

A nonlinear, time-dependent, magnetically driven dynamo theory that shows how magnetically dominated configurations can relax to become helical on the largest scale available is presented. Coupled time-dependent differential equations for large-scale magnetic helicity, small-scale magnetic helicity, velocity, and the electromotive force are solved. The magnetic helicity on small scales relaxes to drive significant large-scale helical field growth on dynamical (Alfvén crossing) time scales, independent of the magnitude of finite microphysical transport coefficients, after which the growing kinetic helicity slows the growth to a viscously limited pace. This magnetically driven dynamo complements the nonlinear kinetic helicity-driven dynamo; for the latter, the growing magnetic helicity fluctuations suppress, rather than drive, large-scale magnetic helicity growth. A unified set of equations accommodates both types of dynamos. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the analogy between the dynamo equation for the generation of a magnetic field by the motion of an electrically conducting fluid and the equation for the evolution of vorticity of a viscous fluid. Exponential stretching feature of the underlying instability problem; Instability of the growth rate of an infinitesimal perturbation in a metric with derivatives.