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Lorentz Force (lorentz + force)

Distribution by Scientific Domains
Physics and Astronomy57%

Selected Abstracts

Saturation and time dependence of geodynamo models

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2010
M. Schrinner
SUMMARY In this study we address the question under which conditions a saturated velocity field stemming from geodynamo simulations leads to an exponential growth of the magnetic field in a corresponding kinematic calculation. We perform global self-consistent geodynamo simulations and calculate the evolution of a kinematically advanced tracer field. The self-consistent velocity field enters the induction equation in each time step, but the tracer field does not contribute to the Lorentz force. This experiment has been performed by Cattaneo and Tobias and is closely related to the test field method by Schrinner et al. We find two dynamo regimes in which the tracer field either grows exponentially or approaches a state aligned with the actual self-consistent magnetic field after an initial transition period. Both regimes can be distinguished by the Rossby number and coincide with the dipolar and multipolar dynamo regimes identified by Christensen and Aubert. Dipolar dynamos with low Rossby number are kinematically stable whereas the tracer field grows exponentially in the multipolar dynamo regime. This difference in the saturation process for dynamos in both regimes comes along with differences in their time variability. Within our sample of 20 models, solely kinematically unstable dynamos show dipole reversals and large excursions. The complicated time behaviour of these dynamos presumably relates to the alternating growth of several competing dynamo modes. On the other hand, dynamos in the low Rossby number regime exhibit a rather simple time dependence and their saturation merely results in a fluctuation of the fundamental dynamo mode about its critical state. [source]

An Euler system source term that develops prototype Z-pinch implosions intended for the evaluation of shock-hydro methods

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2009
J. W. Banks
Abstract In this paper, a phenomenological model for a magnetic drive source term for the momentum and total energy equations of the Euler system is described. This body force term is designed to produce a Z-pinch like implosion that can be used in the development and evaluation of shock-hydrodynamics algorithms that are intended to be used in Z-pinch simulations. The model uses a J × B Lorentz force, motivated by a 0-D analysis of a thin shell (or liner implosion), as a source term in the equations and allows for arbitrary current drives to be simulated. An extension that would include the multi-physics aspects of a proposed combined radiation hydrodynamics (rad-hydro) capability is also discussed. The specific class of prototype problems that are developed is intended to illustrate aspects of liner implosions into a near vacuum and with idealized pre-fill plasma effects. In this work, a high-resolution flux-corrected-transport method implemented on structured overlapping meshes is used to demonstrate the application of such a model to these idealized shock-hydrodynamic studies. The presented results include an asymptotic solution based on a limiting-case thin-shell analytical approximation in both (x, y) and (r, z). Additionally, a set of more realistic implosion problems that include density profiles approximating plasma pre-fill and a set of perturbed liner geometries that excite a hydro-magnetic like Rayleigh,Taylor instability in the implosion dynamics are demonstrated. Finally, as a demonstration of including and evaluating multiphysics effects in the Euler system, a simple radiation model is included and self-convergence results for two types of (r, z) implosions are presented. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Numerical analysis of deformed free surface under AC magnetic fields

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2004
Haruhiko Kohno
Abstract A novel numerical scheme for the analysis of large deformation of electrically conducting liquid under alternating current magnetic fields is presented. The main features are characterized by two numerical tools; the level set method to calculate deformed free surface stably and the hybrid finite element method and boundary element method to discretize the electromagnetic field efficiently. Two-dimensional numerical simulation of conducting drop deformation is carried out to demonstrate the effectiveness of the present scheme, and the oscillatory behaviour, which depends on the magnitude of surface tension and Lorentz force, is investigated. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Analysis of the effect of a mean velocity field on a mean field dynamo

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Alejandra Kandus
ABSTRACT We study semi-analytically and in a consistent manner the generation of a mean velocity field by helical magnetohydrodynamical (MHD) turbulence, and the effect that this field can have on a mean field dynamo. Assuming a prescribed, maximally helical small-scale velocity field, we show that large-scale flows can be generated in MHD turbulent flows via small-scale Lorentz force. These flows back-react on the mean electromotive force of a mean field dynamo through new terms, leaving the original , and , terms explicitly unmodified. Cross-helicity plays the key role in interconnecting all the effects. In the minimal , closure that we chose to work with, the effects are stronger for large relaxation times. [source]

Electron transport properties in InAs four-terminal ballistic junctions under weak magnetic fields

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2009
M. Koyama
Abstract We report on the electron transport properties based on ballistic electrons under magnetic fields in four-terminal ballistic junctions fabricated on an InAs/AlGaSb heterostructure. The four-terminal junction structure is composed of two longitudinal stems with two narrow wires slanted with 30 degree from the perpendicular axis. The electron focusing peak was obtained with the bend resistance measurement. Then it was investigated the nonlinear electron transport property of potential difference between longitudinal stems due to ballistic electrons with applying direct current from narrow wires. Observed nonlinearity showed clear rectification effects which have negative polarity regardless of input voltage polarity. Although this nonlinearity was qualitatively changed due to the Lorentz force under magnetic fields, the degradation of ballistic effects on nonlinear properties were observed when the current increased to higher strength. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

A self-consistent treatment of the electromotive force in magnetohydrodynamics for large diffusivities

ASTRONOMISCHE NACHRICHTEN, Issue 7 2010
A. Courvoisier
Abstract The coupled equations that describe the effect of large-scale magnetic and velocity fields on forced high-diffusivity magnetohydrodynamic flows are investigated through an extension of mean field electrodynamics. Our results generalise those of Rädler & Brandenburg (2010), who consider a similar situation but assume that the effect of the Lorentz force on the momentum equation can be neglected. New mean coupling terms are shown to appear, which can lead to large-scale growth of magnetic and velocity fields even when the usual a-effects are absent (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Large-scale magnetic flux concentrations from turbulent stresses

ASTRONOMISCHE NACHRICHTEN, Issue 1 2010
A. Brandenburg
Abstract In this study we provide the first numerical demonstration of the effects of turbulence on the mean Lorentz force and the resulting formation of large-scale magnetic structures. Using three-dimensional direct numerical simulations (DNS) of forced turbulence we show that an imposed mean magnetic field leads to a decrease of the turbulent hydromagnetic pressure and tension. This phenomenon is quantified by determining the relevant functions that relate the sum of the turbulent Reynolds and Maxwell stresses with the Maxwell stress of the mean magnetic field. Using such a parameterization, we show by means of two-dimensional and three-dimensional mean-field numerical modelling that an isentropic density stratified layer becomes unstable in the presence of a uniform imposed magnetic field. This large-scale instability results in the formation of loop-like magnetic structures which are concentrated at the top of the stratified layer. In three dimensions these structures resemble the appearance of bipolar magnetic regions in the Sun. The results of DNS and mean-field numerical modelling are in good agreement with theoretical predictions. We discuss our model in the context of a distributed solar dynamo where active regions and sunspots might be rather shallow phenomena (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]