265 research outputs found
Turbulent diffusion and galactic magnetism
Using the test-field method for nearly irrotational turbulence driven by
spherical expansion waves it is shown that the turbulent magnetic diffusivity
increases with magnetic Reynolds numbers. Its value levels off at several times
the rms velocity of the turbulence multiplied by the typical radius of the
expansion waves. This result is discussed in the context of the galactic
mean-field dynamo.Comment: 2 pages, 1 figure, to appear in "Magnetic Fields in Diffuse Media",
proceedings of Joint Discussion at the 2009 XXVII IAU General Assembly in Rio
de Janeiro from 12 to 14 August, 200
Magnetic helicity fluxes and their effect on stellar dynamos
Magnetic helicity fluxes in turbulently driven alpha^2 dynamos are studied to
demonstrate their ability to alleviate catastrophic quenching. A
one-dimensional mean-field formalism is used to achieve magnetic Reynolds
numbers of the order of 10^5. We study both diffusive magnetic helicity fluxes
through the mid-plane as well as those resulting from the recently proposed
alternate dynamic quenching formalism. By adding shear we make a parameter scan
for the critical values of the shear and forcing parameters for which dynamo
action occurs. For this dynamo we find that the preferred mode
is antisymmetric about the mid-plane. This is also verified in 3-D direct
numerical simulations.Comment: 5 pages, 6 figures, proceedings of IAU Symp. 286, Comparative
Magnetic Minima: characterizing quiet times in the Sun and star
Towards Dark Energy from String-Theory
We discuss vacuum energy in string and M-theory with a focus on heterotic
M-theory. In the latter theory a mechanism is described for maintaining zero
vacuum energy after supersymmetry breaking. Higher-order corrections can be
expected to give a sufficiently small amount of vacuum energy to possibly
account for dark energy.Comment: 20 pages, 1 figure; v2: references adde
A numerical approach for fluid deformable surfaces with conserved enclosed volume
We consider surface finite elements and a semi-implicit time stepping scheme
to simulate fluid deformable surfaces. Such surfaces are modeled by
incompressible surface Navier-Stokes equations with bending forces. We here
consider closed surfaces and enforce conservation of the enclosed volume. The
numerical approach builds on higher order surface parameterizations, a
Taylor-Hood element for the surface Navier-Stokes part, appropriate
approximations of the geometric quantities of the surface and a Lagrange
multiplier for the constraint. The considered computational examples highlight
the solid-fluid duality of fluid deformable surfaces and demonstrate
convergence properties, partly known to be optimal for different sub-problems
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