41 research outputs found
Transport in rough self-affine fractures
Transport properties of three-dimensional self-affine rough fractures are
studied by means of an effective-medium analysis and numerical simulations
using the Lattice-Boltzmann method. The numerical results show that the
effective-medium approximation predicts the right scaling behavior of the
permeability and of the velocity fluctuations, in terms of the aperture of the
fracture, the roughness exponent and the characteristic length of the fracture
surfaces, in the limit of small separation between surfaces. The permeability
of the fractures is also investigated as a function of the normal and lateral
relative displacements between surfaces, and is shown that it can be bounded by
the permeability of two-dimensional fractures. The development of channel-like
structures in the velocity field is also numerically investigated for different
relative displacements between surfaces. Finally, the dispersion of tracer
particles in the velocity field of the fractures is investigated by analytic
and numerical methods. The asymptotic dominant role of the geometric
dispersion, due to velocity fluctuations and their spatial correlations, is
shown in the limit of very small separation between fracture surfaces.Comment: submitted to PR
The Theory of Brown Dwarfs and Extrasolar Giant Planets
Straddling the traditional realms of the planets and the stars, objects below
the edge of the main sequence have such unique properties, and are being
discovered in such quantities, that one can rightly claim that a new field at
the interface of planetary science and and astronomy is being born. In this
review, we explore the essential elements of the theory of brown dwarfs and
giant planets, as well as of the new spectroscopic classes L and T. To this
end, we describe their evolution, spectra, atmospheric compositions, chemistry,
physics, and nuclear phases and explain the basic systematics of
substellar-mass objects across three orders of magnitude in both mass and age
and a factor of 30 in effective temperature. Moreover, we discuss the
distinctive features of those extrasolar giant planets that are irradiated by a
central primary, in particular their reflection spectra, albedos, and transits.
Aspects of the latest theory of Jupiter and Saturn are also presented.
Throughout, we highlight the effects of condensates, clouds, molecular
abundances, and molecular/atomic opacities in brown dwarf and giant planet
atmospheres and summarize the resulting spectral diagnostics. Where possible,
the theory is put in its current observational context.Comment: 67 pages (including 36 figures), RMP RevTeX LaTeX, accepted for
publication in the Reviews of Modern Physics. 30 figures are color. Most of
the figures are in GIF format to reduce the overall size. The full version
with figures can also be found at:
http://jupiter.as.arizona.edu/~burrows/papers/rm
High Blocking Temperature of Magnetization and Giant Coercivity in the Azafullerene Tb 2 @C 79 N with a Single-Electron TerbiumâTerbium Bond
The azafullerene Tb 2 @C 79 N is found to be a single-molecule magnet with a high 100-s blocking temperature of magnetization of 24 K and large coercivity. Tb magnetic moments with an easy-axis single-ion magnetic anisotropy are strongly coupled by the unpaired spin of the single-electron TbâTb bond. Relaxation of magnetization in Tb 2 @C 79 N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time Ï QTM =16 462±1230 s. At higher temperature, relaxation follows the Orbach mechanism with a barrier of 757±4 K, corresponding to the excited states, in which one of the Tb spins is flipped. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA11sci