6,528 research outputs found
Some are more equal than others : the role of ‘keystone’ species in the degradation of recalcitrant substrates
Peer reviewedPublisher PD
Internally heated porous convection: an idealised model for Enceladus' hydrothermal activity
Recent planetary data and geophysical modelling suggest that hydrothermal
activity is ongoing under the ice crust of Enceladus, one of Saturn's moons.
According to these models, hydrothermal flow in the porous, rocky core of the
satellite is driven by tidal deformation that induces dissipation and
volumetric internal heating. Despite the effort in the modelling of Enceladus'
interior, systematic understanding---and even basic scaling laws---of
internally-heated porous convection and hydrothermal activity are still
lacking. In this article, using an idealised model of an internally-heated
porous medium, we explore numerically and theoretically the flows that develop
close and far from the onset of convection. In particular, we quantify
heat-transport efficiency by convective flows as well as the typical extent and
intensity of heat-flux anomalies created at the top of the porous layer. With
our idealised model, we derive simple and general laws governing the
temperature and hydrothermal velocity that can be driven in the oceans of icy
moons. In the future, these laws could help better constraining models of the
interior of Enceladus and other icy satellites.Comment: 23 pages, 13 figure
High-Rayleigh-number convection in porous-fluid layers
We present a numerical study of convection in a horizontal layer comprising a
fluid-saturated porous bed overlain by an unconfined fluid layer. Convection is
driven by a vertical, destabilising temperature difference applied across the
whole system, as in the canonical Rayleigh-B\'enard problem. Numerical
simulations are carried out using a single-domain formulation of the two-layer
problem based on the Darcy-Brinkman equations. We explore the dynamics and heat
flux through the system in the limit of large Rayleigh number, but small Darcy
number, such that the flow exhibits vigorous convection in both the porous and
the unconfined fluid regions, while the porous flow still remains strongly
confined and governed by Darcy's law. We demonstrate that the heat flux and
average thermal structure of the system can be predicted using previous results
of convection in individual fluid or porous layers. We revisit a controversy
about the role of subcritical "penetrative convection" in the porous medium,
and confirm that such induced flow does not contribute to the heat flux through
the system. Lastly, we briefly study the temporal coupling between the two
layers and find that the turbulent fluid convection above acts as a low-pass
filter on the longer-timescale variability of convection in the porous layer.Comment: Accepted for publication in Journal of Fluid Mechanics, 25 pages, 13
figure
Relative hyperbolicity and similar properties of one-generator one-relator relative presentations with powered unimodular relator
A group obtained from a nontrivial group by adding one generator and one
relator which is a proper power of a word in which the exponent-sum of the
additional generator is one contains the free square of the initial group and
almost always (with one obvious exception) contains a non-abelian free
subgroup. If the initial group is involution-free or the relator is at least
third power, then the obtained group is SQ-universal and relatively hyperbolic
with respect to the initial group.Comment: 11 pages. A Russian version of this paper is at
http://mech.math.msu.su/department/algebra/staff/klyachko/papers.htm V3:
revised following referee's comment
Slope of the Isgur-Wise function in the heavy mass limit of quark models \`a la Bakamjian-Thomas
The slope of the Isgur-Wise function for ground state mesons is evaluated for
the heavy mass limit of quark models \`a la Bakamjian-Thomas, which has been
previously discussed by us in general terms. A full calculation in various
spectroscopic models with relativistic kinetic energy gives a rather stable
result , much lower than previous estimates. Attention is
paid to a careful comparison of this result with the ones of QCD fundamental
methods (lattice QCD, QCD sum rules) and with experimental data.Comment: 15 pages, Latex, AMS-LaTe
Clustering Algorithm in Vehicular Ad-hoc Networks: A Brief Summary
An Intelligent Transportation System (ITS) application requires vehicles to be connected to each other and to roadside units to share information, thus reducing fatalities and improving traffic congestion. Vehicular Ad hoc Networks (VANETs) is one of the main forms of network designed for ITS in which information is broadcasted amongst vehicular nodes. However, the broadcast reliability in VANETs face a number of challenges - dynamic routing being one of the major issues. Clustering, a technique used to group nodes based on certain criteria, has been suggested as a solution to this problem. This paper gives a summary of the core criteria of some of the clustering algorithms issues along with a performance comparison and a development evolution roadmap, in an attempt to understand and differentiate different aspects of the current research and suggest future research insights
Endovascular treatment of intractable epistaxis — results of a 4-year local audit
Objective. Transcatheter embolisation is an accepted and effective treatment for intractable epistaxis. We analysed our success and complication rates and compared these with results from other published series.
Design. Retrospective review.
Setting. Unitas Interventional Unit, Centurion.
Methods. Case record review (57 procedures) and telephonic interviews (36 traceable respondents). Outcome measures. A numerical audit of the success and complication rates for embolisation procedures performed during the 4-year period between July 1999 and June 2003.
Results. A total of 57 endovascular embolisation procedures were performed for intractable epistaxis in 51 patients during this period. Eight patients (15.7%) developed a re-bleed between 1 and 33 days after embolisation, of whom 5 were reembolised, giving a primary short-term success rate of 86.3% and secondary assisted success rate of 94.1%. Thirty-five of 36 respondents (97.2%) reported no further epistaxis during the long-term follow-up period of 1 - 47 months. The mortality rate was 0%, the major morbidity rate was 2% (1 stroke) and the minor morbidity rate was 25%.
Conclusion. Our success and complication rates are acceptable and compare favourably with those reported in other large series.
S Afr Med J 2004; 94: 373-378
Smearing of charge fluctuations in a grain by spin-flip assisted tunneling
We investigate the charge fluctuations of a grain (large dot) coupled to a
lead via a small quantum dot in the Kondo regime. We show that the strong
entanglement of charge and spin flips in this setup can result in a stable
SU(4) Kondo fixed point, which considerably smears out the Coulomb staircase
behavior already in the weak tunneling limit. This behavior is robust enough to
be experimentally observable.Comment: 4 pages, 1 figure, final version for PRB Rapid Com
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