61 research outputs found
Miscible displacement fronts of shear thinning fluids inside rough fractures
The miscible displacement of a shear-thinning fluid by another of same
rheological properties is studied experimentally in a transparent fracture by
an optical technique imaging relative concentration distributions. The fracture
walls have complementary self-affine geometries and are shifted laterally in
the direction perpendicular to the mean flow velocity {\bf U} : the flow field
is strongly channelized and macro dispersion controls the front structure for
P\'{e}clet numbers above a few units. The global front width increases then
linearly with time and reflects the velocity distribution between the different
channels. In contrast, at the local scale, front spreading is similar to Taylor
dispersion between plane parallel surfaces. Both dispersion mechanisms depend
strongly on the fluid rheology which shifts from Newtonian to shear-thinning
when the flow rate increases. In the latter domain, increasing the
concentration enhances the global front width but reduces both Taylor
dispersion (due to the flattening of the velocity profile in the gap of the
fracture) and the size of medium scale front structures
Characterization of fracture aperture field heterogeneity by electrical resistance measurement
We use electrical resistance measurements to characterize the aperture field in a rough fracture. This is done by performing displacement xperiments using two miscible fluids of different electrical resistivity and monitoring the time variation of the overall fracture resistance. Two fractures have been used: their complementary rough walls are identical but have different relative shear displacements which create âchannelâ or âbarrierâ structures in the aperture field, respectively parallel or perpendicular to the mean flow velocity âU. In the âchannelâ geometry, the resistance displays an initial linear variation followed by a tail part which reïŹects the velocity contrast between slow and fast ïŹow channels. In the âbarrierâ geometry, a change in the slope between two linear zones suggests the existence of domains of different characteristic aperture along the fracture. These variations are well reproduced analytically and numerically using simple ïŹow models. For each geometry, we present then a data inversion procedure that allows one to extract the key features of the heterogeneity from the resistance measurement.Fil: Boschan, Alejandro. Universidad de Buenos Aires. Facultad de IngenierĂa. Departamento de FĂsica. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de IngenierĂa. Departamento de FĂsica. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Chertcoff, Ricardo HĂ©ctor. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad de Buenos Aires. Facultad de IngenierĂa. Departamento de FĂsica. Grupo de Medios Porosos; ArgentinaFil: Hulin, J. P.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Auradou, H.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; Franci
Two-Photon 2s<->1s Transitions during Recombination of Hydrogen in the Universe
Based on the standard cosmological model, we calculate the correction to the
rate of two-photon 2s1s transitions in the hydrogen atom under primordial
hydrogen plasma recombination conditions that arises when the induced
transitions under equilibrium background radiation with a blackbody spectrum
and plasma recombination radiation are taken into account.Comment: 20 pages, 9 figure
CMB constraints on the fine structure constant
We study constraints on time variation of the fine structure constant alpha
from cosmic microwave background (CMB) taking into account simultaneous change
in alpha and the electron mass m_e which might be implied in unification
theories. We obtain the constraints -0.097 < Delta alpha/alpha < 0.034 at 95%
C.L. using WMAP data only, and -0.042 < Delta alpha/alpha < 0.026 combining
with the constraint on the Hubble parameter by the HST Hubble Key Project.
These are improved by 15% compared with constraints assuming only alpha varies.
We discuss other relations between variations in alpha and m_e but we do not
find evidence for varying alpha.Comment: 19 pages, 8 figure
How exactly did the Universe become neutral?
We present a refined treatment of H, He I, and He II recombination in the
early Universe. The difference from previous calculations is that we use
multi-level atoms and evolve the population of each level with redshift by
including all bound-bound and bound-free transitions. In this framework we
follow several hundred atomic energy levels for H, He I, and He II combined.
The main improvements of this method over previous recombination calculations
are: (1) allowing excited atomic level populations to depart from an
equilibrium distribution; (2) replacing the total recombination coefficient
with recombination to and photoionization from each level directly at each
redshift step; and (3) correct treatment of the He I atom, including the
triplet and singlet states. We find that the ionization fraction x_e = n_e/n_H
is approximately 10% smaller at redshifts <~800 than in previous calculations,
due to the non-equilibrium of the excited states of H, which is caused by the
strong but cool radiation field at those redshifts. In addition we find that He
I recombination is delayed compared with previous calculations, and occurs only
just before H recombination. These changes in turn can affect the predicted
power spectrum of microwave anisotropies at the few percent level. Other
improvements such as including molecular and ionic species of H, including
complete heating and cooling terms for the evolution of the matter temperature,
including collisional rates, and including feedback of the secondary spectral
distortions on the radiation field, produce negligible change to x_e. The lower
x_e at low z found in this work affects the abundances of H molecular and ionic
species by 10-25%. However this difference is probably not larger than other
uncertainties in the reaction rates.Comment: 24 pages, including 18 figures, using emulateapj.sty, to appear in
ApJ, the code recfast can be obtained at
http://www.astro.ubc.ca/people/scott/recfast.html (in FORTRAN) and
http://cfa-www.harvard.edu/~sasselov/rec/ (in C
The Adsorption of H2O on TiO2 and SnO2(110) Studied by First-Principles Calculations
First-principles calculations based on density functional theory and the
pseudopotential method have been used to investigate the energetics of HO
adsorption on the (110) surface of TiO and SnO. Full relaxation of all
atomic positions is performed on slab systems with periodic boundary
conditions, and the cases of full and half coverage are studied. Both molecular
and dissociative (HO OH + H) adsorption are treated,
and allowance is made for relaxation of the adsorbed species to unsymmetrical
configurations. It is found that for both TiO and SnO an unsymmetrical
dissociated configuration is the most stable. The symmetrical molecularly
adsorbed configuration is unstable with respect to lowering of symmetry, and is
separated from the fully dissociated configuration by at most a very small
energy barrier. The calculated dissociative adsorption energies for TiO and
SnO are in reasonable agreement with the results of thermal desorption
experiments. Calculated total and local electronic densities of states for
dissociatively and molecularly adsorbed configurations are presented and their
relation with experimental UPS spectra is discussed
Cryopyrin-Associated Periodic Syndrome: An Update on Diagnosis and Treatment Response
Cryopyrin-associated periodic syndrome (CAPS) is a rare hereditary inflammatory disorder encompassing a continuum of three phenotypes: familial cold autoinflammatory syndrome, Muckle-Wells syndrome, and neonatal-onset multisystem inflammatory disease. Distinguishing features include cutaneous, neurological, ophthalmologic, and rheumatologic manifestations. CAPS results from a gain-of-function mutation of the NLRP3 gene coding for cryopyrin, which forms intracellular protein complexes known as inflammasomes. Defects of the inflammasomes lead to overproduction of interleukin-1, resulting in inflammatory symptoms seen in CAPS. Diagnosis is often delayed and requires a thorough review of clinical symptoms. Remarkable advances in our understanding of the genetics and the molecular pathway that is responsible for the clinical phenotype of CAPS has led to the development of effective treatments. It also has become clear that the NLRP3 inflammasome plays a critical role in innate immune defense and therefore has wider implications for other inflammatory disease states
Transient mechanics of foams and emulsions
Systems far from equilibrium have numerous practical uses, but challenge our understanding of their underlying physics. Materials like foams, emulsions, suspensions and granular matter can show liquidlike properties or get trapped in a solidlike jammed state. The phase transition between the flowing and static state is often referred to as the âjamming transitionâ. This work focuses on the mechanical behavior of amorphous viscoelastic materials, close to the jamming point. In many traditional solids, the relation between stress and strain is well described by a linear proportionality, known as Hookeâs law. In jammed solids, by contrast, the stressstrain relation quickly becomes nonlinear, making them much harder to model. Here we ask how and why the linear response breaks. To answer the questions, we investigate the breakdown of linear response as a function of deformation rate and amplitude.Engineering Thermodynamic
Jamming and irreversibility
We investigate irreversibility in soft frictionless disk packings on approach to the unjamming transition. Using simulations of shear reversal tests, we study the relationship between plastic work and irreversible rearrangements of the contact network. Infinitesimal strains are reversible, while any finite strain generates plastic work and contact changes in a sufficiently large packing. The number of irreversible contact changes grows with strain, and the stressâstrain curve displays a crossover from linear to increasingly nonlinear response when the fraction of irreversible contact changes approaches unity.Engineering Thermodynamic
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