105 research outputs found
Strong Coupling Correction in Superfluid He in Aerogel
Effects of impurity scatterings on the strong coupling (SC) contribution,
stabilizing the ABM (axial) pairing state, to the quartic term of the
Ginzburg-Landau (GL) free energy of superfluid He are theoretically studied
to examine recent observations suggestive of an anomalously small SC effect in
superfluid He in aerogels. To study the SC corrections, two approaches are
used. One is based on a perturbation in the short-range repulsive interaction,
and the other is a phenomenological approach used previously for the bulk
liquid by Sauls and Serene [Phys.Rev.B 24, 183 (1981)]. It is found that the
impurity scattering favors the BW pairing state and shrinks the region of the
ABM pairing state in the T-P phase diagram. In the phenomenological approach,
the resulting shrinkage of the ABM region is especially substantial and, if
assuming an anisotropy over a large scale in aerogel, leads to justifying the
phase diagrams determined experimentally.Comment: 19 pages, 9 figures, Accepted for publication in Phys. Rev.
Effects of viscous dissipation and boundary conditions on forced convection in a channel occupied by a saturated porous medium
Forced convection with viscous dissipation in a parallel plate channel filled by a saturated porous medium is investigated numerically. Three different viscous dissipation models are examined. Two different sets of wall conditions are considered: isothermal and isoflux. Analytical expressions are also presented for the asymptotic temperature profile and the asymptotic Nusselt number. With isothermal walls, the Brinkman number significantly influences the developing Nusselt number but not the asymptotic one. At constant wall heat flux, both the developing and the asymptotic Nusselt numbers are affected by the value of the Brinkman number. The Nusselt number is sensitive to the porous medium shape factor under all conditions considered
Effects of viscous dissipation and boundary conditions on forced convection in a channel occupied by a saturated porous medium
Forced convection with viscous dissipation in a parallel plate channel filled by a saturated porous medium is investigated numerically. Three different viscous dissipation models are examined. Two different sets of wall conditions are considered: isothermal and isoflux. Analytical expressions are also presented for the asymptotic temperature profile and the asymptotic Nusselt number. With isothermal walls, the Brinkman number significantly influences the developing Nusselt number but not the asymptotic one. At constant wall heat flux, both the developing and the asymptotic Nusselt numbers are affected by the value of the Brinkman number. The Nusselt number is sensitive to the porous medium shape factor under all conditions considered
On the stability and uniqueness of the flow of a fluid through a porous medium
© 2016, The Author(s). In this short note, we study the stability of flows of a fluid through porous media that satisfies a generalization of Brinkman’s equation to include inertial effects. Such flows could have relevance to enhanced oil recovery and also to the flow of dense liquids through porous media. In any event, one cannot ignore the fact that flows through porous media are inherently unsteady, and thus, at least a part of the inertial term needs to be retained in many situations. We study the stability of the rest state and find it to be asymptotically stable. Next, we study the stability of a base flow and find that the flow is asymptotically stable, provided the base flow is sufficiently slow. Finally, we establish results concerning the uniqueness of the flow under appropriate conditions, and present some corresponding numerical results
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
Statistical Mechanics of Nonuniform Magnetization Reversal
The magnetization reversal rate via thermal creation of soliton pairs in
quasi-1D ferromagnetic systems is calculated. Such a model describes e.g. the
time dependent coercivity of elongated particles as used in magnetic recording
media. The energy barrier that has to be overcome by thermal fluctuations
corresponds to a soliton-antisoliton pair whose size depends on the external
field. In contrast to other models of first order phase transitions such as the
phi^4 model, an analytical expression for this energy barrier is found for all
values of the external field. The magnetization reversal rate is calculated
using a functional Fokker-Planck description of the stochastic magnetization
dynamics. Analytical results are obtained in the limits of small fields and
fields close to the anisotropy field. In the former case the hard-axis
anisotropy becomes effectively strong and the magnetization reversal rate is
shown to reduce to the nucleation rate of soliton-antisoliton pairs in the
overdamped double sine-Gordon model. The present theory therefore includes the
nucleation rate of soliton-antisoliton pairs in the double sine-Gordon chain as
a special case. These results demonstrate that for elongated particles, the
experimentally observed coercivity is significantly lower than the value
predicted by the standard theories of N\'eel and Brown.Comment: 21 pages RevTex 3.0 (twocolumn), 6 figures available on request, to
appear in Phys Rev B, Dec (1994
Non-AIDS defining cancers in the D:A:D Study-time trends and predictors of survival : a cohort study
BACKGROUND:Non-AIDS defining cancers (NADC) are an important cause of morbidity and mortality in HIV-positive individuals. Using data from a large international cohort of HIV-positive individuals, we described the incidence of NADC from 2004-2010, and described subsequent mortality and predictors of these.METHODS:Individuals were followed from 1st January 2004/enrolment in study, until the earliest of a new NADC, 1st February 2010, death or six months after the patient's last visit. Incidence rates were estimated for each year of follow-up, overall and stratified by gender, age and mode of HIV acquisition. Cumulative risk of mortality following NADC diagnosis was summarised using Kaplan-Meier methods, with follow-up for these analyses from the date of NADC diagnosis until the patient's death, 1st February 2010 or 6 months after the patient's last visit. Factors associated with mortality following NADC diagnosis were identified using multivariable Cox proportional hazards regression.RESULTS:Over 176,775 person-years (PY), 880 (2.1%) patients developed a new NADC (incidence: 4.98/1000PY [95% confidence interval 4.65, 5.31]). Over a third of these patients (327, 37.2%) had died by 1st February 2010. Time trends for lung cancer, anal cancer and Hodgkin's lymphoma were broadly consistent. Kaplan-Meier cumulative mortality estimates at 1, 3 and 5 years after NADC diagnosis were 28.2% [95% CI 25.1-31.2], 42.0% [38.2-45.8] and 47.3% [42.4-52.2], respectively. Significant predictors of poorer survival after diagnosis of NADC were lung cancer (compared to other cancer types), male gender, non-white ethnicity, and smoking status. Later year of diagnosis and higher CD4 count at NADC diagnosis were associated with improved survival. The incidence of NADC remained stable over the period 2004-2010 in this large observational cohort.CONCLUSIONS:The prognosis after diagnosis of NADC, in particular lung cancer and disseminated cancer, is poor but has improved somewhat over time. Modifiable risk factors, such as smoking and low CD4 counts, were associated with mortality following a diagnosis of NADC
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