449 research outputs found
Fully compressible simulation of low-speed premixed reacting flows
Low speed premixed combustion flows in industrial applications are generally simulated using the "incompressible" Navier-Stokes algorithms, which belong to the family of fractional step methods, or segregated methods. The approximations used for the combustion modelling in the framework of the segregated mathematical formulation, often represent important limitations for applying the combustion numerical simulation to a wider class of problems of engineering interest. Recent developments of preconditioning techniques allow to apply the same complete system of Navier-Stokes equations to a wide variety of fluid flow problems characterized by the whole range of Reynolds, Mach, Grashof, Prandtl and Damkoeler numbers. The present work describes the development of a fully "compressible" mathematical model for the simulation of low-speed turbulent premixed reactive flows. Issues on flow and fluid compressibility as well as on the two mathematical alternative formulations, are discussed. Also discussed are issues related to coupling the flamelet premixed combustion model (based on the solution of a transport equation for the progress variable) with one-equation turbulence models, instead of the classical two-equation K – ε model. In this work the model by Spalart & Allmaras is used. The several advantages brought about by the use of the fully compressible formulation are discussed based on the results obtained on a test case taken from literature
A Singular Perturbation Analysis for \\Unstable Systems with Convective Nonlinearity
We use a singular perturbation method to study the interface dynamics of a
non-conserved order parameter (NCOP) system, of the reaction-diffusion type,
for the case where an external bias field or convection is present. We find
that this method, developed by Kawasaki, Yalabik and Gunton for the
time-dependant Ginzburg-Landau equation and used successfully on other NCOP
systems, breaks down for our system when the strength of bias/convection gets
large enough.Comment: 5 pages, PostScript forma
On the positive mass theorem for manifolds with corners
We study the positive mass theorem for certain non-smooth metrics following
P. Miao's work. Our approach is to smooth the metric using the Ricci flow. As
well as improving some previous results on the behaviour of the ADM mass under
the Ricci flow, we extend the analysis of the zero mass case to higher
dimensions.Comment: 21 pages, incorporated referee's comment
A truncated ultrasound screening procedure for atheroma of the cervical arteries in asymptomatic diabetic patients: Evidence from a retrospective study
AimTo demonstrate that ultrasound screening of diabetic patients presenting with no cerebrovascular symptoms for evaluation of atheroma of the cervical arteries can be limited to the carotid arteries. Methods We retrospectively analyzed the results of cervical artery ultrasound imaging of diabetic patients with no cerebrovascular symptoms. This diabetic population was divided into two subpopulations according to whether or not the vertebral and subclavian artery findings were normal or abnormal. Results Of the 760 patients who fulfilled the criteria for study inclusion, the ultrasound imaging findings of the vertebral and subclavian arteries were normal in 712 cases. Review of the files of the 48 remaining patients showed that findings for either the vertebral or subclavian arteries did not lead to any changes in patient management because of associated risk factors, carotid atheroma or peripheral arterial disease. Conclusion A vascular risk evaluation in diabetic patients could include ultrasound imaging assessment for cervical artery atheroma and our data suggest that such an evaluation could be focused solely on the carotid arteries
Random Walks in Logarithmic and Power-Law Potentials, Nonuniversal Persistence, and Vortex Dynamics in the Two-Dimensional XY Model
The Langevin equation for a particle (`random walker') moving in
d-dimensional space under an attractive central force, and driven by a Gaussian
white noise, is considered for the case of a power-law force, F(r) = -
Ar^{-sigma}. The `persistence probability', P_0(t), that the particle has not
visited the origin up to time t, is calculated. For sigma > 1, the force is
asymptotically irrelevant (with respect to the noise), and the asymptotics of
P_0(t) are those of a free random walker. For sigma < 1, the noise is
(dangerously) irrelevant and the asymptotics of P_0(t) can be extracted from a
weak noise limit within a path-integral formalism. For the case sigma=1,
corresponding to a logarithmic potential, the noise is exactly marginal. In
this case, P_0(t) decays as a power-law, P_0(t) \sim t^{-theta}, with an
exponent theta that depends continuously on the ratio of the strength of the
potential to the strength of the noise. This case, with d=2, is relevant to the
annihilation dynamics of a vortex-antivortex pair in the two-dimensional XY
model. Although the noise is multiplicative in the latter case, the relevant
Langevin equation can be transformed to the standard form discussed in the
first part of the paper. The mean annihilation time for a pair initially
separated by r is given by t(r) \sim r^2 ln(r/a) where a is a microscopic
cut-off (the vortex core size). Implications for the nonequilibrium critical
dynamics of the system are discussed and compared to numerical simulation
results.Comment: 10 pages, 1 figur
Application of the quantum spin glass theory to image restoration
Quantum fluctuation is introduced into the Markov random fields (MRF's) model
for image restoration in the context of Bayesian approach. We investigate the
dependence of the quantum fluctuation on the quality of BW image restoration by
making use of statistical mechanics. We find that the maximum posterior
marginal (MPM) estimate based on the quantum fluctuation gives a fine
restoration in comparison with the maximum a posterior (MAP) estimate or the
thermal fluctuation based MPM estimate.Comment: 19 pages, 9 figures, 1 table, RevTe
Dynamical Scaling: the Two-Dimensional XY Model Following a Quench
To sensitively test scaling in the 2D XY model quenched from
high-temperatures into the ordered phase, we study the difference between
measured correlations and the (scaling) results of a Gaussian-closure
approximation. We also directly compare various length-scales. All of our
results are consistent with dynamical scaling and an asymptotic growth law , though with a time-scale that depends on the
length-scale in question. We then reconstruct correlations from the
minimal-energy configuration consistent with the vortex positions, and find
them significantly different from the ``natural'' correlations --- though both
scale with . This indicates that both topological (vortex) and
non-topological (``spin-wave'') contributions to correlations are relevant
arbitrarily late after the quench. We also present a consistent definition of
dynamical scaling applicable more generally, and emphasize how to generalize
our approach to other quenched systems where dynamical scaling is in question.
Our approach directly applies to planar liquid-crystal systems.Comment: 10 pages, 10 figure
Studying nonlinear effects on the early stage of phase ordering using a decomposition method
Nonlinear effects on the early stage of phase ordering are studied using
Adomian's decomposition method for the Ginzburg-Landau equation for a
nonconserved order parameter. While the long-time regime and the linear
behavior at short times of the theory are well understood, the onset of
nonlinearities at short times and the breaking of the linear theory at
different length scales are less understood. In the Adomian's decomposition
method, the solution is systematically calculated in the form of a polynomial
expansion for the order parameter, with a time dependence given as a series
expansion. The method is very accurate for short times, which allows to
incorporate the short-time dynamics of the nonlinear terms in a analytical and
controllable way.Comment: 11 pages, 1 figure, to appear in Phys Lett
Local variation in endoparasite intensities of bank voles (Clethrionomys glareolus )from ecologically similar sites: morphometric and endocrine correlates
Much interest has centred recently on the role of adaptive trade-offs between the immune system and other components of life history in determining resistance and parasite intensities among hosts. Steroid hormones, particularly glucocorticoids and sex steroids, provide a plausible mechanism for mediating such trade-offs. A basic assumption behind the hypothesis, however, is that steroid activity will generally correlate with reduced resistance and thus greater parasite intensities. Here, we present some findings from a field study of bank voles (Clethrionomys glareolus ) in which we have looked at associations between parasite intensities, anatomical and morphometric measures relating to endocrine function and life history variation in three local populations inhabiting similar but mutually isolated woodland habitats. In general, sites with greater parasite intensities were those in which male C. glareolus had significantly larger adrenal glands, testes and seminal vesicles for their age and body size. Females also showed a site difference in adrenal gland weight. Some aspects of site-related parasite intensity were associated with asymmetry in adrenal gland weight and hind foot length, which may have reflected developmental effects on glucocorticoid activity
Variation in the helminth community structure in bank voles (Clethrionomys glareolus) from three comparable localities in the Mazury Lake District region of Poland
We tested the null hypothesis that populations of hosts trapped in isolated neighbouring locations showing comparable habitat quality, should support similar helminth parasite communities. The study was undertaken in a 2-week period in late summer in NE Poland in a single year, thereby eliminating seasonal and between-year variation in parasite burdens. A total of 139 Clethrionomys glareolus (bank vole) were sampled from 3 forest sites of similar habitat quality. Total species richness was 11 (6 nematodes and 5 cestodes) with 85±6% of the voles carrying at least 1 species and an overall mean species richness of 1±4. At the component community level, the fewest species of helminths were recorded from site 2 (n=6, compared with 9 at each of the other sites), but site 3 had the lowest Berger-Parker Dominance Index and the highest Simpson's Index of Diversity. At the infracommunity level, site 3 had the highest mean no. of helminthspecies}vole, the highest mean Brillouin's Index of Diversity but the lowest mean no. of helminths/vole. Voles from sites 1 and 3 differed in the nematodes that were most common (site 1, Heligmosomum mixtum ± 95%; site 3, Heligmosomoides glareoli ± 79±3%). At site 2 no species exceeded 50% but prevalence of Syphacia petrusewiczi was higher than at the other sites. The prevalence of cestodes was too low to test reliably (12±9%), but the highest prevalence of adult cestodes was recorded at site 1 (22±5%compared with 4±9 and 1±7%for sites 2 and 3 respectively). Host sex did not ifluence infection, but mean species richness increased with age. The different sites were responsible for most of the variation in our data, and the intrinsic factors (sex and age) were less important in shaping the component community structure of helminths. We conclude that even locations in relative close proximity to one another (13±25 km), selected on the basis of similar habitat quality, have rodent populations that differ in their helminth parasite communities, although for reasons other than the factors quantified in the present study
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