932 research outputs found
Pattern Formation and Dynamics in Rayleigh-B\'{e}nard Convection: Numerical Simulations of Experimentally Realistic Geometries
Rayleigh-B\'{e}nard convection is studied and quantitative comparisons are
made, where possible, between theory and experiment by performing numerical
simulations of the Boussinesq equations for a variety of experimentally
realistic situations. Rectangular and cylindrical geometries of varying aspect
ratios for experimental boundary conditions, including fins and spatial ramps
in plate separation, are examined with particular attention paid to the role of
the mean flow. A small cylindrical convection layer bounded laterally either by
a rigid wall, fin, or a ramp is investigated and our results suggest that the
mean flow plays an important role in the observed wavenumber. Analytical
results are developed quantifying the mean flow sources, generated by amplitude
gradients, and its effect on the pattern wavenumber for a large-aspect-ratio
cylinder with a ramped boundary. Numerical results are found to agree well with
these analytical predictions. We gain further insight into the role of mean
flow in pattern dynamics by employing a novel method of quenching the mean flow
numerically. Simulations of a spiral defect chaos state where the mean flow is
suddenly quenched is found to remove the time dependence, increase the
wavenumber and make the pattern more angular in nature.Comment: 9 pages, 10 figure
Defect Dynamics for Spiral Chaos in Rayleigh-Benard Convection
A theory of the novel spiral chaos state recently observed in Rayleigh-Benard
convection is proposed in terms of the importance of invasive defects i.e
defects that through their intrinsic dynamics expand to take over the system.
The motion of the spiral defects is shown to be dominated by wave vector
frustration, rather than a rotational motion driven by a vertical vorticity
field. This leads to a continuum of spiral frequencies, and a spiral may rotate
in either sense depending on the wave vector of its local environment. Results
of extensive numerical work on equations modelling the convection system
provide some confirmation of these ideas.Comment: Revtex (15 pages) with 4 encoded Postscript figures appende
Social contact and the perceived impact of social distancing on health outcomes during the COVID-19 pandemic among community dwelling older adults taking part in the OPAL cohort study
Background: During the COVID-19 pandemic, social distancing and reduced social contact may have affected older adults’ health. Objectives: To evaluate the perceived impact of social distancing on older adults’ health and explore the association between social contact and health outcomes. Design: Cross-sectional and longitudinal analyses of the OPAL cohort study. Subjects: Community dwelling older adults. Methods: We sent questionnaires to participants of an existing cohort study (n = 4328). Questions included the amount and type of social contact, and how often they went outside. Participants rated the impact of social distancing on their health. Sociodemographic factors and quality of life were available from previous questionnaires. We examined quality of life prior to and during the pandemic and explored the cross-sectional relationship between social contact and health using logistic regression. Results: There were 3856/4328 (89%) questionnaires returned. EQ-5D scores changed little compared to pre-pandemic scores but 25% of participants reported their overall health had worsened. The telephone was the most used method of contact (78%). Video calls were used least with 35% of participants not using them or having no access to them. 13% of respondents never went outside. Lower levels of contact were associated with increased risk of reporting worse health (Odds ratio (OR) 1.04 (95% CI 1.01–1.08)). Those experiencing financial strain and who spent less time outside experienced the largest increase in risk of reporting perceived worsened overall health. Those reporting a strain to get by financially were 4 times more likely to report worsened health than those who described themselves as quite comfortably off (OR 4.00 (95% CI 1.86–8.16)). Participants who reported never going outside were twice as likely to report worsened health compared to those who went outside daily (OR 2.00 (95% CI 1.57–2.54)). Conclusions: Less contact with other people was associated with perceived worsening in overall health. Although many older people reported using online technology, such as video calls, a substantial proportion were not using them. Older people facing financial strain were more likely to report worsened health, highlighting the impact of social inequalities during the pandemic. Going outside less was also associated with perceived worsened health
Mean flow and spiral defect chaos in Rayleigh-Benard convection
We describe a numerical procedure to construct a modified velocity field that
does not have any mean flow. Using this procedure, we present two results.
Firstly, we show that, in the absence of mean flow, spiral defect chaos
collapses to a stationary pattern comprising textures of stripes with angular
bends. The quenched patterns are characterized by mean wavenumbers that
approach those uniquely selected by focus-type singularities, which, in the
absence of mean flow, lie at the zig-zag instability boundary. The quenched
patterns also have larger correlation lengths and are comprised of rolls with
less curvature. Secondly, we describe how mean flow can contribute to the
commonly observed phenomenon of rolls terminating perpendicularly into lateral
walls. We show that, in the absence of mean flow, rolls begin to terminate into
lateral walls at an oblique angle. This obliqueness increases with Rayleigh
number.Comment: 14 pages, 19 figure
Cross-Newell equations for hexagons and triangles
The Cross-Newell equations for hexagons and triangles are derived for general
real gradient systems, and are found to be in flux-divergence form. Specific
examples of complex governing equations that give rise to hexagons and
triangles and which have Lyapunov functionals are also considered, and explicit
forms of the Cross-Newell equations are found in these cases. The general
nongradient case is also discussed; in contrast with the gradient case, the
equations are not flux-divergent. In all cases, the phase stability boundaries
and modes of instability for general distorted hexagons and triangles can be
recovered from the Cross-Newell equations.Comment: 24 pages, 1 figur
Induced defect nucleation and side-band instabilities in hexagons with rotation and mean flow
The combined effect of mean flow and rotation on hexagonal patterns is
investigated using Ginzburg-Landau equations that include nonlinear gradient
terms as well as the nonlocal coupling provided by the mean flow. Long-wave and
short-wave side-band instabilities are determined. Due to the nonlinear
gradient terms and enhanced by the mean flow, the penta-hepta defects can
become unstable to the induced nucleation of dislocations in the defect-free
amplitude, which can lead to the proliferation of penta-hepta defects and
persistent spatio-temporal chaos. For individual penta-hepta defects the
nonlinear gradient terms enhance climbing or gliding motion, depending on
whether they break the chiral symmetry or not.Comment: 24 pages, 15 figures, submitted to Physica
Mean flow in hexagonal convection: stability and nonlinear dynamics
Weakly nonlinear hexagon convection patterns coupled to mean flow are
investigated within the framework of coupled Ginzburg-Landau equations. The
equations are in particular relevant for non-Boussinesq Rayleigh-B\'enard
convection at low Prandtl numbers. The mean flow is found to (1) affect only
one of the two long-wave phase modes of the hexagons and (2) suppress the
mixing between the two phase modes. As a consequence, for small Prandtl numbers
the transverse and the longitudinal phase instability occur in sufficiently
distinct parameter regimes that they can be studied separately. Through the
formation of penta-hepta defects, they lead to different types of transient
disordered states. The results for the dynamics of the penta-hepta defects shed
light on the persistence of grain boundaries in such disordered states.Comment: 33 pages, 20 figures. For better
figures:http://astro.uchicago.edu/~young/hexmeandi
Access to HIV care in the context of universal test and treat: challenges within the ANRS 12249 TasP cluster-randomized trial in rural South Africa
Introduction: We aimed to quantify and identify associated factors of linkage to HIV care following home-based HIV counselling and testing (HBHCT) in the ongoing ANRS 12249 treatment-as-prevention (TasP) cluster-randomized trial in rural KwaZulu-Natal, South Africa.
Methods: Individuals ]16 years were offered HBHCT; those who were identified HIV positive were referred to cluster-based TasP clinics and offered antiretroviral treatment (ART) immediately (five clusters) or according to national guidelines (five clusters). HIV care was also available in the local Department of Health (DoH) clinics. Linkage to HIV care was defined as TasP or DoH clinic attendance within three months of referral among adults not in HIV care at referral. Associated factors were identified using multivariable logistic regression adjusted for trial arm.
Results: Overall, 1323 HIV-positive adults (72.9% women) not in HIV care at referral were included, of whom 36.9% (n488) linked to care B3 months of referral (similar by sex). In adjusted analyses (n1222), individuals who had never been in HIV care before referral were significantly less likely to link to care than those who had previously been in care (B33% vs. 42%, pB0.001). Linkage to care was lower in students (adjusted odds-ratio [aOR] 0.47; 95% confidence interval [CI] 0.240.92) than in employed adults, in adults who completed secondary school (aOR0.68; CI 0.490.96) or at least some secondary school (aOR0.59; CI 0.410.84) versus 5 primary school, in those who lived at 1 to 2 km (aOR0.58; CI 0.440.78) or 25 km from the nearest TasP clinic (aOR0.57; CI 0.410.77) versus B1 km, and in those who were referred to clinic after ]2 contacts (aOR0.75; CI 0.580.97) versus those referred at the first contact. Linkage to care was higher in adults who reported knowing an HIV-positive family member (aOR1.45; CI 1.121.86) versus not, and in those who said that they would take ART as soon as possible if they were diagnosed HIV positive (aOR2.16; CI 1.134.10) versus not.
Conclusions: Fewer than 40% of HIV-positive adults not in care at referral were linked to HIV care within three months of HBHCT in the TasP trial. Achieving universal test and treat coverage will require innovative interventions to support linkage to HIV care
Bifurcations in annular electroconvection with an imposed shear
We report an experimental study of the primary bifurcation in
electrically-driven convection in a freely suspended film. A weakly conducting,
submicron thick smectic liquid crystal film was supported by concentric
circular electrodes. It electroconvected when a sufficiently large voltage
was applied between its inner and outer edges. The film could sustain rapid
flows and yet remain strictly two-dimensional. By rotation of the inner
electrode, a circular Couette shear could be independently imposed. The control
parameters were a dimensionless number , analogous to the Rayleigh
number, which is and the Reynolds number of the
azimuthal shear flow. The geometrical and material properties of the film were
characterized by the radius ratio , and a Prandtl-like number . Using measurements of current-voltage characteristics of a large number of
films, we examined the onset of electroconvection over a broad range of
, and . We compared this data quantitatively to
the results of linear stability theory. This could be done with essentially no
adjustable parameters. The current-voltage data above onset were then used to
infer the amplitude of electroconvection in the weakly nonlinear regime by
fitting them to a steady-state amplitude equation of the Landau form. We show
how the primary bifurcation can be tuned between supercritical and subcritical
by changing and .Comment: 17 pages, 12 figures. Submitted to Phys. Rev. E. Minor changes after
refereeing. See also http://mobydick.physics.utoronto.c
Identity ambiguity and the promises and practices of hybrid e-HRM project teams
The role of IS project team identity work in the enactment of day-to-day relationships with their internal clients is under-researched. We address this gap by examining the identity work undertaken by an electronic human resource management (e-HRM) 'hybrid' project team engaged in an enterprise-wide IS implementation for their multi-national organisation. Utilising social identity theory, we identify three distinctive, interrelated dimensions of project team identity work (project team management, team 'value propositions' (promises) and the team's 'knowledge practice'). We reveal how dissonance between two perspectives of e-HRM project identity work (clients' expected norms of project team's service and project team's expected norms of themselves) results in identity ambiguity. Our research contributions are to identity studies in the IS project management, HR and hybrid literatures and to managerial practice by challenging the assumption that hybrid experts are the panacea for problems associated with IS projects
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