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 $V$ 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 ${\cal R}$, analogous to the Rayleigh number, which is $\propto V^2$ and the Reynolds number ${\cal R}e$ of the azimuthal shear flow. The geometrical and material properties of the film were characterized by the radius ratio $\alpha$, and a Prandtl-like number ${\cal P}$. Using measurements of current-voltage characteristics of a large number of films, we examined the onset of electroconvection over a broad range of $\alpha$, ${\cal P}$ and ${\cal R}e$. 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 $\alpha$ and ${\cal R}e$.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