Thermally Induced Fluctuations Below the Onset of Rayleigh-B\'enard Convection

We report quantitative experimental results for the intensity of noise-induced fluctuations below the critical temperature difference $\Delta T_c$ for Rayleigh-B\'enard convection. The structure factor of the fluctuating convection rolls is consistent with the expected rotational invariance of the system. In agreement with predictions based on stochastic hydrodynamic equations, the fluctuation intensity is found to be proportional to $1/\sqrt{-\epsilon}$ where $\epsilon \equiv \Delta T / \Delta T_c -1$. The noise power necessary to explain the measurements agrees with the prediction for thermal noise. (WAC95-1)Comment: 13 pages of text and 4 Figures in a tar-compressed and uuencoded file (using uufiles package). Detailed instructions of unpacking are include

Finite size effects near the onset of the oscillatory instability

A system of two complex Ginzburg - Landau equations is considered that applies at the onset of the oscillatory instability in spatial domains whose size is large (but finite) in one direction; the dependent variables are the slowly modulated complex amplitudes of two counterpropagating wavetrains. In order to obtain a well posed problem, four boundary conditions must be imposed at the boundaries. Two of them were already known, and the other two are first derived in this paper. In the generic case when the group velocity is of order unity, the resulting problem has terms that are not of the same order of magnitude. This fact allows us to consider two distinguished limits and to derive two associated (simpler) sub-models, that are briefly discussed. Our results predict quite a rich variety of complex dynamics that is due to both the modulational instability and finite size effects

Influence of the Soret effect on convection of binary fluids

Convection in horizontal layers of binary fluids heated from below and in particular the influence of the Soret effect on the bifurcation properties of extended stationary and traveling patterns that occur for negative Soret coupling is investigated theoretically. The fixed points corresponding to these two convection structures are determined for realistic boundary conditions with a many mode Galerkin scheme for temperature and concentration and an accurate one mode truncation of the velocity field. This solution procedure yields the stable and unstable solutions for all stationary and traveling patterns so that complete phase diagrams for the different convection types in typical binary liquid mixtures can easily be computed. Also the transition from weakly to strongly nonlinear states can be analyzed in detail. An investigation of the concentration current and of the relevance of its constituents shows the way for a simplification of the mode representation of temperature and concentration field as well as for an analytically manageable few mode description.Comment: 30 pages, 12 figure

Equal pay by gender and by nationality: a comparative analysis of Switzerland's unequal equal pay policy regimes across time

What explains the adoption of two different policies on equal pay by gender (EPG) and by nationality (EPN) in Switzerland? And why is the liberal, litigation-based, equal pay policy regime set up by the Gender Equality Act of 1996 much less effective than the neocorporatist ‘accompanying measures' to the Bilateral European Union-Switzerland Agreement on Free Movement of Persons adopted in 1999 to ensure equal pay for workers of different national origins? The formation of two different policy regimes cannot be explained by different levels of political will. Equally, different ‘varieties of capitalism' cannot explain the setup of the two different equal pay policy regimes within the very same country. Instead, our qualitative comparative analysis across time suggests that the differences can be best explained by a particular constellation of attributes, namely the use of different policy frames—i.e. ‘anti-discrimination' in the EPG and ‘unfair competition' in the EPN case—and the different setting of interest politics epitomised by the opposite stances adopted by Switzerland's employer associations in the two case

Diagnostic yield and accuracy of coronary CT angiography after abnormal nuclear myocardial perfusion imaging

We aimed to determine the diagnostic yield and accuracy of coronary CT angiography (CCTA) in patients referred for invasive coronary angiography (ICA) based on clinical concern for coronary artery disease (CAD) and an abnormal nuclear stress myocardial perfusion imaging (MPI) study. We enrolled 100 patients (84 male, mean age 59.6 +/- 8.9 years) with an abnormal MPI study and subsequent referral for ICA. Each patient underwent CCTA prior to ICA. We analyzed the prevalence of potentially obstructive CAD (>= 50% stenosis) on CCTA and calculated the diagnostic accuracy of >= 50% stenosis on CCTA for the detection of clinically significant CAD on ICA (defined as any >= 70% stenosis or >= 50% left main stenosis). On CCTA, 54 patients had at least one >= 50% stenosis. With ICA, 45 patients demonstrated clinically significant CAD. A positive CCTA had 100% sensitivity and 84% specificity with a 100% negative predictive value and 83% positive predictive value for clinically significant CAD on a per patient basis in MPI positive symptomatic patients. In conclusion, almost half (48%) of patients with suspected CAD and an abnormal MPI study demonstrate no obstructive CAD on CCTA

Convection in Binary Fluid Mixtures. II. Localized Traveling Waves. (Physical Review E, in press)

Nonlinear, spatially localized structures of traveling convection rolls are investigated in quantitative detail as a function of Rayleigh number for two different Soret coupling strengths (separation ratios) with Lewis and Prandtl numbers characterizing ethanol-water mixtures. A finite-difference method was used to solve the full hydrodynamic field equations numerically. Structure and dynamics of these localized traveling waves (LTW) are dominated by the concentration field. Like in the spatially extended convective states ( cf. accompanying paper), the Soret-induced concentration variations strongly influence, via density changes, the buoyancy forces that drive convection. The spatio-temporal properties of this feed-back mechanism, involving boundary layers and concentration plumes, show that LTW's are strongly nonlinear states. Light intensity distributions are determined that can be observed in side-view shadowgraphs. Detailed analyses of all fields are made using colour-coded isoplots, among others. In the frame comoving with their drift velocity, LTW's display a nontrivial spatio-temporal symmetry consisting of time-translation by half an oscillation period combined with vertical reflection through the horizontal midplane of the layer. A time-averaged concentration current is driven by a phase difference between the waves of concentration and vertical velocity in the bulk of the LTW state. The associated large-scale concentration redistribution stabilizes the LTW and controls its drift velocity into the quiescent fluid by generating a buoyancy-reducing concentration "barrier" ahead of the leading LTW front. The selection of the width of the LTW's is investigated and comparisons with experiments are presented.Comment: 18 pages and 6 figures as uuencoded Postscript file (using uufiles) 1 color figure as uuencoded Postscript file, a high resolution version of the color figure (about 10MB) can be requested from [email protected] or [email protected].: (Barten)present address: PSI, CH-5232 Villigen PSI, Switzerlan