270 research outputs found
Experimental evidence of symmetry-breaking supercritical transition in pipe flow of shear-thinning fluids
Experimental results reveal that the asymmetric flow of shear-thinning fluid through a cylindrical pipe, which was previously associated with the laminar-turbulent transition process, appears to have the characteristics of a nonhysteretic, supercritical instability of the laminar base state. Contrary to what was previously believed, classical transition is found to be responsible for returning symmetry to the flow. An absence of evidence of the instability in simulations (either linear or nonlinear) suggests that an element of physics is lacking in the commonly used rheological model for inelastic shear-thinning fluids. These unexpected discoveries raise new questions regarding the stability of these practically important fluids and how they can be successfully modeled
Nature of Correlated Motion of Electrons in the Parent Cobaltate Superconductors
Recently discovered class of cobaltate superconductors (Na0.3CoO2.nH2O) is a
novel realization of interacting quantum electron systems in a triangular
network with low-energy degrees of freedom. We employ angle-resolved
photoemission spectroscopy to uncover the nature of microscopic electron motion
in the parent superconductors for the first time. Results reveal a large
hole-like Fermi surface (consistent with Luttinger theorem) generated by the
crossing of super-heavy quasiparticles. The measured quasiparticle parameters
collectively suggest a two orders of magnitude departure from the conventional
Bardeen-Cooper-Schrieffer electron dynamics paradigm and unveils cobaltates as
a rather hidden class of relatively high temperature superconductors.Comment: 5 pages, 4 figures, 1 tabl
Classifying medical histories in US medicare beneficiaries using fixed vs all‐available look‐back approaches
Purpose: Evaluate use of fixed and all‐available look‐backs to identify eligibility criteria and confounders among Medicare beneficiaries. Methods: We identified outpatient visits (2007‐2012) with recently documented (≤180 days) cardiovascular risk and classified patients according to whether the exposure (statin) was initiated within 14 days. We selected each beneficiary's first eligible visit (in each treatment group) that met criteria during the respective look‐backs: continuous enrollment (1 or 3 years for fixed look‐back; 180 days for all‐available), no cancer history, and no statin claims. We estimated crude and standardized mortality ratio weighted hazard ratios (HRs) for the effect of statin initiation on incident 6‐month cancer (a known null effect) and 2‐year mortality, separately, adjusting for covariates assessed by using each look‐back. Results: Analyzing short‐term cancer, the estimated HR from the all‐available approach (HR = 0.90, 95% CI: 0.83, 0.98) was less biased than the 1‐year look‐back (HR = 0.79, 95% CI: 0.73, 0.84), which included beneficiaries with prevalent cancer. The 3‐year look‐back (HR = 1.05, 95% CI: 0.90, 1.21) was somewhat less biased than the all‐available estimate but less precise due the exclusion of a large proportion of observations without sufficient continuous enrollment (62.0% and 59.9% of initiators and non‐initiators, respectively). All approaches produced similar estimates of the effect on all‐cause mortality. Alternative look‐backs did not differ in their ability to control confounding. Conclusions: The all‐available look‐back performed nearly as well as the 3‐year fixed, which produced the least biased point estimate. If 3‐year look‐backs are infeasible (eg, due to power/sample), all‐available look‐backs may be preferable to short (1‐year) fixed look‐backs
Multiple inlet sudden expansion flow of power-law fluids
The flow of power-law fluids through two-dimensional planar sudden expansion geometries with multiple inlets was simulated numerically using an in-house finite volume code. The fluids are modelled using the power-law model, and a range of power-law index values (0.4 ≤ n ≤ 1.4) was investigated to cover shear-thinning, Newtonian and shear-thickening inelastic behaviour. The effect of the generalised Reynolds number and the spacing between inlets on the flow behaviour and stability was analysed. The main characteristics of the vortices formed near the wall are similar to those found in single inlet sudden expansions, with symmetric wall vortices forming at low Reynolds numbers, with transitions to steady asymmetric, third eddy flow regimes and time-dependent flow at higher generalised Reynolds numbers. In some cases, the steady asymmetric and third eddy regimes are absent and the flow transitions directly from symmetric flow to time dependent flow (as seen for the geometry with the largest spacing). The stabilising nature of shear-thinning behaviour was observed and the opposite effect was seen for shear-thickening behaviour. In addition, intermediate vortices were seen to form between inlets which grow in size within the symmetric regime as the generalised Reynolds number is increased. New scalings which help collapse the data are introduced and a strong effect of inlet spacing on all flow transitions is shown, with the ratio of the wall distance to inlet height being a determining factor on the corner vortices length when the spacing between inlets is sufficiently large
Flows of Non-Newtonian Fluids in a Sudden Expansion with Multiple Inlets: A Numerical Study
A series of numerical simulations for inelastic and elastic non-Newtonian fluids through a two-dimensional planar sudden expansion is conducted using an in-house finite volume solver1.The geometries investigated are composed by three horizontal inlets of equal widths where the effect of their spacing is investigated. The obtained flow responses are further compared with the equivalent of a single-inlet configuration. Initially, flows of inelastic non-Newtonian fluids described by the power-law model are investigated for the considered multi-inlet configurations, demonstrating the importance of the spacing ratio between inlets and its influence on the resulting flow2. It was found that the spacing ratio between the inlets controls the formation of internal vortices, and leads to vortex interactions which affect the critical conditions causing the flow to become asymmetric and later time dependent. The study is then expanded to consider viscoelastic fluids of constant viscosity described by the upper-convected Maxwell (UCM)model. Creeping flow conditions (i.e Reynolds number (Re) tending to zero) are considered first, to investigate the effects of elasticity in the absence of inertia, where vortex-stabilising effects due to elasticity have been observed. This is in agreement with the observations of other studies employing single-inlet planar sudden expansion configurations3 4. For these conditions, intermediate vortices at the sections between the inlets are not observed, however interesting interactions are reported for varying Weissenberg numbers (Wi). Finally, to account for inertial effects and investigate the influence of vortex interactions, numerical simulations are performed at constant elasticity number (El = Wi/Re), which is representative of realistic experimental conditions
Water-like anomalies for core-softened models of fluids: One dimension
We use a one-dimensional (1d) core-softened potential to develop a physical
picture for some of the anomalies present in liquid water. The core-softened
potential mimics the effect of hydrogen bonding. The interest in the 1d system
stems from the facts that closed-form results are possible and that the
qualitative behavior in 1d is reproduced in the liquid phase for higher
dimensions. We discuss the relation between the shape of the potential and the
density anomaly, and we study the entropy anomaly resulting from the density
anomaly. We find that certain forms of the two-step square well potential lead
to the existence at T=0 of a low-density phase favored at low pressures and of
a high-density phase favored at high pressures, and to the appearance of a
point at a positive pressure, which is the analog of the T=0 ``critical
point'' in the Ising model. The existence of point leads to anomalous
behavior of the isothermal compressibility and the isobaric specific heat
.Comment: 22 pages, 7 figure
Intra-molecular coupling as a mechanism for a liquid-liquid phase transition
We study a model for water with a tunable intra-molecular interaction
, using mean field theory and off-lattice Monte Carlo simulations.
For all , the model displays a temperature of maximum
density.For a finite intra-molecular interaction ,our
calculations support the presence of a liquid-liquid phase transition with a
possible liquid-liquid critical point for water, likely pre-empted by
inevitable freezing. For J=0 the liquid-liquid critical point disappears at
T=0.Comment: 8 pages, 4 figure
Real-world on-treatment and initial treatment absolute risk differences for dabigatran vs warfarin in older US adults
Purpose: Trials and past observational work compared dabigatran and warfarin in patients with atrial fibrillation, but few reported estimates of absolute harm and benefit under real-world adherence patterns, particularly in older adults that may have differing benefit-harm profiles. We aimed to estimate risk differences for ischemic stroke, death, and gastrointestinal bleeding after initiating dabigatran and warfarin in older adults (a) when patients adhere to treatment and (b) under real-world adherence patterns. Methods: In a 20% sample of nationwide Medicare claims from 2010 to 2015, we identified beneficiaries aged 66 years and older initiating warfarin and dabigatran. We followed individuals from initiation until death or October 2015 (initial treatment, IT) and separately censored individuals' follow-up after drug switches and gaps in supply (on-treatment, OT). We applied inverse probability of treatment and standardized morbidity ratio weights, as well as inverse probability of censoring weights, to estimate two-year risk differences (RDs) for dabigatran vs warfarin. Results: We identified 10,717 dabigatran and 74,891 warfarin initiators. Weighted OT RDs suggested decreased ischemic stroke risk for dabigatran vs warfarin; IT RDs indicated increased or no change in ischemic stroke risk. Regardless of follow-up approach and weighting strategy, risk of death appeared lower and risk of gastrointestinal bleeding appeared higher when comparing dabigatran vs warfarin. Conclusions: Dabigatran use was associated with lower risks of mortality and ischemic stroke in routine care when older adults stayed on treatment. IT analyses suggested that these benefits may be diminished under real-world patterns of switching and discontinuation
Reweighting oranges to apples: Transported RE-LY trial versus nonexperimental effect estimates of anticoagulation in atrial fibrillation
Background: Results from trials and nonexperimental studies are often directly compared, with little attention paid to differences between study populations. When target and trial population data are available, accounting for these differences through transporting trial results to target populations of interest provides useful perspective. We aimed to compare two-year risk differences (RDs) for ischemic stroke, mortality, and gastrointestinal bleeding in older adults with atrial fibrillation initiating dabigatran and warfarin when using trial transport methods versus nonexperimental methods. Methods: We identified Medicare beneficiaries who initiated warfarin or dabigatran from a 20% nationwide sample. To transport treatment effects observed in the randomized evaluation of long-term anticoagulation trial, we applied inverse odds weights to standardize estimates to two Medicare target populations of interest, initiators of: (1) dabigatran and (2) warfarin. Separately, we conducted a nonexperimental study in the Medicare populations using standardized morbidity ratio weighting to control measured confounding. Results: Comparing dabigatran to warfarin, estimated two-year RDs for ischemic stroke were similar with trial transport and nonexperimental methods. However, two-year mortality RDs were closer to the null when using trial transport versus nonexperimental methods for the dabigatran target population (transported RD: -0.57%; nonexperimental RD: -1.9%). Estimated gastrointestinal bleeding RDs from trial transport (dabigatran initiator RD: 1.8%; warfarin initiator RD: 1.9%) appeared more harmful than nonexperimental results (dabigatran initiator RD: 0.14%; warfarin initiator RD: 0.57%). Conclusions: Differences in study populations can and should be considered quantitatively to ensure results are relevant to populations of interest, particularly when comparing trial with nonexperimental findings. See video abstract: http://links.lww.com/EDE/B703
Single-mode neodymium fibre lasers
A laser medium in the form of a single mode optical fibre offers a number of attractive features. For example, using a laser to end pump such a fibre provided with feedback mirrors, allows a very low oscillation threshold to be attained. In the case of Nd doped fibre a GaAs diode laser can be used as the pump, thus providing a very simple and compact laser device. The low threshold performance also suggests that other, weaker transitions may be capable of laser action eg. the 1.3µm 4F3/2 to 4I13/2 transition in Nd or transitions in other dopants which have not previously shown laser action in a glass host. Other possible applications include the use of fibres as amplifiers e.g. as an in-line amplifier in an optical communication system where it could play the role of a repeater. As a power amplifier the fibre device could have advantages over the bulk glass systems by reducing the problems of thermal distortion and thermal fracture. In this paper we report some results obtained with neodymium-doped single mode fibres
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