45,459 research outputs found
On the miscible Rayleigh-Taylor instability: two and three dimensions
We investigate the miscible Rayleigh-Taylor (RT) instability in both 2 and 3
dimensions using direct numerical simulations, where the working fluid is
assumed incompressible under the Boussinesq approximation. We first consider
the case of randomly perturbed interfaces. With a variety of diagnostics, we
develop a physical picture for the detailed temporal development of the mixed
layer: We identify three distinct evolutionary phases in the development of the
mixed layer, which can be related to detailed variations in the growth of the
mixing zone. Our analysis provides an explanation for the observed differences
between two and three-dimensional RT instability; the analysis also leads us to
concentrate on the RT models which (1) work equally well for both laminar and
turbulent flows, and (2) do not depend on turbulent scaling within the mixing
layer between fluids. These candidate RT models are based on point sources
within bubbles (or plumes) and interaction with each other (or the background
flow). With this motivation, we examine the evolution of single plumes, and
relate our numerical results (of single plumes) to a simple analytical model
for plume evolution.Comment: 31 pages, 27 figures, to appear in November issue of JFM, 2001. For
better figures: http://astro.uchicago.edu/~young/ps/jfmtry08.ps.
Penta-hepta defect chaos in a model for rotating hexagonal convection
In a model for rotating non-Boussinesq convection with mean flow we identify
a regime of spatio-temporal chaos that is based on a hexagonal planform and is
sustained by the {\it induced nucleation} of dislocations by penta-hepta
defects. The probability distribution function for the number of defects
deviates substantially from the usually observed Poisson-type distribution. It
implies strong correlations between the defects inthe form of density-dependent
creation and annihilation rates of defects. We extract these rates from the
distribution function and also directly from the defect dynamics.Comment: 4 pages, 5 figures, submitted to PR
Literacy Access through Storytime: An Ethnographic Study of Public Library Storytellers in a Low-Income Neighborhood
While early literacy achievement continues to be stratified by social class in the United States, public libraries often offer programs such as “storytime” in order to bolster the literacy development of youth in their communities. The purpose of the present ethnographic study was to explore how storytellers recruited and maintained participation in this free literacy program in a lower-income neighborhood. Via participant observations, semi-structured interviews, and artifact collection, storytellers recruited new patrons to storytime by (1) appealing to community members to enter the physical space of the library and (2) appealing to library patrons to attend storytime. Once patrons attended storytime, storytellers acted in order to maintain storytime attendance by (1) facilitating meaningful learning experiences, (2) fostering enjoyment through participation, (3) developing nurturing relationships, and (4) offering flexibility in storytime expectations. By exploring a contextualized account of the work of storytellers, the findings suggest important avenues through which public programs may contribute to more equitable access to literacy learning
Generating entanglement with low Q-factor microcavities
We propose a method of generating entanglement using single photons and
electron spins in the regime of resonance scattering. The technique involves
matching the spontaneous emission rate of the spin dipole transition in bulk
dielectric to the modified rate of spontaneous emission of the dipole coupled
to the fundamental mode of an optical microcavity. We call this regime
resonance scattering where interference between the input photons and those
scattered by the resonantly coupled dipole transition result in a reflectivity
of zero. The contrast between this and the unit reflectivity when the cavity is
empty allow us to perform a non demolition measurement of the spin and to non
deterministically generate entanglement between photons and spins. The chief
advantage of working in the regime of resonance scattering is that the required
cavity quality factors are orders of magnitude lower than is required for
strong coupling, or Purcell enhancement. This makes engineering a suitable
cavity much easier particularly in materials such as diamond where etching high
quality factor cavities remains a significant challenge
Making Rasch decisions: The use of Rasch analysis in the construction of preference based health related quality of life instruments
Objective: To set out the methodological process for using Rasch analysis alongside traditional psychometric methods in the development of a health state classification that is amenable to valuation.
Methods: The overactive bladder questionnaire is used to illustrate a four step process for deriving a reduced health state classification from an existing nonpreference based health related quality of life instrument. Step I excludes items that do not meet the initial validation process and step II uses criteria based on Rasch analysis and psychometric testing to select the final items for the health state classification. In step III, item levels are examined and Rasch analysis is used to explore the possibility of reducing the number of item levels. Step IV repeats steps I to III on alternative data sets in order to validate the selection of items for the health state classification.
Conclusions: The techniques described enable the construction of a health state classification amenable for valuation exercises that will allow the derivation of preference weights. Thus, the health related quality of life of patients with conditions, like overactive bladder, can be valued and quality adjustment weights such as quality adjusted life years derived
The use of Rasch analysis as a tool in the construction of a preference based measure: the case of AQLQ
The majority of quality of life instruments are not preference-based measures and so cannot be used within cost utility analysis. The Asthma Quality of Life Questionnaire (AQLQ) is one such instrument. The aim of this study was to develop a health state classification that is amenable to valuation from the AQLQ.
Rasch models were applied to samples of responders to the AQLQ with the aim of i) selecting a number of items for a preference based utility measure (AQL-5D), ii) reducing the number of levels for each item to a more manageable number of levels for establishing AQL-5D. Selection of items for the evaluation survey was supported with conventional psychometric criteria for item selection (feasibility, internal consistency, floor and ceiling effects, responsiveness and regression against overall health).
The role of Rasch analysis in reducing the number of item levels to a preconceived target number of levels proved unsuccessful. However, Rasch analysis proved to be a useful tool in assisting in the initial process of selecting items from an existing HRQL instrument in the construction of AQL-5D. The method is recommended for use alongside conventional psychometric testing to aid in the development of preference-based measures
Holography of the Conformal Window
Inspired by the model of Jarvinen and Kiritsis, we present a simple
holographic model for the on set of chiral symmetry breaking at the edge of the
conformal window in QCD in the Veneziano limit. Our most naive model enforces
the QCD two loop running coupling on a D3/D7 holographic brane system. The mass
of the holographic field, describing the chiral condensate in the model, is
driven below the BF bound when the running is sufficiently strong, triggering
chiral symmetry breaking for N_f/N_c<2.9. This model though contains too great
a remnant of supersymmetry and does not correctly encode the perturbative
anomalous dimensions of QCD. In a second model we impose the QCD anomalous
dimension result and find chiral symmetry breaking sets in for N_f/N_c=4 at a
BKT-type phase transition. In this case the transition is triggered when the
anomalous dimension of the mass operator \gamma_m=1.Comment: 10 pages, 2 figures, v2: minor corrections, improved Figure
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