13,667 research outputs found
A Design Strategy for Deadlock-Free Concurrent Systems
When building concurrent systems, it would be useful to have a collection of reusable processes
to perform standard tasks. However, without knowing certain details of the inner workings of
these components, one can never be sure that they will not cause deadlock when connected to
some particular network.
Here we describe a hierarchical method for designing complex networks of communicating
processeswhich are deadlock-free.We use this to define a safe and simple method for specifying
the communication interface to third party software components. This work is presented using
the CSP model of concurrency and the occam2.1 programming language
Radiation trapping in coherent media
We show that the effective decay rate of Zeeman coherence, generated in a
Rb87 vapor by linearly polarized laser light, increases significantly with the
atomic density. We explain this phenomenon as the result of radiation trapping.
Our study shows that radiation trapping must be taken into account to fully
understand many electromagnetically induced transparency experiments with
optically thick media
Theoretical development of Monte Carlo codes for modeling cumulus cloud fields
Reflected fluxes are calculated for stratocumulus cloud fields as a function of skycover, cloud aspect ratio, and cloud shape. Cloud liquid water volume is held invariant as cloud shape is varied so that the results can be utilized more effectively by general circulation models and climate models. On the basis of required accuracy in the Earth Radiation Budget Experiment program, an order of magnitude value of 10 W/sq m is used to estimate significant differences between plane parallel and broken cloudiness. This limit is exceeded for cloud covers between 10% and 90%, indicating that plane paralel calculations are not satisfactory at most values of cloud cover. The choice of cloud shape also leads to large differences in reflected fluxes. These differences are traced to the anisotropic intensity pattern out the cloud sides, to the size and shape of the holes between clouds, and to variations in cloud area as viewed from the solar direction. An empirical relationship for effective cloud cover is given at solar zenith angle of theta = 60 deg. This relationship allows for the relatively accurate (delta F = 10 - 15 W/sq m.) computation of broken cloud field reflected fluxes from plane parallel calculations
Modeling and parameterization of horizontally inhomogeneous cloud radiative properties
One of the fundamental difficulties in modeling cloud fields is the large variability of cloud optical properties (liquid water content, reflectance, emissivity). The stratocumulus and cirrus clouds, under special consideration for FIRE, exhibit spatial variability on scales of 1 km or less. While it is impractical to model individual cloud elements, the research direction is to model a statistical ensembles of cloud elements with mean-cloud properties specified. The major areas of this investigation are: (1) analysis of cloud field properties; (2) intercomparison of cloud radiative model results with satellite observations; (3) radiative parameterization of cloud fields; and (4) development of improved cloud classification algorithms
Alternative model for the administration and analysis of research-based assessments
Research-based assessments represent a valuable tool for both instructors and
researchers interested in improving undergraduate physics education. However,
the historical model for disseminating and propagating conceptual and
attitudinal assessments developed by the physics education research (PER)
community has not resulted in widespread adoption of these assessments within
the broader community of physics instructors. Within this historical model,
assessment developers create high quality, validated assessments, make them
available for a wide range of instructors to use, and provide minimal (if any)
support to assist with administration or analysis of the results. Here, we
present and discuss an alternative model for assessment dissemination, which is
characterized by centralized data collection and analysis. This model provides
a greater degree of support for both researchers and instructors in order to
more explicitly support adoption of research-based assessments. Specifically,
we describe our experiences developing a centralized, automated system for an
attitudinal assessment we previously created to examine students'
epistemologies and expectations about experimental physics. This system
provides a proof-of-concept that we use to discuss the advantages associated
with centralized administration and data collection for research-based
assessments in PER. We also discuss the challenges that we encountered while
developing, maintaining, and automating this system. Ultimately, we argue that
centralized administration and data collection for standardized assessments is
a viable and potentially advantageous alternative to the default model
characterized by decentralized administration and analysis. Moreover, with the
help of online administration and automation, this model can support the
long-term sustainability of centralized assessment systems.Comment: 7 pages, 1 figure, accepted in Phys. Rev. PE
Killing the Spirit: Doublespeak and Double Jeopardy in a Classroom of Scholars
One of the most difficult tasks we face as human beings is trying to communicate across individual differences, trying to make sure that what we say to someone is interpreted the way we intended. This becomes even more difficult when we attempt to communicate across social differences, gender, race, or class lines, or any situation of unequal power. We have conducted a nine-year longitudinal study of the relationship between pre-college enrichment experiences and the development of academic ethos (scholar identity) in educationally disadvantaged African-American adolescents. The study, Project EXCEL, examines how each participant constructs a definition of scholar and how, or if, that definition affects achievement in a university-sponsored enrichment program in reading, writing, and foreign language study. Student participants enter the program as sophomores and exit at the conclusion of their senior year
NMR study of a bimesogenic liquid crystal with two nematic phases
Recent interest in bimesogenic liquid crystals showing two nematic phases has led us to investigate the nematic mean-field interactions in these nematic phases by using rigid solutes as probes. The nematic potential that is modelled by two independent Maier-Saupe terms is successful in fitting the observed dipolar couplings (order parameters) of para-, meta- and ortho-dichlorobenzene solutes in both the nematic phases of 39 wt% of 4-n-pentyl-4âČ-cyanobiphenyl (5CB) in α,Ï-bis(4-4âČ-cyanobiphenyl)nonane (CB_C9_CB) to better than the 5% level. The derived liquid-crystal potential parameters Gâ and Gâ for each solute in the N and Ntb phases will be discussed. The most interesting observation is that G1 (associated with size and shape interactions) is almost constant in the Ntb phase, whereas Gâ (associated with longer-range electrostatic interactions) has large variation, even changing sign
Contribution of cod liver oil-related nutrients (vitamins A, D, E and eicosapentaenoic acid and docosahexaenoic acid) to daily nutrient intake and their associations with plasma concentrations in the EPIC-Norfolk cohort
Total nutrient intake (TNI) is intake from food and supplements. This provides an assessment of nutrient adequacy and the prevalence of excessive intake, as well as the response with respect to biomarkers. Cod liver oil (CLO) is the most frequently consumed supplement in the UK, containing nutrients that might have varying influences on health. We calculated TNI for vitamins A, D and E, as well as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and assessed associations with the respective blood concentrations
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