4,439 research outputs found
Collaborating and Coordinating with Employers
A 2009 research brief produced for the NTAR Leadership Center, a consortium led by the John H. Heldrich Center for Workforce Development at Rutgers, The State University of New Jersey. Founded in 2007 under a grant/contract with the Office of Disability Employment Policy at the U.S. Department of Labor, the NTAR Leadership Center's mission is to build capacity and leadership at the federal, state, and local levels to enable change across workforce development and disability-specific systems that will increase employment and economic self-sufficiency for adults with disabilities. This brief examines the evolving relationship between disability employment initiatives and employers, and highlights the implications that these collaborations have for effectively formulating broad-scale promotion of disability workforce investment initiatives
Pipe Poiseuille flow of viscously anisotropic, partially molten rock
Laboratory experiments in which synthetic, partially molten rock is subjected
to forced deformation provide a context for testing hypotheses about the
dynamics and rheology of the mantle. Here our hypothesis is that the aggregate
viscosity of partially molten mantle is anisotropic, and that this anisotropy
arises from deviatoric stresses in the rock matrix. We formulate a model of
pipe Poiseuille flow based on theory by Takei and Holtzman [2009a] and Takei
and Katz [2013]. Pipe Poiseuille is a configuration that is accessible to
laboratory experimentation but for which there are no published results. We
analyse the model system through linearised analysis and numerical simulations.
This analysis predicts two modes of melt segregation: migration of melt from
the centre of the pipe toward the wall and localisation of melt into
high-porosity bands that emerge near the wall, at a low angle to the shear
plane. We compare our results to those of Takei and Katz [2013] for plane
Poiseuille flow; we also describe a new approximation of radially varying
anisotropy that improves the self-consistency of models over those of Takei and
Katz [2013]. This study provides a set of baseline, quantitative predictions to
compare with future laboratory experiments on forced pipe Poiseuille flow of
partially molten mantle.Comment: 23 pages, 7 figures. Submitted to Geophysical Journal International
on 25 April 2014. Revised after reviewer comments and resubmitted on 20
August 201
Consequences of viscous anisotropy in a deforming, two-phase aggregate. Why is porosity-band angle lowered by viscous anisotropy?
In laboratory experiments that impose shear deformation on partially molten
aggregates of initially uniform porosity, melt segregates into high-porosity
sheets (bands in cross-section). The bands emerge at 15-20 degrees to the shear
plane. A model of viscous anisotropy can explain these low angles whereas
previous, simpler models have failed to do so. The anisotropic model is
complex, however, and the reason that it produces low-angle bands has not been
understood. Here we show that there are two mechanisms: (i) suppression of the
well-known tensile instability, and (ii) creation of a new, shear-driven
instability. We elucidate these mechanisms using linearised stability analysis
in a coordinate system that is aligned with the perturbations. We consider the
general case of anisotropy that varies dynamically with deviatoric stress, but
approach it by first considering uniform anisotropy that is imposed a priori
and showing the difference between static and dynamic cases. We extend the
model of viscous anisotropy to include a strengthening in the direction of
maximum compressive stress. Our results support the hypothesis that viscous
anisotropy is the cause of low band-angles in experiments.Comment: 32 pages, 8 figures. Accepted for publication in the Journal of Fluid
Mechanics on 4 October 201
Melt-preferred orientation, anisotropic permeability, and melt-band formation in a deforming, partially molten aggregate
Shear deformation of partially molten rock in laboratory experiments causes
the emergence of melt-enriched sheets (bands in cross-section) that are aligned
at about 15-20 degrees to the shear plane. Deformation and deviatoric stress
also cause the coherent alignment of pores at the grain scale. This leads to a
melt-preferred orientation that may, in turn, give rise to an anisotropic
permeability. Here we develop a simple, general model of anisotropic
permeability in partially molten rocks. We use linearised analysis and
nonlinear numerical solutions to investigate its behaviour under simple-shear
deformation. In particular, we consider implications of the model for the
emergence and angle of melt-rich bands. Anisotropic permeability affects the
angle of bands and, in a certain parameter regime, it can give rise to low
angles consistent with experiments. However, the conditions required for this
regime have a narrow range and seem unlikely to be entirely met by experiments.
Anisotropic permeability may nonetheless affect melt transport and the
behaviour of partially molten rocks in Earth's mantle.Comment: 19 pages, 7 figures, accepted for publication in Geophysical Journal
International on 3 September 201
Variations in mid-ocean ridge CO2 emissions driven by glacial cycles
The geological record shows links between glacial cycles and volcanic
productivity, both subaerially and at mid-ocean ridges. Sea-level-driven
pressure changes could also affect chemical properties of mid-ocean ridge
volcanism. We consider how changing sea-level could alter the CO2 emissions
rate from mid-ocean ridges, on both the segment and global scale. We develop a
simplified transport model for a highly incompatible element through a
homogenous mantle; variations in the melt concentration the emission rate of
the element are created by changes in the depth of first silicate melting. The
model predicts an average global mid-ocean ridge CO2 emissions-rate of 53
Mt/yr, in line with other estimates. We show that falling sea level would cause
an increase in ridge CO2 emissions with a lag of about 100 kyrs after the
causative sea level change. The lag and amplitude of the response are sensitive
to mantle permeability and plate spreading rate. For a reconstructed sea-level
time series of the past million years, we predict variations of up to 12% (7
Mt/yr) in global mid-ocean ridge CO2 emissions. The magnitude and timing of the
predicted variations in CO2 emissions suggests a potential role for ridge
carbon emissions in glacial cycles
Daily minimum and maximum temperature simulation over complex terrain
Spatiotemporal simulation of minimum and maximum temperature is a fundamental
requirement for climate impact studies and hydrological or agricultural models.
Particularly over regions with variable orography, these simulations are
difficult to produce due to terrain driven nonstationarity. We develop a
bivariate stochastic model for the spatiotemporal field of minimum and maximum
temperature. The proposed framework splits the bivariate field into two
components of "local climate" and "weather." The local climate component is a
linear model with spatially varying process coefficients capturing the annual
cycle and yielding local climate estimates at all locations, not only those
within the observation network. The weather component spatially correlates the
bivariate simulations, whose matrix-valued covariance function we estimate
using a nonparametric kernel smoother that retains nonnegative definiteness and
allows for substantial nonstationarity across the simulation domain. The
statistical model is augmented with a spatially varying nugget effect to allow
for locally varying small scale variability. Our model is applied to a daily
temperature data set covering the complex terrain of Colorado, USA, and
successfully accommodates substantial temporally varying nonstationarity in
both the direct-covariance and cross-covariance functions.Comment: Published in at http://dx.doi.org/10.1214/12-AOAS602 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A Renaissance Instrument to Support Nonprofits: The Sale of Private Chapels in Florentine Churches
Catholic churches in Renaissance Florence supported themselves overwhelmingly from the contributions of wealthy citizens. The sale of private chapels within churches to individuals was a significant source of church funds, and facilitated a church construction boom. Chapel sales offered three benefits to churches: prices were usually far above cost; donor/purchasers purchased masses and other tie-in services; and they added to the magnificence of the church because donors were required to decorate chapels expensively. Donors purchased chapels for two primary reasons: to facilitate services for themselves and their families, such as masses and church burials, that would speed their departure from Purgatory; and to gain status in the community. Chapels were private property within churches, but were only occasionally used directly by their owners. The expense of chapels and their decorations made them an ideal signal for wealth, particularly since sumptuary laws limited most displays of wealth. To overcome the contributions free-rider problem, these churches sold private benefits not readily available elsewhere, namely status and salvation.
An Economist's Guide to U.S. v. Microsoft
We analyze the central economic issues raised by U.S. v Microsoft. Network effects and economies of scale in applications programs created a barrier to entry for new operating system competitors, which the combination of Netscape Navigator and the Java programming language potentially could have lowered. Microsoft took actions to eliminate this threat to its operating system monopoly, and some of Microsoft's conduct very likely harmed consumers. While we recognize the risks of the government's proposed structural remedy of splitting Microsoft in two, we are pessimistic that a limited conduct remedy would be effective in this case.
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