513 research outputs found
Quantum molecular dynamics simulations for the nonmetal-to-metal transition in fluid helium
We have performed quantum molecular dynamics simulations for dense helium to
study the nonmetal-to-metal transition at high pressures. We present new
results for the equation of state and the Hugoniot curve in the warm dense
matter region. The optical conductivity is calculated via the Kubo-Greenwood
formula from which the dc conductivity is derived. The nonmetal-to-metal
transition is identified at about 1 g/ccm. We compare with experimental results
as well as with other theoretical approaches, especially with predictions of
chemical models.Comment: 4 pages, 5 figure
XUV Opacity of Aluminum between the Cold-Solid to Warm-Plasma Transition
We present calculations of the free-free XUV opacity of warm, solid-density
aluminum at photon energies between the plasma frequency at 15 eV and the
L-edge at 73 eV, using both density functional theory combined with molecular
dynamics and a semi-analytical model in the RPA framework with the inclusion of
local field corrections. As the temperature is increased from room temperature
to 10 eV, with the ion and electron temperatures equal, we calculate an
increase in the opacity in the range over which the degree of ionization is
constant. The effect is less pronounced if only the electron temperature is
allowed to increase. The physical significance of these increases is discussed
in terms of intense XUV-laser matter interactions on both femtosecond and
picosecond time-scales.Comment: 4 pages, 3 figure
Probing the interiors of the ice giants: Shock compression of water to 700 GPa and 3.8 g/ccm
Recently there has been tremendous increase in the number of identified
extra-solar planetary systems. Our understanding of their formation is tied to
exoplanet internal structure models, which rely upon equations of state of
light elements and compounds like water. Here we present shock compression data
for water with unprecedented accuracy that shows water equations of state
commonly used in planetary modeling significantly overestimate the
compressibility at conditions relevant to planetary interiors. Furthermore, we
show its behavior at these conditions, including reflectivity and isentropic
response, is well described by a recent first-principles based equation of
state. These findings advocate this water model be used as the standard for
modeling Neptune, Uranus, and "hot Neptune" exoplanets, and should improve our
understanding of these types of planets.Comment: Accepted to Phys. Rev. Lett.; supplementary material attached
including 2 figures and 2 tables; to view attachments, please download and
extract the gzipped tar source file listed under "Other formats
Sensitivity of Low Sloped Roofs Designs to Initial Water and Air Leakage
Liquid water in low sloped roofs almost
always causes problems. Roofs are
designed only to control the migration of
vapor, if at all. Small amounts of water
leakage/penetration, may cause mold
growth or catastrophic corrosion in current
roofs systems. In a recent paper by the
authors the effect of exterior surface
emissive and absorptive properties was
found to have a significant effect on the
moisture performance of a roof that had a
leak. Depending on the surface
characteristics, roof systems can be
designed to effectively manage water
penetration, but at an energy cost. In the
roofs system examined previously, air
leakage was not included.
In the present study, the authors reinvestigated
the effect of water penetration
and the influence of air leakage on the
hygrothermal performance of a few
selected roofs. The drying potential of a
groove ventilated roof is examined. The
performance concept is based on the fact
that warming up of air in the groove
increases it's ability to transport moisture
to the outside. Solar radiation raises the
temperature of air in the grooves and on
average, during a sunny summer day 0.5 L
of water can be ventilated out of the roof
per 1m width of the roof.
In this paper, one climatic condition was
investigated; a hot and humid Climate
representative of Houston, TX. The
specific questions that the paper addresses
are: What are the vapor and liquid control
dynamic involved in the moisture
migration of a roof in Houston TX? and
how does airflow influence the
performance of a roof that is initially wet ?
A state-of-the-art numerical model was
used to address these issues. Results
showed that the drying potential depends
on the ventilation rates. The roof system
with ventilation grooves dried out faster
from the initially wet stage than the roof
without the ventilation grooves. The total
increase in heat loss of the roof was found
to be between 0 - 5 % depending on the
thickness of the insulation. The ventilation
can cool down the temperature of the roof
in the middle of a hot and sunny day thus
reducing the heat load to the inside
Homeless street children in Nepal : use of allostatic load to assess the burden of childhood adversity.
As challenges to child well-being through economic disadvantage, family disruption, and migration or displacement escalate world wide, the need for cross-culturally robust understanding of childhood adversity proportionately increases. Toward this end, developmental risk was assessed in four contrasting groups of 107 Nepali children ages 10–14 years that represent distinctive, common conditions in which contemporary children grow up. Relative cumulative burden (allostatic load) indexed by multiple dimensions of physical and psychosocial stress was ascertained among homeless street boys and three family-based groups, from poor urban squatter settlements, urban middle class, and a remote rural village. Biomarkers of stress and vulnerability to stress included growth status, salivary cortisol, antibodies to Epstein–Barr virus, acute phase inflammatory responses (alpha1-antichymotrypsin), and cardiovascular fitness and reactivity (flex heart rate and pressor response). Individual biomarkers of risk and allostatic load differed markedly among groups, were highest in villagers, and varied by components of allostatic load. Such data suggest a need for critical appraisal of homelessness and migration as a risk factor to youth, given prevailing local conditions such as rural poverty, and represents the only multidimensional study of childhood allostatic load and developmental risk in non-Western settings
Integrated Hygrothermal Performance of Building Envelopes and Systems in Hot and Humid Climates
In hot and humid climates the interior and exterior
environmental loads that building envelopes must
respond to are larger than many other climatic
conditions. Moisture-originated failures in low-rise
residential buildings have put a significant pressure to
change construction codes in North America.
Solutions to moisture induced problems may be
difficult when several interacting mechanisms of
moisture transport are present. A new approach to
building envelope durability assessment has been
introduced in North America; a moisture engineering
approach. This requires system information about the
wall systems as constructed along with aging
characteristics coupled with advanced modeling that
0 term allow the designer to predict the Iong-term performances of building envelope systems. This
permits the comparison and ranking of individual
building envelope systems with respect to total
hygrothermal performance.
Critical information can be obtained by investigating
the one to one relationships of a building envelope to
interior and exterior environments, however, the total
behavior of the actual whole building is not
accounted for. This paper goes one step further, by
incorporating the individual hygrothermal
performances of all walls, roof, floor and mechanical
systems. The direct and indirect coupling of the
building envelope and indoor environment with
HVAC system are included in the analysis. The full
house hygrothermal performance of an aerated
concrete wall system are examined for a hot and
humid climate. The hour by hour drying potential of
each system was then numerically analyzed using
weather conditions of Miami (hot and humid
climate). The results clearly demonstrate the limited
drying potential for the wall system in that climate.
Furthermore, the selected exterior thermal insulation
strategies and interior vapor control strategies in this
study clearly show the critical behavior of the full
house with respect to drying initial construction
moisture. The results show the importance of the
total hygrothermal behavior of the whole house to the
coupling between the various envelope parts, interior
and exterior environments and HVAC system. From
these results moisture control strategies are identified
for the whole house hygrothermal performance
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