3,648 research outputs found
Can a Kasner Universe with a Viscous Cosmological Fluid be Anisotropic?
A Bianchi type -I metric of Kasner form is considered, when the space is
filled with a viscous fluid. Whereas an ideal (nonviscous) fluid permits the
Kasner metric to be anisotropic provided that the fluid satisfies the
Zel'dovich equation of state, the viscous fluid does not permit the Kasner
metric to be anisotropic at all. In the latter case, we calculate the Kasner
(isotropic) metric expressed by the fluid's density, pressure, and bulk
viscosity, at some chosen instant . The equation of state is also
calculated. The present paper is related to a recent Comment of Cataldo and del
Campo [Phys. Rev. D, scheduled to April 15, 2000], on a previous work of the
present authors [Phys. Rev. D {\bf 56}, 3322 (1997)].Comment: 8 pages, LaTeX, no figures. To appear in PR
Melting in multilayer adsorbed films
We present both an improved model and new experimental data concerning the problem of melting in multilayer adsorbed films. The model treats in a mutually consistent manner all interfaces in a stratified film. This results in the prediction of substrate freezing, a phenomenon thermodynamically analogous to surface melting. We also compare the free energies of stratified films to those of homogeneous films. This leads to an orderly classification of multilayer phase diagrams in the vicinity of the bulk triple point. The results of the model are compared with the experimentally known systems. Of these, only methane/graphite exhibits melting from homogeneous solid to homogeneous liquid in multilayer films. The systems Ne/graphite and Ar/graphite, studied by Zhu and Dash, exhibit surface melting and substrate freezing instead. We observe experimentally, by means of pulsed nuclear magnetic resonance, that melting in methane adsorbed on graphite extends below the film thickness at which the latent heat of melting is known to vanish. The multilayer melting curve in this system is a first-order prewetting transition, extending from triple-point dewetting at bulk coexistence down to a critical point where the latent heat vanishes at about four layers, and apparently extending to thinner films as a higher-order, two-dimensional phase transition. It would therefore seem that methane/graphite is an ideal system in which to study the evolution of melting from two dimensions to three dimensions
Bound states of 3He at the edge of a 4He drop on a cesium surface
We show that small amounts of 3He atoms, added to a 4He drop deposited on a
flat cesium surface at zero temperature, populate bound states localized at the
contact line. These edge states show up for drops large enough to develop well
defined surface and bulk regions together with a contact line, and they are
structurally different from the well-known Andreev states that appear at the
free surface and at the liquid-solid interface of films. We illustrate the
one-body density of 3He in a drop with 1000 4He atoms, and show that for
sufficiently large number of impurities, the density profiles spread beyond the
edge, coating both the curved drop surface and its flat base and eventually
isolating it from the substrate.Comment: 10 pages and 7 figures. Submitted to PR
Current and new formulations of the plant growth regulator Primo MAXX® (trinexapac-ethyl) for turfgrass : Comparison of PRIMO MAXX® (A11825A) and Primo MAXX® NG (A19238C) on golf course fairways and greens
Anomaly in the stability limit of liquid helium 3
We propose that the liquid-gas spinodal line of helium 3 reaches a minimum at
0.4 K. This feature is supported by our cavitation measurements. We also show
that it is consistent with extrapolations of sound velocity measurements.
Speedy [J. Phys. Chem. 86, 3002 (1982)] previously proposed this peculiar
behavior for the spinodal of water and related it to a change in sign of the
expansion coefficient alpha, i. e. a line of density maxima. Helium 3 exhibits
such a line at positive pressure. We consider its extrapolation to negative
pressure. Our discussion raises fundamental questions about the sign of alpha
in a Fermi liquid along its spinodal.Comment: 5 pages, 3 figure
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Microwave signatures of ice hydrometeors from ground-based observations above Summit, Greenland
Multi-instrument, ground-based measurements provide unique and comprehensive
data sets of the atmosphere for a specific location over long periods of time
and resulting data compliment past and existing global satellite
observations. This paper explores the effect of ice hydrometeors on
ground-based, high-frequency passive microwave measurements and attempts to
isolate an ice signature for summer seasons at Summit, Greenland, from 2010
to 2013. Data from a combination of passive microwave, cloud radar,
radiosonde, and ceilometer were examined to isolate the ice signature at
microwave wavelengths. By limiting the study to a cloud liquid water path of
40 g m−2 or less, the cloud radar can identify cases where the
precipitation was dominated by ice. These cases were examined using liquid
water and gas microwave absorption models, and brightness temperatures were
calculated for the high-frequency microwave channels: 90, 150, and 225 GHz.
By comparing the measured brightness temperatures from the microwave
radiometers and the calculated brightness temperature using only gas and
liquid contributions, any residual brightness temperature difference is due
to emission and scattering of microwave radiation from the ice hydrometeors
in the column. The ice signature in the 90, 150, and 225 GHz channels for
the Summit Station summer months was isolated. This measured ice signature
was then compared to an equivalent brightness temperature difference
calculated with a radiative transfer model including microwave
single-scattering properties for several ice habits. Initial model results
compare well against the 4 years of summer season isolated ice signature in
the high-frequency microwave channels
Proton tracking in a high-granularity Digital Tracking Calorimeter for proton CT purposes
Radiation therapy with protons as of today utilizes information from x-ray CT
in order to estimate the proton stopping power of the traversed tissue in a
patient. The conversion from x-ray attenuation to proton stopping power in
tissue introduces range uncertainties of the order of 2-3% of the range,
uncertainties that are contributing to an increase of the necessary planning
margins added to the target volume in a patient. Imaging methods and
modalities, such as Dual Energy CT and proton CT, have come into consideration
in the pursuit of obtaining an as good as possible estimate of the proton
stopping power. In this study, a Digital Tracking Calorimeter is benchmarked
for proof-of-concept for proton CT purposes. The Digital Tracking Calorimeteris
applied for reconstruction of the tracks and energies of individual high energy
protons. The presented prototype forms the basis for a proton CT system using a
single technology for tracking and calorimetry. This advantage simplifies the
setup and reduces the cost of a proton CT system assembly, and it is a unique
feature of the Digital Tracking Calorimeter. Data from the AGORFIRM beamline at
KVI-CART in Groningen in the Netherlands and Monte Carlo simulation results are
used to in order to develop a tracking algorithm for the estimation of the
residual ranges of a high number of concurrent proton tracks. The range of the
individual protons can at present be estimated with a resolution of 4%. The
readout system for this prototype is able to handle an effective proton
frequency of 1 MHz by using 500 concurrent proton tracks in each readout frame,
which is at the high end range of present similar prototypes. A future further
optimized prototype will enable a high-speed and more accurate determination of
the ranges of individual protons in a therapeutic beam.Comment: 21 pages, 8 figure
Atmospheric Ice Accretion on Railway Overhead Powerline Conductors- A Numerical Case Study
Ice accretion on railway overhead contact wires/conductors can cause various critical operational and safety issues such as overloading, arc formation, mass imbalance, and wire galloping. The focus of this multiphase numerical study is to understand and analyze the ice accretion physics on railway overhead powerline conductors at various operating conditions. In this regard, two different geometric shape conductors of 12 mm diameter, 1) a grooved shape contact wire (like an actual railway conductor); 2) a standard circular shape contact wire are used. Computational Fluid Dynamics (CFD) based numerical simulations are carried out for both geometric configurations at different operating parameters such as wind speed, Liquid Water Content (LWC), cloud droplet size distribution, Median Volume Diameter (MVD), and atmospheric temperature. Analysis shows that variation in the operating weather parameters for both geometric configurations considerably affects the ice accretion, both in terms of accreted ice thickness and mass
Path integral Monte Carlo simulation of helium at negative pressures
Path integral Monte Carlo (PIMC) simulations of liquid helium at negative
pressure have been carried out for a temperature range from the critical
temperature to below the superfluid transition. We have calculated the
temperature dependence of the spinodal line as well as the pressure dependence
of the isothermal sound velocity in the region of the spinodal. We discuss the
slope of the superfluid transition line and the shape of the dispersion curve
at negative pressures.Comment: 6 pages, 7 figures, submitted to Physical Review B Revised: new
reference, replaced figure
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