397 research outputs found
Comparison between an integral and algebraic formulation for the eddy diffusivity using the Copenhagen experimental dataset
In this work an algebraic formulation to evaluate the eddy diffusivities in the Convective Boundary Layer (CBL) is derived. The expression depends on the turbulence properties (z height dependence) and the distance from the source. It is based on the turbulent kinetic energy spectra and Taylor’s statistical diffusion theory. It has been tested and compared through an experimental dataset, with
another complex integral formulation taken from the literature. The Agreement between the complex integral formulation and simple algebraic expression points out that this new parameterization is valid and can be used as a surrogate for eddy diffusivities in the inhomogeneous convective turbulence present in the CBL. The validation shows that the proposed algebraic vertical eddy diffusivity is suitable for application in advanced air quality regulatory models
Structural and optical studies of FeSb2 under high pressure
Nanostructured orthorhombic FeSb2 and an amorphous phase were formed by
mechanical alloying starting from a mixture of high purity elemental Fe and Sb
powders. The effects of high pressures on structural and optical properties
were studied using X-ray diffraction (XRD) and Raman spectroscopy (RS). XRD
patterns showed the presence of the orthorhombic FeSb2 phase up to the maximum
pressure applied (28.2 GPa). The XRD patterns showed also an increase in the
amount of the amorphous phase with increasing pressure up to 23.3 GPa. At 14.3
GPa, together with the former phases, a new phase was observed and indexed to a
tetragonal FeSb2 phase, but its volume fraction is small at least up to 23.3
GPa. For the orthorhombic FeSb2 phase, the pressure dependence of the volume
fitted to a Birch-Murnaghan equation of state gave a bulk modulus = 74.2 +- 3.0
GPa and its pressure derivative = 7.5 +- 0.6. RS measurements were performed
from atmospheric pressure up to 45.2 GPa. For the orthorhombic FeSb2 phase, the
Raman active mode was observed up to the maximum pressure applied, while the
mode disappeared at 16.6 GPa. For pressures higher than 21 GPa, the Raman
active mode of a tetragonal FeSb2 phase was observed, confirming ab initio
calculations reported in the literature.Comment: 31 pages, 11 figures and 2 tables. Already submitted for publicatio
Synchrotron radiation photoionization mass spectrometry of laser ablated species
The present paper describes an experimental apparatus suitable to create and study free clusters by combining laser ablation and synchrotron radiation. First tests on sulfur samples, S, showed the production, through laser ablation, of neutral Sn clusters (n = 1–8). These clusters were ionized using synchrotron radiation at photon energies from 160 eV to 175 eV, across the S 2p core edge. The feasibility of such combined ablation–synchrotron radiation experiments is demonstrated, opening new possibilities on the investigation of free clusters and radical
Turbulent dispersion from tall stack in the unstable boundary layer: a comparison between Gaussian and K-diffusion modelling for non buoyant emissions
Most air quality dispersionmo dels used for regulatory applications are based onGaussianan d K-diffusionform ulations. The reliability of such models strongly depends on how dispersion parameters and eddy diffusivities are computed on the basis of the update understanding of the Planetary Boundary Layer (PBL) meteorology. In this paper, we compare the performances in simulating pollutants
released from continuous point source, by using some Gaussian and K-diffusion models with different assumptions concerning the parameterisation of the dispersionpro
cesses. Results show that the Gaussianmo del, inwhic h the dispersion parameters are directly related to spectral peak of turbulence energy, gives the best overall performances. This could be due to a more realistic description of spreading processes occurring into the PBL. This suggests that, in the context of the regulatory applications, this model cangiv e the best combinationb etweengroun d level
concentration estimates and computer requirements
Creation of NOON states by double Fock-state/Bose-Einstein condensates
NOON states (states of the form where and
are single particle states) have been used for predicting violations of
hidden-variable theories (Greenberger-Horne-Zeilinger violations) and are
valuable in metrology for precision measurements of phase at the Heisenberg
limit. We show theoretically how the use of two Fock state/Bose-Einstein
condensates as sources in a modified Mach Zender interferometer can lead to the
creation of the NOON state in which and refer to arms of the
interferometer and is the total number of particles in the two condensates.
The modification of the interferometer involves making conditional ``side''
measurements of a few particles near the sources. These measurements put the
remaining particles in a superposition of two phase states, which are converted
into NOON states by a beam splitter. The result is equivalent to the quantum
experiment in which a large molecule passes through two slits. The NOON states
are combined in a final beam splitter and show interference. Attempts to detect
through which ``slit'' the condensates passed destroys the interference.Comment: 8 pages 5 figure
Linear Response, Validity of Semi-Classical Gravity, and the Stability of Flat Space
A quantitative test for the validity of the semi-classical approximation in
gravity is given. The criterion proposed is that solutions to the
semi-classical Einstein equations should be stable to linearized perturbations,
in the sense that no gauge invariant perturbation should become unbounded in
time. A self-consistent linear response analysis of these perturbations, based
upon an invariant effective action principle, necessarily involves metric
fluctuations about the mean semi-classical geometry, and brings in the
two-point correlation function of the quantum energy-momentum tensor in a
natural way. This linear response equation contains no state dependent
divergences and requires no new renormalization counterterms beyond those
required in the leading order semi-classical approximation. The general linear
response criterion is applied to the specific example of a scalar field with
arbitrary mass and curvature coupling in the vacuum state of Minkowski
spacetime. The spectral representation of the vacuum polarization function is
computed in n dimensional Minkowski spacetime, and used to show that the flat
space solution to the semi-classical Einstein equations for n=4 is stable to
all perturbations on distance scales much larger than the Planck length.Comment: 22 pages: This is a significantly expanded version of gr-qc/0204083,
with two additional sections and two new appendices giving a complete,
explicit example of the semi-classical stability criterion proposed in the
previous pape
Completeness of Communicable Disease Reporting, North Carolina, USA, 1995–1997 and 2000–2006
Reporting proportions were <50% for 49 of the 53 diseases evaluated
A new staggered algorithm for thermomechanical coupled problems
This study presents a new staggered coupled strategy to deal with thermomechanical problems. The proposed strategy is based on the isothermal split methodology, i.e. the mechanical problem is solved at constant temperature and the thermal problem is solved for a fixed configuration. Nevertheless, the procedure for this strategy is divided into two phases within each increment: the prediction and the correction phases, while the interchange of information is performed on both. This allows taking advantage of automatic time-step control techniques, previously implemented for the mechanical problem, which is the main feature that distinguishes it from the classical strategies. The aim of the proposed strategy is to reduce the computational cost without compromising the accuracy of the results. The new coupling strategy is validated using three numerical examples, comparing its accuracy and performance with the ones obtained with the classical (commonly employed) strategies for solving thermomechanical problems. Moreover, the influence of the time-step size on the accuracy is analysed. The results indicate that the proposed strategy presents accuracy close to the one obtained with the implicit coupling algorithm, while the computational cost is only slightly higher than the one required by the explicit strategy. (C) 2017 Elsevier Ltd. All rights reserved.The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) under projects P2020-PTDC/EMS-TEC/0702/2014 (POCI-01-0145-FEDER-016779) and P2020-PTDC/EMS-TEC/6400/2014 (POCI-01-0145-FEDER-016876) by UE/FEDER through the program COMPETE 2020. The second author is also grateful to the FCT for the Postdoctoral grant SFRH/BPD/101334/2014.info:eu-repo/semantics/publishedVersio
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