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  • American Institute of Physics (AIP)  (6)
  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 59 (1988), S. 783-786 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Experiments with an oscillatory vane in a wind tunnel reveal that the vane's oscillation frequency is directly proportional to the square root of the dynamic pressure of the flow. The linear relationship is observed for flow speeds up to the speed of sound in air, argon, and helium. The vane's oscillations are a measure of mass flow, or if used on an aircraft, of indicated air speed. In constant density media the oscillation frequency is directly proportional to flow speed, and a simple count of the oscillations is a measure of distance traveled. At given ambient temperature and pressure, the vane's frequency is the same at the speed of sound for all gases, irrespective of their molecular weight.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In order to test the reliability of plane-wave and Gaussian-orbital based DFT methods for calculating reaction energies and activation barriers, detailed calculations are performed for several reactions involving gas phase silanes and a simple model of H2 desorption from the Si(100)2×1 surface. This study is motivated in particular by apparent discrepancies between the results of cluster-model and slab-model calculations of the activation energy for H2 desorption from the Si(100)2×1 surface. The DFT results obtained with several different exchange-correlation functionals are compared with the results of calculations with the generally reliable QCISD(T) method and, where possible, with experiment. It is found that the functionals usually employed in plane-wave DFT calculations significantly underestimate the activation energies. The Becke3LYP functional, on the other hand, is found to give reaction and activation energies close to experiment and to those from QCISD(T) calculations. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 6175-6181 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A differential equation describing the behavior of an oscillatory vane for flow-speed measurements is obtained. It differs from the classical equation for an ordinary flow-direction vane in that it contains a positive feedback term representing the flipovers of the fin. No analytical solution to the equation exists, but an oscillatory vane's response may be determined to a good approximation by simple calculation of second-order parameters and by use of a relevant coefficient whose value has been obtained from a numerical solution and is provided here in a table for a variety of vanes. In agreement with experiments, the vane's oscillation frequency is shown to be directly proportional to flow speed, and furthermore, to have a square-root dependence on the density of the flowing medium, the physical dimensions of the vane, and the reciprocal of its moment of inertia.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
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  • 5
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In order to address the problem of three-body interactions in gas–surface scattering, we considered the collision of a He atom with the (0001) surface of graphite coated by a monolayer of Xe. To eliminate the uncertainties connected with errors in the two-body He–Xe interaction, we determined the latter by crossed-beam differential collision cross-section measurements performed at two energies (67.2 and 22.35 meV). These scattering data together with room-temperature bulk diffusion data are then fitted with a Hartree–Fock–dispersion–type function to yield an interaction potential that explains most of the properties of this system within the experimental errors and represents an improvement on previously published He–Xe potentials. Helium diffraction measurements are then carried out from the Xe overlayer and the dependence of the specular intensity from the angle of incidence is carefully determined. Further, a He–surface potential is constructed by adding together the following terms: (1) the He–Xe pairwise sum, (2) the long-range He–(0001)C interaction, (3) the three-body contribution generated by the Axilrod–Teller–Muto term, (4) the so-called surface-mediated three-body interaction He–Xe–(0001)C first considered by A. D. McLachlan [Mol. Phys. 7, 381 (1964)], and finally (5) a small correction which is meant to take into account the nonstationary nature of the surface. Using this potential, well-converged close-coupling scattering calculations are carried out, and their results compared with the data. In general, good agreement is obtained. The agreement can, however, be improved by (a) an increase of about 30% in the contribution of three-body forces, (b) the lowering of the He–graphite long-range attraction coefficient by about 15%, or (c) a reduction of the two-body interaction well depth of 1.6% (the experimental error) together with any combination of the factors under (a) and (b) reduced by an adequate amount. Elimination of the contribution of the graphite surface by studying Xe multilayers is hindered by the uncertainties in the "thermal correction'' [point (5) above] which, due to the multilayer increased "softness,'' becomes an appreciable source of uncertainty.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The binding energies and configurations for single Si adatoms on the Si(100) surface are investigated theoretically. Detailed comparisons between previously published and new calculations using classical potentials, semiempirical formulations, and density functional theory (DFT) are made. The DFT calculations used both the plane-wave-pseudopotential approach in a periodic slab geometry and the Gaussian-orbital based all-electron approach employing cluster geometries. In the local-density approximation excellent agreement between the cluster and slab results was obtained. Inclusion of gradient corrections to the exchange-correlation energy significantly improves absolute binding energies and changes relative energies by as much as 0.3–0.5 eV depending on the particular exchange-correlation functional used. Binding energies and relative energies obtained using the classical potentials disagree with the gradient corrected DFT energies at about the 0.6–0.9 eV level, and most find qualitatively different local minima from those found in the DFT calculations. The semiempirical approaches give results intermediate in quality between those of the classical potentials and the ab initio calculations. Analysis of the energies and binding site geometries provides insight into the shortcomings of some of the classical potentials. © 1995 American Institute of Physics.
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