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  • 1
    Electronic Resource
    Electronic Resource
    Time-resolved electron energy loss spectroscopy study of water desorption from Ag(011) (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7964-7971 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present time-resolved electron energy loss spectroscopy (EELS) results for the desorption of submonolayer coverages of H2O and D2O from Ag(011). The measurements were performed isothermally and effectively at constant coverage. At coverages above a few percent of a monolayer, we observe that the desorption is characterized by zero-order kinetics. That is, the desorption rate is independent of coverage. We hypothesize that this results from the formation of two-dimensional water islands on the surface at all coverages and temperatures measured. The desorption process is governed in part by the dissociation of water from the edges of these islands. Rapid diffusion on the surface maintains a quasiequilibrium between the molecules bound to islands and isolated adsorbed water molecules, thereby leading to pseudo-zero-order kinetics. This result is interpreted upon general thermodynamic grounds, and also using a simple kinetic model which leads to pseudo-zero-order desorption kinetics. An unusual hydrogen kinetic isotope effect is reported which lends support to our interpretation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457214
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  • 2
    Electronic Resource
    Electronic Resource
    Substrate-mediated dispersion interaction effects in the properties of a physisorbed gas (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 4152-4159 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory of substrate-mediated dispersion interactions is applied to analyze a set of quasiequilibrium measurements of adsorption isobars and desorption isotherms of the CH4/Ag(011) physisorption system. The theory provides for the construction of an adsorbate intermolecular potential from the gas phase potential, with the spacing between the metal surface and the adsorbed layer as the sole adjustable parameter. The best fits to the kinetic and thermodynamic data yield a consistent intermolecular potential, with a well depth of approximately two-thirds of the free space value. The limited dynamic range of the measurements does not permit a determination of the form of the intermolecular potential. However, the good agreement of the theory with both data sets lends strong support for the quasiequilibrium model, and supports the proposition that the properties of physisorbed layers can be predicted using gas phase potentials while accounting for substrate-mediated dispersion interaction effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466111
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  • 3
    Electronic Resource
    Electronic Resource
    Compressibility effects on large structures in free shear flows (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 1251-1258 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Space-time correlations were used to study compressibility effects on large structures in mixing layers. Two high Reynolds number mixing layers with Mc=0.51 (case 1) and 0.86 (case 2) were studied. The results indicate that the structures in case 1 are similar to those in the subsonic case, but less organized. The structures in case 2 are highly three dimensional, with a good spatial but a poor temporal organization. The streamwise correlations showed a decay rate 4 to 5 times greater for case 2 relative to case 1. While the spanwise correlations for case 1 showed trends similar to incompressible mixing layers, the behavior of case 2 was very different. The pressure fluctuations in the fully developed region of case 2 displayed significant rms variation in the spanwise direction with a well-defined pattern. Based on these measurements, the structures in case 2 seem to be of a horseshoe type, transversely spanning the mixing layer with the head in the low-speed side and the legs inclined in both the x-y and the x-z planes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858243
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  • 4
    Electronic Resource
    Electronic Resource
    Compressibility effects in free shear layers (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1231-1240 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High Reynolds number compressible free shear layers were studied experimentally to explore the effects of compressibility on the turbulence field. Previous preliminary results reported by the authors showed that the level and the lateral extent of turbulence fluctuations are reduced as the compressibility, which is characterized by a convective Mach number, is increased. The two convective Mach numbers used in the previous study were relatively close, Mc=0.51 and 0.64, and as a result the conclusions were not concrete. The present results with Mc=0.86 strongly support the earlier results, showing even higher reductions in the level and the lateral extent of Reynolds stresses. The higher-order moments of turbulence fluctuations such as skewness and flatness are reported, which show that the intermittency resulting from the excursion of large-scale structures into the free streams at the edge of shear layers was significantly reduced (both in the level and the extent) because of increased Mc. In the developing region of shear layers, development of mean flow and turbulence fluctuation profiles are reported that have similar trends seen in incompressible shear layers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857816
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  • 5
    Electronic Resource
    Electronic Resource
    On streamwise vortices in high Reynolds number supersonic axisymmetric jets (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 187-202 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Pitot pressure measurements and flow visualizations were used to investigate streamwise vortices previously observed in underexpanded jets. A simple model was developed, which gives reasonable agreement with the pressure measurements. A converging nozzle and converging–diverging nozzle of design Mach number 1.5 were used to generate jet flows of equivalent Mach numbers up to 2.5 (stagnation to ambient pressure ratios up to 17.1). By operating the nozzles fully expanded, overexpanded, and underexpanded, insight was gained into both the occurrence and cause for formation of the vortices. Spatially stationary streamwise vortices were found to exist in the near-field region around the circumference of underexpanded jets in the vicinity of the jet boundary. Short exposure visualizations show the vortices persist much farther downstream with a loss of spatial organization. Visualizations suggest adjacent vortices have streamwise vorticity of opposite sign, so the action of adjacent vortices is to either pump jet fluid radially outward or entrain ambient fluid radially inward toward the jet. The downstream extent, strength, and number of vortices around the jet circumference increase with degree of underexpansion. A large number of vortices is found near the nozzle exit. Fewer vortices of larger scale are found farther downstream, indicative of a merging process. The absence of the vortices in fully expanded and overexpanded jets suggests the vortices are a consequence of a Taylor–Goertler-type instability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858803
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  • 6
    Electronic Resource
    Electronic Resource
    Chaotic motion of an array of vortices above a flat wall (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 704-717 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The trajectories of three inviscid point vortices above a flat wall were investigated numerically. The behavior of the trajectories was characterized by detailed power spectra, phase portraits, Poincaré sections, and estimates of the Lyapunov exponent spectrum. For certain initial conditions, the trajectories are regular and confined to a two-torus in phase space. However, chaotic behavior that sets in as one of the vortices is initially placed closer to the unstable point of the system. In the chaotic regime, the system also exhibits large scale intermittency. Analytical expressions for the near- and far-field noise, generated by the motion of the vortices, were also obtained. It was found that in the presence of a flat boundary, this motion does not contribute to any dipole field noise radiation, but is characterized by a quadrupole structure to leading order. The near-field pressure was computed numerically and it was found that the chaotic behavior of the vortex trajectories results in broadband noise.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857364
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  • 7
    Electronic Resource
    Electronic Resource
    Streamwise structures in a turbulent supersonic boundary layer (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 1081-1083 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Flow visualizations in a high Reynolds number, Mach 3, fully developed turbulent boundary layer indicate that the upper half of the boundary layer is populated with elongated longitudinal structures. These structures are robust with considerable streamwise but very limited spanwise extent, and are randomly distributed in space and time. Possible mechanisms for the generation of these structures are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868279
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  • 8
    Electronic Resource
    Electronic Resource
    Dispersion compensation high-resolution electron energy-loss spectrometer for time-resolved surface studies (1991)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1256-1262 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new high-resolution electron energy-loss spectrometer based on the dispersion compensation (DC) concept has been designed, built, and tested. The parallel processing inherent in DC has allowed us to attain signal levels two orders of magnitude in excess of conventional designs while operating at comparable resolution. This is consistent with expectations based on simple model calculations. The resolving power of the spectrometer is not as good as theoretically predicted probably due to uncontrolled beam angle effects. Initial tests on a variety of systems are presented. We analyze the strengths and weaknesses of the design and suggest future improvements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1142008
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  • 9
    Electronic Resource
    Electronic Resource
    A molecular filter based velocimetry technique for high speed flows (1994)
    Springer
    Experiments in fluids 18 (1994), S. 107-118 
    Details
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A molecular absorption filter-based diagnostic technique has been developed to obtain instantaneous, planar velocity measurements in high speed flows. This paper presents the details of the technique and its application in high Reynolds number compressible mixing layers with convective Mach numbers of 0.51 and 0.86. Pressure broadening was used to “tune” the absorption profile of the filter to a given flow field/optical arrangement. It is shown that two filters can be used when background scattering is significant: one as the velocity-discriminating filter, the other to eliminate unwanted background light from the reference camera. Collected images show instantaneous variations in velocity within both the highly organized roller type structures of the M c = 0.51 mixing layer and the unorganized structures found in both mixing layers. An uncertainty analysis showed that the measurement uncertainty was ±8.6% for the current measurements, but improvements to reduce the uncertainty to ±3% are suggested.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00209367
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  • 10
    Electronic Resource
    Electronic Resource
    Two-component planar Doppler velocimetry in the compressible turbulent boundary layer (1998)
    Springer
    Experiments in fluids 24 (1998), S. 323-332 
    Details
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract  Non-intrusive Planar Doppler Velocimetry (PDV) has been extended to two-component measurements in a Mach 3, flat plate turbulent boundary layer and the distorted boundary layers downstream of 7° and 14° expansions. PDV results were compared to redundant Laser Doppler Velocimetry (LDV) measurements. Mean velocity results obtained with the two techniques agree to within ±5%. PDV measurements were obtained within 0.4 mm of the surface while LDV could be employed only to within approximately 2.0 mm, highlighting a near-wall resolution advantage for PDV. Effects including those associated with separate filtered and reference cameras led to PDV uncertainties of the same order as the encountered moderate turbulence intensities, precluding an investigation of instantaneous turbulence fields. Despite these difficulties, the current multi-component measurements in distorted, compressible boundary layers highlight the potential of PDV and represent progress in its ongoing evolution. Sources of error and improvements required for quantitative turbulence measurements are discussed. Further advances can be expected from ongoing development efforts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003480050179
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