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  • Articles: DFG German National Licenses  (21)
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  • Articles: DFG German National Licenses  (21)
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  • 11
    Electronic Resource
    Electronic Resource
    The role of mixing in traveling-wave convection (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1468-1468 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In binary fluid mixtures with negative separation ratio, the conduction state has a vertical concentration gradient, due to the Soret effect, which opposes the destabilizing effect of the vertical temperature gradient. The first convective states are traveling waves (TW) with a phase velocity which is much lower than that of the linear instability. Recent perturbation theory calculations and numerical simulations of this nonlinear, traveling-wave state predict that the linear concentration gradient in the conduction state is eliminated in the interior of the fluid by convective mixing, and the concentration gradient remains only in the boundary layers. It is the persistence of the concentration gradient in these boundary layers which leads to the small but nonzero wave speed. As the Rayleigh number R is increased, convective stirring of the mixture decreases the concentration boundary layers, and the TW phase speed goes continuously to zero. We report experimental studies of these nonlinear, traveling-wave states in ethanol/water mixtures in an annular geometry. The measured TW phase speed as a function of R is in excellent agreement with the theoretical predictions over the entire TW branch. The experiments also confirm that this transition to stationary convection is not hysteretic. The role of mixing and the transport of concentration in the dynamics of nonlinear fronts and pulses observed in this system will also be discussed. [Work supported by DARPA URI Contract No. N00014-86-K-0758.]
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858002
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  • 12
    Electronic Resource
    Electronic Resource
    Spatial coupling in heterogeneous catalysis (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 8209-8215 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Spatial coupling mechanisms are studied in the heterogeneous catalytic oxidation of carbon monoxide over platinum at atmospheric pressure under oscillatory conditions. Experiments are conducted in a continuous flow reactor, and the reaction rate is monitored using both infrared imaging and thermocouples. The catalysts are in the form of platinum annular thin films on washer-shaped quartz substrates, and they provide highly repeatable oscillatory behavior. Oscillations are typically spatially synchronized with the entire catalyst "flashing'' on and off uniformly. Spatial coupling is investigated by introducing various barriers which split the annular ring in half. Infrared images show that coupling through the gas phase dominates coupling via the diffusion of CO on the surface or heat diffusion through the substrate. The introduction of a localized heat perturbation to the catalyst surface does not induce a transition in the reaction rate. Thus, it is likely that the primary mode of communication is through the gas-phase diffusion of reactants. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470185
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    Paper (German National Licenses)
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  • 13
    Electronic Resource
    Electronic Resource
    Spatio-temporal dynamics of oscillatory heterogeneous catalysis: CO oxidation on platinum (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8614-8625 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reaction-rate oscillations in the oxidation of carbon monoxide on the surface of platinum catalysts are studied in a continuous flow reactor at atmospheric pressure using infrared imaging. Small-amplitude temperature oscillations (0.2–8 K) result in approximately isothermal conditions, where changes in rate constants, for typical activation energies and temperatures, are small. The catalysts are in the form of platinum thin films on quartz substrates and provide highly repeatable oscillatory behavior. The platinum films are fabricated in the form of annular rings which provide a quasi-one-dimensional geometry in order to simplify comparison to theoretical models. Time-series measurements by means of thermocouples are used to characterize the oscillations. The infrared images show that most oscillations are spatially synchronized to within the 0.25 s time resolution of the experiment. The images also show that "fine structure'' oscillations (i.e., small-amplitude, high frequency oscillations superimposed on larger-amplitude waveforms) are associated with spatially desynchronized patterns. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468963
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  • 14
    Electronic Resource
    Electronic Resource
    Erratum: "Effects of local reactant concentration perturbations in oscillatory catalysis" [J. Chem. Phys. 108, 5565 (1998)] (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1619-1619 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480659
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  • 15
    Electronic Resource
    Electronic Resource
    Effects of local reactant concentration perturbations in oscillatory catalysis (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 5565-5570 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reaction-rate oscillations in CO oxidation over heterogeneous platinum thin-film catalysts were studied at atmospheric pressure where spatial coupling is due to gas-phase diffusion of reactant concentration gradients. The catalyst was perturbed locally by the introduction of CO directly above the surface, while the resulting spatiotemporal behavior was monitored using infrared imaging. The transient response of the system was studied in both the steady-state and oscillatory regimes, and comparisons are made between the observed behavior and predictions from models for the oscillations. Using these perturbations, global behavior was initiated using a localized effect. Similarities and differences between the induced oscillations and those occurring naturally are discussed, as are tests to vary the phase of the oscillatory cycle. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475946
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  • 16
    Electronic Resource
    Electronic Resource
    Observation of parametric instabilities in the lower-hybrid range of frequencies in the high-density tokamak (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 983-994 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Parametric decay processes have been studied using rf probes and CO2 laser scattering during the lower-hybrid wave heating and current drive experiments in the Alcator C tokamak. The most important process is believed to be the nonresonant decay into ion-cyclotron quasimodes and/or that into electron-Landau quasimodes. At lower densities ω0/ωlh(0)〉2 [where ω0 is the frequency of the injected pump wave and ωlh(0) is the lower-hybrid frequency evaluated at the plasma center], where efficient current drive and electron heating are obtained, parametric decay is absent or very weak. At higher densities [ω0/ωlh(0) (approximately-less-than)2] strong parametric decay is observed which correlates well with ion tail formation near the plasma edge. Above these densities the electron tail is no longer formed and no significant heating or current drive have been obtained. Parametric decay may be responsible, at least partially, for loss of wave power near the plasma periphery.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865070
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  • 17
    Electronic Resource
    Electronic Resource
    Light scattering studies of the ion acoustic instability in a positive column plasma (1985)
    Springer
    Journal of statistical physics 39 (1985), S. 755-767 
    Details
    ISSN: 1572-9613
    Keywords: Light scattering ; plasma instability ; ion-acoustic waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Light scattering is used to study the amplitude, spectrum, and angular distribution of the saturated state of the ion acoustic instability in a He positive column plasma. The ion acoustic waves are driven unstable by the electron current in the column. The properties of the saturated state are studied as a function of the concentration of hydrogen impurities which are found to be present in positive column plasmas. At concentrations of a few percent, the hydrogen ions can cause linear wave damping. Their role in saturating the instability by nonlinear processes is studied by varying the hydrogen concentration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01008364
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  • 18
    Electronic Resource
    Electronic Resource
    Pinning and long-time-scale behavior in traveling-wave convection (1991)
    Springer
    Journal of statistical physics 64 (1991), S. 903-912 
    Details
    ISSN: 1572-9613
    Keywords: Convection ; traveling waves ; pattern formation and dynamics ; binary fluid mixtures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We study nonlinear traveling-wave (TW) and stationary states of convection in experiments in ethanol-water mixtures. While the TW phase velocity as a function of Rayleigh number has been recently shown to be in agreement with the predictions of theory and numerical calculations, we find that this velocity is temporally modulated at frequencies corresponding to the travel time of a single convection roll and of a roll pair past a point stationary in the convection cell. This modulation could be due to the pinning of the convection pattern by experimental inhomogeneities. For large Rayleigh numbers where stationary overturning convection is expected, we sometimes observe extremely slow unidirectional TW states. For larger Rayleigh numbers, this slow TW state starts and stops intermittently on a characteristic time scale of several days. The possible origin of these phenomena and their potential utility are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01048803
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  • 19
    Electronic Resource
    Electronic Resource
    Accumulation and storage of low energy positrons (1989)
    Springer
    Hyperfine interactions 44 (1989), S. 185-200 
    Details
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We describe an experiment in progress which is designed to efficiently accumulate and store positrons from a radioactive source. The potential uses of such a collection of positrons is briefly discussed, as well as the limits of these accumulation and storage methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02398669
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  • 20
    Electronic Resource
    Electronic Resource
    Annihilation rates of positrons on aromatic molecules (1994)
    Springer
    Hyperfine interactions 89 (1994), S. 271-278 
    Details
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have measured positron annihilation rates for a selection of hydrocarbon molecules based on six carbon atoms, including substituted benzenes. These measurements illustrate the importance of molecular symmetry in determining the annihilation rate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02064512
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