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  • 1
    Electronic Resource
    Electronic Resource
    Transport of a passive scalar at a shear-free boundary in fully developed turbulent open channel flow (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 11 (1999), S. 2607-2625 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct numerical simulations of fully developed turbulence in an open channel geometry were performed in which a passive scalar was introduced. The simulations were intended to explore transport at free surfaces in two cases for which (1) the free surface was maintained at constant temperature and (2) the interfacial flux was fixed. These cases can be considered models for mass and evaporative heat transport where buoyancy and surface deformation effects are negligible. Significant differences were found in the thermal fields in these two cases. The turbulent statistics reveal that the surface flux in the constant temperature case was significantly more intermittent compared to the surface temperature field in the constant flux case. The surface temperature field in the latter case formed large patches of warm fluid, reminiscent of the so-called fish scale patterns revealed in recent infrared imagery of the air–water interface. The wake-like structure of the patches was evident despite the absence of surface shear. A model of surface renewal based on the existence of two disparate time scales (a fast hydrodynamic scale, and a slow, diffusional scale) was introduced to explain these differences in a heuristic manner. The model appears successful in explaining, in a qualitative sense, the surface thermal structure in each case. Correlations between the surface thermal fields (flux or temperature) and the subsurface hydrodynamics were also computed. A model based on the hypothesis that hairpin eddies are the dominant kinematic structure responsible for surface renewal is consistent with the correlations. However, these results cannot rule out the importance of other turbulent structures in free surface heat and mass transport processes. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870123
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  • 2
    Electronic Resource
    Electronic Resource
    Differential diffusion in low Reynolds number water jets (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 10 (1998), S. 1135-1146 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental data on differential diffusion between two species with large and quite disparate Schmidt numbers were obtained in a turbulent water jet by optically measuring the two species concentrations simultaneously. Experimental conditions were chosen so that the species were dilute and did not affect the water density thereby avoiding inertial effects. Differential diffusion was found to be significant in magnitude, even in the absence of these effects. Schmidt number ratios of 4 and 18 were considered. Differential diffusion was found to be statistically significant and to manifest at scales larger than the computed Batchelor scale. In some instances the concentration signal for the species with larger diffusivity was simply a blurred version of the other, while in other instances structures present in one signal were completely absent from the other. This second observation, presumably a more complex effect due to diffusion across velocity gradients, is discussed. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869638
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  • 3
    Electronic Resource
    Electronic Resource
    Infrared imaging of the surface temperature field of water during film spreading (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 12 (2000), S. 597-602 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Deposition of a spontaneously-spreading film on a clean water surface creates a front which propagates radially outward from the point of deposition. This rapidly spreading film was used as a tool to quickly change the boundary condition of a water surface from one which is shear-free, to a boundary condition which supports shear. Infrared images of a water surface experiencing evaporative convection were recorded as this film spread. These images were converted to surface temperature fields. The amount of turbulent structure present in these fields changes dramatically across the front. Ahead of the front, significant variations at large and small spatial scales are evident, while behind the front the small scale structures are eliminated. The time scale at which this damping occurs is short and has not been reported on heretofore. In addition to being relevant to free surface turbulence, these results demonstrate the utility of infrared imaging in the study of spreading films.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870265
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  • 4
    Electronic Resource
    Electronic Resource
    Conditional scalar dissipation rates in turbulent wakes, jets, and boundary layers (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3207-3215 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The expected value Eχ≡E(χ,θ=θ0) of the dissipation rate χ of a passive scalar θ conditioned on the scalar value θ=θ0 has been measured in three varieties of turbulent shear flows: heated wakes, dyed liquid jets, and the atmospheric surface layer. The quantity Eχ depends fairly strongly on θ0 and on the flow. For the wake, Eχ exhibits two peaks—one on the low-temperature end and the other on the high-temperature end—and the peaks are separated by an approximately flat region. The relative strength of the two peaks varies with the spatial position. Measured in the turbulent part alone, Eχ tends to have only one peak on the hot side, but is still nonuniform. The related quantity, Eθ‘≡(∇2θ,θ=θ0), which is the expected value of the Laplacian of the scalar conditioned on the scalar concentration, has also been measured on the wake centerline and shows a simpler dependence on θ0 than Eχ. For jets, Eχ has a single peak on the high-concentration side. This feature appears to be essentially independent of the use of Taylor's hypothesis and on whether or not the dissipation rate χ is approximated by only one of its components. It is, however, sensitive to the resolution of measurement. For the temperature fluctuation in the atmospheric boundary layer, the peak in Eχ on the cold side is far weaker than that on the hot side. From this combination of experiments, it is argued that the different shapes of Eχ in different flows are related to differences in the nature of the scalar pdf itself and, for the high-Schmidt-number dyes in water flows, on whether or not the finest scales of the scalar are resolved.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858677
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  • 5
    Electronic Resource
    Electronic Resource
    Gas transport across an air/water interface populated with capillary waves (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 2529-2541 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An experimental study of gas transport across an air/water interface, populated by a field of standing capillary waves is presented. The experiments were conducted in a small tank containing distilled water, enriched with carbon dioxide. The capillary waves were of the Faraday type, generated by providing a small vertical vibration to the water tank. The frequency of excitation was varied from 200 to 400 Hz, giving wavelengths from 3.62 to 2.26 mm (linear estimate). The gas transport rate across the interface increased by almost two orders of magnitude as the wave slope was increased from zero to slightly above 0.2 m/m. A unique aspect of these experiments is that capillary waves were isolated from the obfuscating effects of turbulence, aerosol generation, and other phenomena typically present in wind/wave tunnel experiments. Consequently the large enhancement in gas transfer was due to the effects of capillary waves alone, demonstrating their importance in gas exchange processes. The maximum mass transfer coefficients obtained in these experiments are not achieved in typical wind/wave tunnel experiments below wind speeds of 10 m/s. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869370
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  • 6
    Electronic Resource
    Electronic Resource
    Determining liquid substrate cleanliness using infrared imaging (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 4408-4414 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Experiments conducted in a modified Langmuir trough are described. Measurements of surface tension were obtained using a Wilhelmy plate, while infrared (IR) images of the water surface adjacent to the Wilhelmy plate were simultaneously recorded. A contaminating surfactant film was allowed to form on the surface while the experiments were being conducted. The data reveal a substantial change in the IR imagery due to the film in all cases. The difference between the appearance of the clean and surfactant-covered regions of the surface is clear and distinct. For some of these experiments the change in the IR imagery is accompanied by no observable change in surface tension. This, along with several other aspects of this work, suggest that IR imagery may be a superior tool, compared to traditional surface tension measurements, for ascertaining the cleanliness of a liquid substrate. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1416106
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  • 7
    Electronic Resource
    Electronic Resource
    An experimental investigation of the surface temperature field during evaporative convection (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 13 (2001), S. 428-439 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Measurements of the surface temperature field are presented for a water surface undergoing evaporation. These temperature fields were measured using an infrared camera for a range of heat fluxes q″=30–500 W/m2. Experiments were conducted for water surfaces with and without a surfactant monolayer. A statistical analysis of the data is presented which shows the effect of heat flux and surfactants on the root mean square and skewness of the field. The data reveals a linear increase in the rms with increasing heat flux, which is similar for clean and surfactant conditions. In contrast, the skewness is markedly different for the clean and surfactant-covered cases. For clean surface conditions, the skewness attains large, negative values, becoming increasingly negative as q″ increases. When the surface is covered with a surfactant monolayer, however, the skewness exhibits small, negative values which approach zero as the heat flux increases. This behavior is reflected in the pdf which is clearly asymmetric in the clean case and virtually symmetric in the surfactant case. A physical mechanism is presented to explain these results. Temporal power spectra are presented which reveal the role of heat flux and surfactants on the temporal evolution of the surface temperature field. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1337064
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  • 8
    Electronic Resource
    Electronic Resource
    Capillary wave gas exchange in the presence of surfactants (1999)
    Springer
    Experiments in fluids 27 (1999), S. 332-338 
    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  The effect of surfactants on gas exchange across an air/water interface populated with capillary waves, is considered. Experiments were conducted on capillary waves having a wavelength of 2.87 mm in the presence of oleyl alcohol and stearic acid, as well as on surfaces which were surfactant-free. The presence of these surfactants decreased the gas exchange rate by at most a factor of two when the energy delivered to the tank was held constant. Thus, even in the presence of surfactants, pure capillary waves still caused significant gas exchange, indicating that partially damped capillary waves may play an important role in air/sea gas exchange. When the gas exchange coefficient was plotted as a function of mean square slope, the presence of surfactants was found to negligibly affect the gas exchange rate, with the possible exception of the high wave slope regime for stearic acid. This result suggests that it is principally the kinematics of wave motion which accounts for the enhancement of transport due to the capillary waves investigated here. Moreover, these results agree with those obtained from polychromatic, wind-generated waves, suggesting that, for non-breaking waves, knowledge of the statistics of the wave field may be all that is required to parameterize the gas exchange coefficient.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003480050357
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  • 9
    Electronic Resource
    Electronic Resource
    Photobleaching of disodium fluorescein in water (1995)
    Springer
    Experiments in fluids 18 (1995), S. 445-447 
    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 Photobleaching of disodium fluorescein dissolved in water is experimentally investigated using laser induced fluorescence (LIF). It is demonstrated that significant photobleaching occurs on the millisecond time scale, resulting in a large decrease in the fluorescence signal emanating from a constant concentration sample. The importance of avoiding photobleaching when using LIF with disodium fluorescein for concentration measurements in water flow experiments is demonstrated. A half-life for photobleaching is introduced and measured for disodium fluorescein and is shown to be a more appropriate measure than the traditional ‘bleaching quantum efficiency’. It is demonstrated that the photobleaching of disodium fluorescein is at least partially reversible.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00208467
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  • 10
    Electronic Resource
    Electronic Resource
    Measurement of surfactant properties using a circular capillary wave field (2000)
    Springer
    Experiments in fluids 29 (2000), S. 509-518 
    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  Measurements of the decay coefficient are presented for three different surfactants as a function of surface concentration. These measurements were obtained using a circular field of capillary waves. Data was purposely obtained close to the center of the tank, where the wave amplitude is large, and meniscus effects are expected to be small. The theory for linear, standing waves in a circular geometry is developed, without the usual assumption |kr|≫1. The surfactants investigated were: stearic acid, oleyl alcohol and hemicyanine. Measurements of the decay coefficient for hemicyanine have not been obtained heretofore. Several intrinsic benefits of the circular geometry are noted. The potential for improved accuracy in the measurement of surfactant properties using this geometry is suggested.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003480000119
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