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  • 1
    Electronic Resource
    Electronic Resource
    Spectral analysis of hydrodynamic tracer dispersion with an electrochemical measurement technique (1989)
    Springer
    Experiments in fluids 7 (1989), S. 329-334 
    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 We report tracer dispersion measurements in a capillary tube performed in the frequency domain using an electrochemical technique. Tracer Fe+++ ions are produced by oxidizing Fe++ ions at an emission anode; the inverse reaction allows to detect the tracer on a measurement electrode at the outlet of the sample. The amplitude and phase of the steady state signal detected at the outlet of the sample are measured as a function of the frequency of a sinusoidal concentration modulation induced at the inlet of the tube. Measurement results at two flow velocities are in agreement with predictions of the Taylor-Aris model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00198451
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  • 2
    Electronic Resource
    Electronic Resource
    Tracer dispersion in porous media with a double porosity (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 46-57 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Tracer dispersion in double porosity packings of porous grains obtained by grinding sintered glass samples of various porosities and permeabilities is studied experimentally. The dispersivity lD increases faster with velocity than for single porosity packings: this variation is predicted by modeling the medium as a one-dimensional (1-D) sequence of identical cells with one slow and one fast path in parallel. The grain size and the permeability contrast between the inside and the outside of the grains can also be determined. At low grain porosities, the transit time τi inside individual grains becomes of the order of the mean transit time across the whole sample and non-Gaussian dispersion curves are observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858788
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  • 3
    Electronic Resource
    Electronic Resource
    Tracer dispersion in 2-D fractures with flat and rough walls in a radial flow geometry (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1952-1962 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Tracer dispersion is studied in a plane fracture geometry with a radial flow between closely spaced parallel walls with either two smooth surfaces or one smooth and one rough surface. An echo dispersion technique in which the fluid is first injected into the fracture during a time Tinv and then pumped back through a detector is used in the experiments and is complemented by Monte-Carlo-type numerical simulations. For the smooth wall case, the Taylor dispersion mechanism is dominant when longitudinal molecular diffusion is negligible: it is verified numerically and experimentally that its transition to irreversibility only depends on the ratio of Tinv to the transverse diffusion time τm across the fracture thickness. In addition, the variation of Δ¯T¯2/(4Tinvτm) with respect to Tinv/τm is the same as for a flow of parallel geometry (Δ¯T¯2 being the mean square deviation of the transit time). Longitudinal molecular diffusion increases the global dispersion like Tinv/(PeQτm) at low Péclet numbers PeQ and long times due to the longitudinal velocity gradient. When one of the walls is rough, one expects to have a geometrical dispersion locally proportional to the velocity. A corresponding linear variation of Δ¯T¯2/(4Tinvτm) with respect to (square root of)Tinv/PeQ is observed experimentally.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858822
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  • 4
    Electronic Resource
    Electronic Resource
    Tracer dispersion in rough channels: A two-dimensional numerical study (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1333-1343 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Numerical results are presented on the hydrodynamic dispersion of passive tracer in flow through a two-dimensional channel bounded by parallel plates, one having substantial rugosities perpendicular to the flow. The simulation is based on the numerical solution of the microscopic Stokes and convention-diffusion equations, over a range of Péclet numbers and of degrees of rugosity of the channel. One observes a tenfold increase of the effective dispersion between the smooth plate case (classical Taylor dispersion) and the maximum rugosity case, attributable to the trapping of tracer inside deep rugosities. The results are analyzed in terms of tracer exchange models, involving a characteristic exchange time between the low- and high-velocity zones and their relative volume fractions, as a function of the Péclet number and the relative depth of the rugosities. The simulations are compared to "echo'' dispersion experiments in which the tracer is first injected into the model and then, upon reversal of the fluid flow, drawn back through the inlet.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858569
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  • 5
    Electronic Resource
    Electronic Resource
    Lattice-gas automata: A model for the simulation of dispersion phenomena (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 507-512 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dispersion of a tracer in two-dimensional (2-D) parallel flow between two parallel plates has been studied numerically using a lattice-gas model with two different species of particles. After a stepwise change of concentration at one end of the model, the tracer distribution in the flow evolves theoretically, as predicted, toward a Gaussian profile in a time lapse in agreement with the predictions of the Taylor model. The mean square width of the front increases linearly with time after the stabilization period, as required for a diffusive spreading process. The longitudinal dispersion coefficient D(parallel) has been determined in a range of Peclet number values Pe between 4.3 and 35.4. It varies as the square of the velocity, in agreement with the Taylor–Aris model; the molecular diffusion coefficient value (Dm=0.62 in lattice and simulation step units) obtained from the proportionality coefficient is in good agreement with the values obtained by other independent methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857421
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  • 6
    Electronic Resource
    Electronic Resource
    Experimental measurement of dispersion processes at short times using a pulsed field gradient NMR technique (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 481-490 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Dispersion at short times is studied using a PFG-NMR (pulsed field gradient NMR) technique inside a fixed bed of nonconsolidated spherical beads saturated with water flowing at a constant velocity. This allows measurement of the probability distribution of the displacement of water molecules along the magnetic field gradient during a preset measurement time Δ: the mean displacement of the water molecules is varied between 0.1 and 7.3 times the bead diameter by varying Δ between 20 and 100 ms and the bead diameter between 800 and 81 μm. At short times, the displacement of the molecules is small enough so that the local displacement is proportional to the local velocity component along the magnetic field gradient. At mean displacements larger than 5 bead diameters, the displacement distribution is Gaussian and centered about the mean displacement; the width of the distribution corresponds to the macroscopic dispersion coefficient as measured by other techniques. At intermediate displacements, this distribution displays two peaks corresponding to a combination of the two processes. The main features of this transition can be reproduced by a simple Monte-Carlo simulation modeling the porous medium as a set of finite length tubes with random orientations. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869208
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  • 7
    Electronic Resource
    Electronic Resource
    Transition to irreversibility for the dispersion of a tracer in porous media (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 681-687 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The transition from reversibility to irreversibility for tracer dispersion in an unconsolidated porous medium made of 2 and 4 mm diam glass beads has been studied; a low dead volume electrochemical technique has been used to produce and detect the tracer. It is shown that the transition occurs over a small number of layers (ten) at the inlet of the sample; this distance depends only on the ratio of the penetration distance and the bead diameter and varies very little with the Peclet number. For large penetration distances, the dispersion length values obtained in classical transmission dispersion measurements are recovered (dispersion has become fully irreversible). At very small distances, dispersion is largely reversible and dispersion coefficient values 20 or 80 times smaller than the asymptotic value have been measured.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857721
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  • 8
    Electronic Resource
    Electronic Resource
    Experimental study of tracer dispersion in sintered glass porous materials of variable compaction (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 1690-1698 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental measurements of tracer dispersion in sintered glass samples of variable compaction are reported. The results are compared to measurements in nonconsolidated packings of the same glass beads used to prepare the sintered material. While normal "Gaussian'' dispersion is obtained in nonconsolidated samples (37% porosity), clear-cut anomalies (such as dispersion "tails'' at long times) are observed in sintered materials even with a very low degree of compaction (34% porosity). Experimental curves are analyzed with a "capacitance''-type model. The effective dispersion coefficient increases by a factor of 60 when the porosity decreases from 37% to 14% at higher degrees of compaction. The characteristic exchange time between the low-velocity regions and the main flow varies linearly with the transit time through the sample. These results are interpreted by the presence of inhomogeneities with a size of several grain diameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866234
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  • 9
    Electronic Resource
    Electronic Resource
    "Echo'' tracer dispersion in porous media (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 1341-1347 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The reversibility of tracer dispersion in porous media is investigated experimentally using a "dispersion echo'' technique in which tracer is injected into the sample and then pumped back. In homogeneous materials, the "echo'' dispersion coefficient is the same as for "transmission'' measurements at all Peclet numbers, and the dispersion is fully irreversible. In heterogeneous materials (giving non-Gaussian transmission curves), the dispersion is partly reversible; "echo'' dispersion curves are Gaussian with a dispersion coefficient lower than in transmission experiments. It is shown that the dispersion irreversibility corresponds to the combined effects of geometrical disorder in the flow field and of molecular diffusion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857309
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  • 10
    Electronic Resource
    Electronic Resource
    Pulsed gradient NMR measurements and numerical simulation of flow velocity distribution in sphere packings (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 293-301 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The displacement of water molecules associated with the flow of water inside a nonconsolidated packing of 800 μm OD glass spheres has been measured by a pulsed gradient NMR technique. Using a stimulated spin-echo sequence, mean displacements of up to 300 μm corresponding to measurement times of up to 200 ms can be analyzed. The measurement can be quantitatively calibrated using the pure molecular self-diffusion of water at zero flow conditions. For molecular displacements much smaller than the pore size, the distribution of the flow velocity component along the mean flow direction is determined at Reynolds numbers high enough so that longitudinal molecular diffusion is negligible. An exponential decay of the probability distribution of the displacements is observed at large distances. The results are very similar to those obtained by numerical solution of the Stokes equation in random sphere packings. At longer displacement distances, a secondary peak of the displacement distribution is observed: It is interpreted as the first step toward the transition toward classical dispersion at displacements much larger than the pore size. The influence of molecular diffusion and of the heterogeneities of the magnetic permeability also are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868839
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