Articles de revues : « Tracers dispersion »

1

Istók, Balázs, et Gergely Kristóf. « Dispersion and Travel Time of Dissolved and Floating Tracers in Urban Sewers ». Slovak Journal of Civil Engineering 22, n^o 1 (1 mars 2014) : 1–8. http://dx.doi.org/10.2478/sjce-2014-0001.

Texte intégral

Résumé :

Abstract Environmental impacts of oil spills affecting urban sewage networks can be eliminated if timely intervention is taken. The design of such actions requires knowledge of the transport of surface pollutants in open channels. In this study we investigated the travel time and dispersion of pollutants by means of tracer experiments in sewage networks and a creek. The travel time of surface tracers has been found to be significantly shorter than that of a bulk flow tracer. The ratio of the travel times of a bulk flow tracer and surface tracers agreed with the known correlations obtained for rivers. An increasing tendency in the ratio of travel times has been observed for increasing bulk flow velocity. A segment-wise dispersion model was implemented in the existing hydraulic model of a sewer system. The simulation results were compared with the experimental observations. The dispersion rate of the bulk flow tracer has been found to obey Taylor’s mixing theory for long channels and was more intensive than that of surface tracers in community sewage channels.

Styles APA, Harvard, Vancouver, ISO, etc.

2

Ayuba, Ibrahim, Lateef T. Akanji, Jefferson L. Gomes et Gabriel K. Falade. « Investigation of Drift Phenomena at the Pore Scale during Flow and Transport in Porous Media ». Mathematics 9, n^o 19 (7 octobre 2021) : 2509. http://dx.doi.org/10.3390/math9192509.

Texte intégral

Résumé :

This paper reports an analytical study conducted to investigate the behaviour of tracers undergoing creeping flow between two parallel plates in porous media. A new coupled model for the characterisation of fluid flow and transport of tracers at pore scale is formulated. Precisely, a weak-form solution of radial transport of tracers under convection–diffusion-dominated flow is established using hypergeometric functions. The velocity field associated with the radial transport is informed by the solution of the Stokes equations. Channel thickness as a function of velocities, maximum Reynolds number of each thickness as a function of maximum velocities and concentration profile for different drift and dispersion coefficients are computed and analysed. Analysis of the simulation results reveals that the dispersion coefficient appears to be a significant factor controlling the concentration distribution of the tracer at pore scale. Further analysis shows that the drift coefficient appears to influence tracer concentration distribution but only after a prolonged period. This indicates that even at pore scale, tracer drift characteristics can provide useful information about the flow and transport properties of individual pores in porous media.

Styles APA, Harvard, Vancouver, ISO, etc.

3

Davis, P. M., T. C. Atkinson et T. M. L. Wigley. « Longitudinal dispersion in natural channels : 2. The roles of shear flow dispersion and dead zones in the River Severn, U.K. » Hydrology and Earth System Sciences 4, n^o 3 (30 septembre 2000) : 355–71. http://dx.doi.org/10.5194/hess-4-355-2000.

Texte intégral

Résumé :

Abstract. The classical one-dimensional advection-diffusion equation (ADE) gives an inadequate description of tracer cloud evolution in the River Severn, U.K. A solute transport model incorporating the effects of tracer storage in dead zones is presented in which the channel is conceived as being divided into two parallel regions. The bulk flow region occurs in the central part. Its longitudinal dispersive properties are described by the ADE. Adjacent to this, an additional cross-sectional area is defined in which tracer can be stored temporarily in regions of slowly moving water called dead zones. Exchange between the two regions follows a first order rate equation. Applying the model to the River Severn shows that a dispersing cloud’s evolution occurs in two distinct stages with a rapid transitional phase. Initially, shear-dispersion is dominant while the tracer particles mix fully over the bulk flow. Once this has occurred, dead zone storage accounts well for the non-Fickian evolution of the cloud. After the transitional phase the dead zone storage mechanism clearly dominates over shear-dispersion. Overall, the combined shear flow dispersion – dead zone model (D-DZM) provides a much better, physically consistent description of the tracer cloud’s evolution than the simple classical ADE approach can do alone. Keywords: Channels; dispersion; dead zones; tracers; River Severn

Styles APA, Harvard, Vancouver, ISO, etc.

4

Richards, K. J., Y. Jia et C. F. Rogers. « Dispersion of Tracers by Ocean Gyres ». Journal of Physical Oceanography 25, n^o 5 (mai 1995) : 873–87. http://dx.doi.org/10.1175/1520-0485(1995)025<0873:dotbog>2.0.co;2.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

5

Lee, Mei-Man, A. J. George Nurser, Andrew C. Coward et Beverly A. de Cuevas. « Effective Eddy Diffusivities Inferred from a Point Release Tracer in an Eddy-Resolving Ocean Model ». Journal of Physical Oceanography 39, n^o 4 (1 avril 2009) : 894–914. http://dx.doi.org/10.1175/2008jpo3902.1.

Texte intégral

Résumé :

Abstract This study uses tracer experiments in a global eddy-resolving ocean model to examine two diagnostic methods for inferring effective eddy isopycnic diffusivity from point release tracers. The first method is based on the growth rate of the area occupied by the tracers (the equivalent variance). During the period when tracer dispersion is dominated by stirring, the equivalent variance is found to increase at a rate between the second power law (for a pure shearing flow regime) and the exponential law (for a pure stretching flow regime). The second method is based on the length of the tracer contours. In the framework of equivalent radius, the two methods of inferring eddy diffusivity can be understood as two different averagings over the tracer patch. Over a shorter period of tracer dispersion the two methods give different eddy diffusivities, and only over a longer time when tracer dispersion approaches the final stage of diffusion do they give a similar value of diffusivity. A new diagnostic quantity called stirring efficiency is introduced to indicate different flow regimes by measuring the efficiency of stirring against mixing. The new diagnostic quantity has the advantage that it can be calculated directly from the gradients of tracer distribution without needing to estimate strain rate or background diffusivity.

Styles APA, Harvard, Vancouver, ISO, etc.

6

Smith, Ronald. « Effect of islands upon dispersion in rivers ». Journal of Fluid Mechanics 292 (10 juin 1995) : 249–70. http://dx.doi.org/10.1017/s0022112095001510.

Texte intégral

Résumé :

A general formulation is given for the dispersion of conservative tracers in steady flow in multi-connected channels. A multi-index is used to distinguish the different routes for tracer between the source and the observation position. For each route exact formulae are obtained for the time integral, time centroid, and cross-channel average of the temporal variance. The total concentration is the superposition of the contributions from the different routes.

Styles APA, Harvard, Vancouver, ISO, etc.

7

HASZPRA, TÍMEA, PÉTER KISS, TAMÁS TÉL et IMRE M. JÁNOSI. « ADVECTION OF PASSIVE TRACERS IN THE ATMOSPHERE : BATCHELOR SCALING ». International Journal of Bifurcation and Chaos 22, n^o 10 (octobre 2012) : 1250241. http://dx.doi.org/10.1142/s0218127412502410.

Texte intégral

Résumé :

Extensive numerical experiments are performed on tracer dispersion in global reanalysis wind fields. Particle trajectories are computed both along an isobaric (500 hPa) and an isentropic (315 K) surface in a time interval of one year. Besides mean quantities such as advection of the center of mass and growth of tracer clouds, special attention is paid to the evaluation of particle pair separation dynamics. The characteristic behavior for intermediate time scales is Batchelor's dispersion along both surfaces, where the zonal extent of the tracer cloud increases linearly in time. The long-time evolution after 70–80 days exhibits a slower, diffusive dispersion (Taylor regime), in agreement with expectations. Richardson–Obukhov scaling (superdiffusion with an exponent of 3/2) could not be identified in the numerical tests. The results confirm the classical prediction by Batchelor that the initial pair-separation determines subsequent time evolution of tracers. The quantitative dependence on the initial distance differs however from the prediction of the theory.

Styles APA, Harvard, Vancouver, ISO, etc.

8

Yuan (原), Yuxuan (宇轩), Mark R. Krumholz et Blakesley Burkhart. « Understanding biases in measurements of molecular cloud kinematics using line emission ». Monthly Notices of the Royal Astronomical Society 498, n^o 2 (18 août 2020) : 2440–55. http://dx.doi.org/10.1093/mnras/staa2432.

Texte intégral

Résumé :

ABSTRACT Molecular line observations using a variety of tracers are often used to investigate the kinematic structure of molecular clouds. However, measurements of cloud velocity dispersions with different lines, even in the same region, often yield inconsistent results. The reasons for this disagreement are not entirely clear, since molecular line observations are subject to a number of biases. In this paper, we untangle and investigate various factors that drive linewidth measurement biases by constructing synthetic position–position–velocity cubes for a variety of tracers from a suite of self-gravitating magnetohydrodynamic simulations of molecular clouds. We compare linewidths derived from synthetic observations of these data cubes to the true values in the simulations. We find that differences in linewidth as measured by different tracers are driven by a combination of density-dependent excitation, whereby tracers that are sensitive to higher densities sample smaller regions with smaller velocity dispersions, opacity broadening, especially for highly optically thick tracers such as CO, and finite resolution and sensitivity, which suppress the wings of emission lines. We find that, at fixed signal-to-noise ratio, three commonly used tracers, the J = 4 → 3 line of CO, the J = 1 → 0 line of C18O, and the (1,1) inversion transition of NH3, generally offer the best compromise between these competing biases, and produce estimates of the velocity dispersion that reflect the true kinematics of a molecular cloud to an accuracy of $\approx 10{{\ \rm per\ cent}}$ regardless of the cloud magnetic field strengths, evolutionary state, or orientations of the line of sight relative to the magnetic field. Tracers excited primarily in gas denser than that traced by NH3 tend to underestimate the true velocity dispersion by $\approx 20{{\ \rm per\ cent}}$ on average, while low-density tracers that are highly optically thick tend to have biases of comparable size in the opposite direction.

Styles APA, Harvard, Vancouver, ISO, etc.

9

Govender, Elaine, Athanasios Kotsiopoulos et Sue T. L. Harrison. « A Study of Permeability and Diffusion at the Agglomerate-Scale in Heap (Bio)Leaching Systems ». Advanced Materials Research 1130 (novembre 2015) : 316–20. http://dx.doi.org/10.4028/www.scientific.net/amr.1130.316.

Texte intégral

Résumé :

Multiple mini-column reactors, loaded with identically constructed ore samples representing grab samples of a larger heap, were used to study the behaviour of solution tracers to elucidate solution diffusion, dispersion and transport. The tracers were either introduced to the ore bed as a pulse, included during agglomeration of the ore or introduced to the system by submerging the ore bed. These methods of tracer introduction allowed for the characterisation of flow interchange in unsteady state systems. The resulting concentration-time distribution curves were analysed to allow characterisation of flow dispersion and diffusion, which facilitates exchange between the fast flowing and largely stagnant liquid phases. Preliminary results support the presence of distinct stagnant and flowing regions within the agglomerated ore bed. Agglomeration with the tracer promotes increased retention on the ore; potentially enhancing microbial transport via increased solution exchange after the initial period of attachment.

Styles APA, Harvard, Vancouver, ISO, etc.

10

Fast, Jerome D., K. Jerry Allwine, Russell N. Dietz, Kirk L. Clawson et Joel C. Torcolini. « Dispersion of Perfluorocarbon Tracers within the Salt Lake Valley during VTMX 2000 ». Journal of Applied Meteorology and Climatology 45, n^o 6 (1 juin 2006) : 793–812. http://dx.doi.org/10.1175/jam2371.1.

Texte intégral

Résumé :

Abstract Six perfluorocarbon tracer experiments were conducted in Salt Lake City, Utah, during October 2000 as part of the Vertical Transport and Mixing (VTMX) field campaign. Four tracers were released at different sites to obtain information on dispersion during stable conditions within down-valley flow, canyon outflow, and interacting circulations in the downtown area. Some of the extensive tracer data that were collected are presented in the context of the meteorological field campaign measurements. Tracer measurements at building-top sites in the downtown area and along the lower slopes of the Wasatch Front indicated that vertical mixing processes transported material up to at least 180 m above the valley floor, although model simulations suggest that tracers were transported upward to much higher elevations. Tracer data provided evidence of downward mixing of canyon outflow, upward mixing within down-valley flow, horizontal transport above the surface stable layer, and transport within horizontal eddies produced by the interaction of canyon and down-valley flows. Although point meteorological measurements are useful in evaluating the forecasts produced by mesoscale models, the tracer data provide valuable information on how the time-varying three-dimensional mean and turbulent motions over urban and valley spatial scales affect dispersion. Although the mean tracer transport predicted by the modeling system employed in this study was qualitatively similar to the measurements, improvements are needed in the treatment of turbulent vertical mixing.

Styles APA, Harvard, Vancouver, ISO, etc.

11

Farazmand, Mohammad, et Themistoklis Sapsis. « Surface Waves Enhance Particle Dispersion ». Fluids 4, n^o 1 (19 mars 2019) : 55. http://dx.doi.org/10.3390/fluids4010055.

Texte intégral

Résumé :

We study the horizontal dispersion of passive tracer particles on the free surface of gravity waves in deep water. For random linear waves with the JONSWAP spectrum, the Lagrangian particle trajectories are computed using an exact nonlinear model known as the John–Sclavounos equation. We show that the single-particle dispersion exhibits an unusual super-diffusive behavior. In particular, for large times t, the variance of the tracer ⟨ | X ( t ) | 2 ⟩ increases as a quadratic function of time, i.e., ⟨ | X ( t ) | 2 ⟩ ∼ t 2 . This dispersion is markedly faster than Taylor’s single-particle dispersion theory which predicts that the variance of passive tracers grows linearly with time for large t. Our results imply that the wave motion significantly enhances the dispersion of fluid particles. We show that this super-diffusive behavior is a result of the long-term correlation of the Lagrangian velocities of fluid parcels on the free surface.

Styles APA, Harvard, Vancouver, ISO, etc.

12

Badin, Gualtiero, Amit Tandon et Amala Mahadevan. « Lateral Mixing in the Pycnocline by Baroclinic Mixed Layer Eddies ». Journal of Physical Oceanography 41, n^o 11 (1 novembre 2011) : 2080–101. http://dx.doi.org/10.1175/jpo-d-11-05.1.

Texte intégral

Résumé :

Abstract Using a process study model, the effect of mixed layer submesoscale instabilities on the lateral mixing of passive tracers in the pycnocline is explored. Mixed layer eddies that are generated from the baroclinic instability of a front within the mixed layer are found to penetrate into the pycnocline leading to an eddying flow field that acts to mix properties laterally along isopycnal surfaces. The mixing of passive tracers released on such isopycnal surfaces is quantified by estimating the variance of the tracer distribution over time. The evolution of the tracer variance reveals that the flow undergoes three different turbulent regimes. The first regime, lasting about 3–4 days (about 5 inertial periods) exhibits near-diffusive behavior; dispersion of the tracer grows nearly linearly with time. In the second regime, which lasts for about 10 days (about 14 inertial periods), tracer dispersion exhibits exponential growth because of the integrated action of high strain rates created by the instabilities. In the third regime, tracer dispersion follows Richardson’s power law. The Nakamura effective diffusivity is used to study the role of individual dynamical filaments in lateral mixing. The filaments, which carry a high concentration of tracer, are characterized by the coincidence of large horizontal strain rate with large vertical vorticity. Within filaments, tracer is sheared without being dispersed, and consequently the effective diffusivity is small in filaments. While the filament centers act as barriers to transport, eddy fluxes are enhanced at the filament edges where gradients are large.

Styles APA, Harvard, Vancouver, ISO, etc.

13

Faller, Alan J. « A random-flight evaluation of the constants of relative dispersion in idealized turbulence ». Journal of Fluid Mechanics 316 (10 juin 1996) : 139–61. http://dx.doi.org/10.1017/s002211209600047x.

Texte intégral

Résumé :

In the idealized problem of homogeneous isotropic stationary inertial-range turbulence the rate of relative dispersion of an ensemble of tracer pairs can be characterized by a constant C0. In order to compute this constant with random-flight equations, however, it is necessary first to know the values of two other constants, C1 and C2, that occur in the two-particle velocity-component relations of Lagrangian tracers (Faller 1992).C1 and C2 are found by an elaborate trial and error procedure in a new two-tracer random-flight model of dispersion that matches input and output values of these two variates. The constant C0 is then computed using the Lagrangian relations and is found to be significantly smaller than when the Eulerian Kármán/Howarth correlations are used.The probability density distribution of tracer separations has a kurtosis slightly larger than that of a comparable Gaussian distribution. At small spacings the frequency of tracer spacings is six to ten times larger than would be expected from a Gaussian distribution. The distribution function for the speed of separation of the Lagrangian tracers has a negative skewness similar to that found for two-point Eulerian velocities.

Styles APA, Harvard, Vancouver, ISO, etc.

14

Lajeunesse, Eric, Olivier Devauchelle et François James. « Advection and dispersion of bed load tracers ». Earth Surface Dynamics 6, n^o 2 (15 mai 2018) : 389–99. http://dx.doi.org/10.5194/esurf-6-389-2018.

Texte intégral

Résumé :

Abstract. We use the erosion–deposition model introduced by Charru et al. (2004) to numerically simulate the evolution of a plume of bed load tracers entrained by a steady flow. In this model, the propagation of the plume results from the stochastic exchange of particles between the bed and the bed load layer. We find a transition between two asymptotic regimes. The tracers, initially at rest, are gradually set into motion by the flow. During this entrainment regime, the plume is strongly skewed in the direction of propagation and continuously accelerates while spreading nonlinearly. With time, the skewness of the plume eventually reaches a maximum value before decreasing. This marks the transition to an advection–diffusion regime in which the plume becomes increasingly symmetrical, spreads linearly, and advances at constant velocity. We analytically derive the expressions of the position, the variance, and the skewness of the plume and investigate their asymptotic regimes. Our model assumes steady state. In the field, however, bed load transport is intermittent. We show that the asymptotic regimes become insensitive to this intermittency when expressed in terms of the distance traveled by the plume. If this finding applies to the field, it might provide an estimate for the average bed load transport rate.

Styles APA, Harvard, Vancouver, ISO, etc.

15

Davis, P. M., et T. C. Atkinson. « Longitudinal dispersion in natural channels : 3. An aggregated dead zone model applied to the River Severn, U.K. » Hydrology and Earth System Sciences 4, n^o 3 (30 septembre 2000) : 373–81. http://dx.doi.org/10.5194/hess-4-373-2000.

Texte intégral

Résumé :

Abstract. An Aggregated Dead Zone (ADZ) model is presented for longitudinal dispersion of tracer in river channels, in which the channel cross-section is divided into two parallel regions: the bulk flow and dead zone storage. Tracer particles in the bulk flow are assumed to obey plug-flow advection at the discharge velocity U without any mixing effects. The dispersive properties of the model are completely embodied in the residence time for tracer storage in the dead zone. The model provides an excellent description and prediction of empirical concentration-time distributions, for times t < x/U. Its physical realism is demonstrated by using it to describe the evolution of a tracer cloud in the River Severn, U.K., and by comparing it with a more complex model which incorporates the additional effects of shear flow dispersion within the bulk flow. The ADZ model is a potentially useful tool for practical prediction of dispersion in natural channels. Keywords: Channels; dispersion; dead zones; tracers; River Severn

Styles APA, Harvard, Vancouver, ISO, etc.

16

Punanova, A., P. Caselli, J. E. Pineda, A. Pon, M. Tafalla, A. Hacar et L. Bizzocchi. « Kinematics of dense gas in the L1495 filament ». Astronomy & ; Astrophysics 617 (septembre 2018) : A27. http://dx.doi.org/10.1051/0004-6361/201731159.

Texte intégral

Résumé :

Context. Nitrogen bearing species, such as NH3, N2H+, and their deuterated isotopologues show enhanced abundances in CO-depleted gas, and thus are perfect tracers of dense and cold gas in star-forming regions. The Taurus molecular cloud contains the long L1495 filament providing an excellent opportunity to study the process of star formation in filamentary environments. Aims. We study the kinematics of the dense gas of starless and protostellar cores traced by the N2D+(2–1), N2H+(1–0), DCO+(2–1), and H13CO+(1–0) transitions along the L1495 filament and the kinematic links between the cores and surrounding molecular cloud. Methods. We measured velocity dispersions, local and total velocity gradients, and estimate the specific angular momenta of 13 dense cores in the four transitions using on-the-fly observations with the IRAM 30-m antenna. To study a possible connection to the filament gas, we used the C18O(1–0) observations. Results. The velocity dispersions of all studied cores are mostly subsonic in all four transitions and are similar and almost constant dispersion across the cores in N2D+(2–1) and N2H+(1–0). A small fraction of the DCO+(2–1) and H13CO+(1–0) lines show transonic dispersion and exhibit a general increase in velocity dispersion with line intensity. All cores have velocity gradients (0.6–6.1 km s−1 pc−1), typical of dense cores in low-mass star-forming regions. All cores show similar velocity patterns in the different transitions, simple in isolated starless cores, and complex in protostellar cores and starless cores close to young stellar objects where gas motions can be affected by outflows. The large-scale velocity field traced by C18O(1–0) does not show any perturbation due to protostellar feedback and does not mimic the local variations seen in the other four tracers. Specific angular momentum J∕M varies in a range (0.6–21.0) × 1020 cm2 s−1, which is similar to the results previously obtained for dense cores. The J∕M measured in N2D+(2–1) is systematically lower than J∕M measured in DCO+(2–1) and H13CO+(1–0). Conclusions. All cores show similar properties along the 10 pc-long filament. N2D+(2–1) shows the most centrally concentrated structure, followed by N2H+(1–0) and DCO+(2–1), which show similar spatial extent, and H13CO+(1–0). The non-thermal contribution to the velocity dispersion increases from higher to lower density tracers. The change of magnitude and direction of the total velocity gradients depending on the tracer used indicates that internal motions change at different depths within the cloud. N2D+ and N2H+ show smaller gradients than the lower density tracers DCO+ and H13CO+, implying a loss of specific angular momentum at small scales. At the level of cloud-core transition, the core’s external envelope traced by DCO+ and H13CO+ is spinning up, which is consistent with conservation of angular momentum during core contraction. C18O traces the more extended cloud material whose kinematics is not affected by the presence of dense cores. The decrease in specific angular momentum towards the centres of the cores shows the importance of local magnetic fields to the small-scale dynamics of the cores. The random distributions of angles between the total velocity gradient and large-scale magnetic field suggests that magnetic fields may become important only in high density gas within dense cores.

Styles APA, Harvard, Vancouver, ISO, etc.

17

Rodriguez, A., A. Sánchez-Arcilla, J. M. Redondo, E. Bahia et J. P. Sierra. « Pollutant dispersion in the nearshore region : modelling and measurements ». Water Science and Technology 32, n^o 9-10 (1 novembre 1995) : 169–78. http://dx.doi.org/10.2166/wst.1995.0682.

Texte intégral

Résumé :

Some results on pollutant dispersion modelling and tracer measurements for the Mediterranean Spanish coast are presented. Two hydrodynamic models have been used to simulate wind and wave induced circulation in the nearshore and surf zones respectively. A “near field” model has been used in order to simulate the initial conditions for the local “far field” dispersion model. Two study cases are presented: The first one shows the mixing of conservative tracers in a Mediterranean surf zone from an experimental and numerical models used to predict bacterial dispersion from the main sea outfalls of Barcelona City. The comparison between dye dispersion experiments and model in the surf zone is good, while the outfall predictions show the importance of accurately modelling the effects of buoyancy on the plume.

Styles APA, Harvard, Vancouver, ISO, etc.

18

Jullien, Marie-Caroline, Patrizia Castiglione et Patrick Tabeling. « Experimental Observation of Batchelor Dispersion of Passive Tracers ». Physical Review Letters 85, n^o 17 (23 octobre 2000) : 3636–39. http://dx.doi.org/10.1103/physrevlett.85.3636.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

19

Rotenberg, B., I. Pagonabarraga et D. Frenkel. « Dispersion of charged tracers in charged porous media ». EPL (Europhysics Letters) 83, n^o 3 (21 juillet 2008) : 34004. http://dx.doi.org/10.1209/0295-5075/83/34004.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

20

Landel, Julien R., C. P. Caulfield et Andrew W. Woods. « Streamwise dispersion and mixing in quasi-two-dimensional steady turbulent jets ». Journal of Fluid Mechanics 711 (12 septembre 2012) : 212–58. http://dx.doi.org/10.1017/jfm.2012.388.

Texte intégral

Résumé :

AbstractWe investigate experimentally and theoretically the streamwise transport and dispersion properties of steady quasi-two-dimensional plane turbulent jets discharged vertically from a slot of width $d$ into a fluid confined between two relatively close rigid boundaries with gap $W\ensuremath{\sim} O(d)$. We model the evolution in time and space of the concentration of passive tracers released in these jets using a one-dimensional time-dependent effective advection–diffusion equation. We make a mixing length hypothesis to model the streamwise turbulent eddy diffusivity such that it scales like $b(z){ \overline{w} }_{m} (z)$, where $z$ is the streamwise coordinate, $b$ is the jet width, ${ \overline{w} }_{m} $ is the maximum time-averaged vertical velocity. Under these assumptions, the effective advection–diffusion equation for $\phi (z, t)$, the horizontal integral of the ensemble-averaged concentration, is of the form ${\partial }_{t} \phi + {K}_{a} {\text{} {M}_{0} \text{} }^{1/ 2} {\partial }_{z} \left(\phi / {z}^{1/ 2} \right)= {K}_{d} {\text{} {M}_{0} \text{} }^{1/ 2} {\partial }_{z} \left({z}^{1/ 2} {\partial }_{z} \phi \right)$, where $t$ is time, ${K}_{a} $ (the advection parameter) and ${K}_{d} $ (the dispersion parameter) are empirical dimensionless parameters which quantify the importance of advection and dispersion, respectively, and ${M}_{0} $ is the source momentum flux. We find analytical solutions to this equation for $\phi $ in the cases of a constant-flux release and an instantaneous finite-volume release. We also give an integral formulation for the more general case of a time-dependent release, which we solve analytically when tracers are released at a constant flux over a finite period of time. From our experimental results, whose concentration distributions agree with the model, we find that ${K}_{a} = 1. 65\pm 0. 10$ and ${K}_{d} = 0. 09\pm 0. 02$, for both finite-volume releases and constant-flux releases using either dye or virtual passive tracers. The experiments also show that streamwise dispersion increases in time as ${t}^{2/ 3} $. As a result, in the case of finite-volume releases more than 50 % of the total volume of tracers is transported ahead of the purely advective front (i.e. the front location of the tracer distribution if all dispersion mechanisms are ignored and considering a ‘top-hat’ mean velocity profile in the jet); and in the case of constant-flux releases, at each instant in time, approximately 10 % of the total volume of tracers is transported ahead of the advective front.

Styles APA, Harvard, Vancouver, ISO, etc.

21

Aniyan, S., A. A. Ponomareva, K. C. Freeman, M. Arnaboldi, O. E. Gerhard, L. Coccato, K. Kuijken et M. Merrifield. « Resolving the Disc–Halo Degeneracy – II : NGC 6946 ». Monthly Notices of the Royal Astronomical Society 500, n^o 3 (9 octobre 2020) : 3579–93. http://dx.doi.org/10.1093/mnras/staa3106.

Texte intégral

Résumé :

ABSTRACT The mass-to-light ratio (M/L) is a key parameter in decomposing galactic rotation curves into contributions from the baryonic components and the dark halo of a galaxy. One direct observational method to determine the disc M/L is by calculating the surface mass density of the disc from the stellar vertical velocity dispersion and the scale height of the disc. Usually, the scale height is obtained from near-IR studies of edge-on galaxies and pertains to the older, kinematically hotter stars in the disc, while the vertical velocity dispersion of stars is measured in the optical band and refers to stars of all ages (up to ∼10 Gyr) and velocity dispersions. This mismatch between the scale height and the velocity dispersion can lead to underestimates of the disc surface density and a misleading conclusion of the submaximality of galaxy discs. In this paper, we present the study of the stellar velocity dispersion of the disc galaxy NGC 6946 using integrated star light and individual planetary nebulae as dynamical tracers. We demonstrate the presence of two kinematically distinct populations of tracers that contribute to the total stellar velocity dispersion. Thus, we are able to use the dispersion and the scale height of the same dynamical population to derive the surface mass density of the disc over a radial extent. We find the disc of NGC 6946 to be closer to maximal with the baryonic component contributing most of the radial gravitational field in the inner parts of the galaxy (Vmax(bar) = 0.76(±0.14)Vmax).

Styles APA, Harvard, Vancouver, ISO, etc.

22

Atkinson, T. C., et P. M. Davis. « Longitudinal dispersion in natural channels : l. Experimental results from the River Severn, U.K. » Hydrology and Earth System Sciences 4, n^o 3 (30 septembre 2000) : 345–53. http://dx.doi.org/10.5194/hess-4-345-2000.

Texte intégral

Résumé :

Abstract. Abstract: A tracer experiment using Rhodamine WT dye was carried out to measure longitudinal dispersion in a 14-km reach of the River Severn in Wales, U.K. The river’s discharge was measured at six points and the depth, width and cross-sectional area were measured at 86 points along the test reach. The channel geometry was close to being statistically uniform. Discharge and velocity were both nearly constant. Dye concentrations were recorded at stations between 210 and 13775 m downstream of injection. Dye was injected over a short interval as a near-uniform line source across the channel. These conditions make the data useful for testing mathematical theories of dispersion. They are presented in full. Keywords: Channels; dispersion; tracers; River Severn

Styles APA, Harvard, Vancouver, ISO, etc.

23

Braconnier, Benjamin, Christophe Preux, Frédéric Douarche et Bernard Bourbiaux. « MUSCL scheme for Single Well Chemical Tracer Test simulation, design and interpretation ». Oil & ; Gas Science and Technology – Revue d’IFP Energies nouvelles 74 (2019) : 10. http://dx.doi.org/10.2516/ogst/2018090.

Texte intégral

Résumé :

Our paper presents an improved numerical scheme to simulate Single Well Chemical Tracer Test (SWCTT) method. SWCTT is mainly applied to determine the residual oil saturation of reservoirs. It consists in injecting an aqueous slug of a primary tracer into the reservoir formation and displacing it at a certain distance from the well. This tracer is partly miscible with oil on the one hand, and generates in situ a secondary tracer on the other hand. As a consequence, a shift is observed between the primary and the secondary tracers arrival times when production is resumed. This time shift is used to evaluate the residual oil saturation. In our paper, we propose a numerical scheme based on a fractional time stepping technique to decouple the resolution of the phases mass conservation equations and the chemical tracers mole conservation equations. For the phases resolution, we use an implicit scheme to ensure stability and robustness. For the chemical tracers, we propose an explicit second-order scheme in time and in space via MUSCL technique to improve the tracers time-shift calculation. The proposed numerical method is implemented on a realistic simulation model consisting of a vertical well crossing a reservoir consisting of a stack of homogeneous layers. By reducing the numerical dispersion, the proposed scheme improves the accuracy of predicted concentration profiles, without significantly increasing the computation time. Finally, the advantages of using a second-order scheme for residual oil saturation assessment are discussed on the basis of a radial 1D mesh convergence study.

Styles APA, Harvard, Vancouver, ISO, etc.

24

Espa, Stefania, G. Querzoli et A. Cenedese. « Dispersion of passive tracers in a confined convective flow ». European Journal of Mechanics - B/Fluids 20, n^o 4 (juillet 2001) : 525–40. http://dx.doi.org/10.1016/s0997-7546(01)01125-6.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

25

Gudiksen, Paul H., et Donald L. Shearer. « The Dispersion of Atmospheric Tracers in Nocturnal Drainage Flows ». Journal of Applied Meteorology 28, n^o 7 (juillet 1989) : 602–8. http://dx.doi.org/10.1175/1520-0450(1989)028<0602:tdoati>2.0.co;2.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

26

Keating, Shane R., K. Shafer Smith et Peter R. Kramer. « Diagnosing Lateral Mixing in the Upper Ocean with Virtual Tracers : Spatial and Temporal Resolution Dependence ». Journal of Physical Oceanography 41, n^o 8 (1 août 2011) : 1512–34. http://dx.doi.org/10.1175/2011jpo4580.1.

Texte intégral

Résumé :

Abstract Several recent studies diagnose lateral stirring and mixing in the upper ocean using altimetry-derived velocity fields to advect “virtual” particles and fields offline. However, the limited spatiotemporal resolution of altimetric maps leads to errors in the inferred diagnostics, because unresolved scales are necessarily imperfectly modeled. The authors examine a range of tracer diagnostics in two models of baroclinic turbulence: the standard Phillips model, in which dispersion is controlled by large-scale eddies, and the Eady model, where dispersion is determined by local scales of motion. These models serve as a useful best- and worst-case comparison and a valuable test of the resolution sensitivity of tracer diagnostics. The effect of unresolved scales is studied by advecting tracers using model velocity fields subsampled in space and time and comparing the derived tracer diagnostics with their “true” value obtained from the fully resolved flow. The authors find that eddy diffusivity and absolute dispersion, which are governed by large-scale dynamics, are insensitive to spatial sampling error in either flow. Measures that depend strongly on small scales, such as relative dispersion and finite-time Lyapunov exponents, are highly sensitive to spatial sampling in the Eady model. Temporal sampling error is found to have a more complicated behavior because of the onset of particle overshoot leading to scrambling of Lagrangian diagnostics. This leads to a potential restriction on the utility of raw altimetry maps for studying mixing in the upper ocean. The authors conclude that offline diagnostics of mixing in ocean flows with an energized submesoscale should be viewed with some caution.

Styles APA, Harvard, Vancouver, ISO, etc.

27

Yi, Shu Ping, Hai Yi Ma et Hua An Wang. « A Preliminary Study on the Transport Behavior for a Potential Disposal Site of LILW in Southern China ». Advanced Materials Research 356-360 (octobre 2011) : 1445–53. http://dx.doi.org/10.4028/www.scientific.net/amr.356-360.1445.

Texte intégral

Résumé :

Near surface disposal of low and intermediate level radioactive waste (LILW) requires a detailed site investigation of the potential sites, in which the transport behavior of solutes in the groundwater system is one of the key processes that needed to be addressed. An investigation is undertaking for the disposal of LILW at a potential site in Southern China. In-situ test have been conducted to study the hydrogeologic characteristics of the site. Firstly, tests including pit permeability tests, injecting tests, water pressure tests, pumping tests and laboratory permeability tests have been performed according to the specific field conditions. Hydraulic conductivities for different layers of rock and soils have been calculated and their recommended values have been presented. And then in-situ dispersion tests have been performed at an area adjacent to the disposal site with non-sorbed tracers. A numerical model has been developed for the site based on data obtained during the site investigation, and has been calibrated with available measured groundwater level and measured tracer concentrations in the dispersion tests. Calibrated results indicate that the longitudinal dispersion coefficient is equal to 5.0×10-3 m2/d. Preliminary predictions have been performed for the groundwater flow and solute advection-dispersion behavior according to the design of the site, in which the center valley will be backfilled with low permeable materials. Predicted results indicate that the groundwater exhibits a lower water table and a smaller hydraulic gradient under designed site condition than under natural condition. All the tracers remain underground in the site and transport slowly mainly through advection and dispersion. Finally, conclusions for the study have been presented and the key tasks for future work have been discussed. This study provides an insight understanding of the hydrogeology characteristics of the disposal site and is useful for the assessment of environment impacts of the site under disposal of LILW.

Styles APA, Harvard, Vancouver, ISO, etc.

28

Yu, Li-Ming, Wei-Hao Bian, Chan Wang, Bi-Xuan Zhao et Xue Ge. « Calibration of the virial factor f in supermassive black hole masses of reverberation-mapped AGNs ». Monthly Notices of the Royal Astronomical Society 488, n^o 2 (1 juillet 2019) : 1519–34. http://dx.doi.org/10.1093/mnras/stz1766.

Texte intégral

Résumé :

ABSTRACT Using a compiled sample of 34 broad-line active galactic nuclei (AGNs) with measured H β time lags from the reverberation mapping (RM) method and measured bulge stellar velocity dispersions σ*, we calculate the virial factor f by assuming that the RM AGNs intrinsically obey the same MBH−σ* relation as quiescent galaxies, where MBH is the mass of the supermassive black hole (SMBH). Considering four tracers of the velocity of the broad-line regions (BLRs), i.e. the H β line width or line dispersion from the mean or rms spectrum, there are four kinds of the factor f. Using the H β full width at half-maximum (FWHM) to trace the BLRs velocity, we find significant correlations between the factor f and some observational parameters, e.g. FWHM, the line dispersion. Using the line dispersion to trace the BLRs velocity, these relations disappear or become weaker. It implies the effect of inclination in BLRs geometry. It also suggests that the variable f in MBH estimated from luminosity and FWHM in a single-epoch spectrum is not negligible. Using a simple model of thick-disc BLRs, we also find that, as the tracer of the BLRs velocity, H β FWHM has some dependence on the inclination, while the line dispersion σH β is insensitive to the inclination. Considering the calibrated FWHM-based factor f from the mean spectrum, the scatter of the SMBH mass is 0.39 dex for our sample of 34 low-redshift RM AGNs. For a high-redshift sample of 30 Sloan Digital Sky Survey RM AGNs with measured stellar velocity dispersions, we find that the SMBH mass scatter is larger than that for our sample of 34 low-redshift RM AGNs. It implies the possibility of evolution of the MBH−σ* relation from high-redshift to low-redshift AGNs.

Styles APA, Harvard, Vancouver, ISO, etc.

29

HAMAKI, Michihiro, Toshiki IWASAKI, Takuya INOUE, Daisuke SATO, Tamaki SUMNER et Yasuyuki SHIMIZU. « EXPERIMENT AND NUMERICAL SIMULATION OF ADVECTION-DISPERSION OF BEDLOAD TRACERS ». Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering) 74, n^o 4 (2018) : I_1033—I_1038. http://dx.doi.org/10.2208/jscejhe.74.i_1033.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

30

Pizzuto, James E. « Dispersion of dyed sand tracers in an oscillatory flow field ». Journal of Geophysical Research 92, n^o C2 (1987) : 1923. http://dx.doi.org/10.1029/jc092ic02p01923.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

31

Wagner, Gregory L., Glenn Flierl, Raffaele Ferrari, Gunnar Voet, Glenn S. Carter, Matthew H. Alford et James B. Girton. « Squeeze Dispersion and the Effective Diapycnal Diffusivity of Oceanic Tracers ». Geophysical Research Letters 46, n^o 10 (21 mai 2019) : 5378–86. http://dx.doi.org/10.1029/2019gl082458.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

32

CHATWIN, P. C., N. T. HAJIAN, N. MOLE et C. D. JONES. « Investigations on the Atmospheric Dispersion of Clouds Containing Charged Tracers ». IMA Journal of Applied Mathematics 42, n^o 2 (1989) : 97–117. http://dx.doi.org/10.1093/imamat/42.2.97.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

33

Croze, Ottavio A., Gaetano Sardina, Mansoor Ahmed, Martin A. Bees et Luca Brandt. « Dispersion of swimming algae in laminar and turbulent channel flows : consequences for photobioreactors ». Journal of The Royal Society Interface 10, n^o 81 (6 avril 2013) : 20121041. http://dx.doi.org/10.1098/rsif.2012.1041.

Texte intégral

Résumé :

Shear flow significantly affects the transport of swimming algae in suspension. For example, viscous and gravitational torques bias bottom-heavy cells to swim towards regions of downwelling fluid (gyrotaxis). It is necessary to understand how such biases affect algal dispersion in natural and industrial flows, especially in view of growing interest in algal photobioreactors. Motivated by this, we here study the dispersion of gyrotactic algae in laminar and turbulent channel flows using direct numerical simulation (DNS) and a previously published analytical swimming dispersion theory. Time-resolved dispersion measures are evaluated as functions of the Péclet and Reynolds numbers in upwelling and downwelling flows. For laminar flows, DNS results are compared with theory using competing descriptions of biased swimming cells in shear flow. Excellent agreement is found for predictions that employ generalized Taylor dispersion. The results highlight peculiarities of gyrotactic swimmer dispersion relative to passive tracers. In laminar downwelling flow the cell distribution drifts in excess of the mean flow, increasing in magnitude with Péclet number. The cell effective axial diffusivity increases and decreases with Péclet number (for tracers it merely increases). In turbulent flows, gyrotactic effects are weaker, but discernable and manifested as non-zero drift. These results should have a significant impact on photobioreactor design.

Styles APA, Harvard, Vancouver, ISO, etc.

34

Croze, Ottavio A., Rachel N. Bearon et Martin A. Bees. « Gyrotactic swimmer dispersion in pipe flow : testing the theory ». Journal of Fluid Mechanics 816 (7 mars 2017) : 481–506. http://dx.doi.org/10.1017/jfm.2017.90.

Texte intégral

Résumé :

Suspensions of microswimmers are a rich source of fascinating new fluid mechanics. Recently we predicted the active pipe flow dispersion of gyrotactic microalgae, whose orientation is biased by gravity and flow shear. Analytical theory predicts that these active swimmers disperse in a markedly distinct manner from passive tracers (Taylor dispersion). Dispersing swimmers display non-zero drift and effective diffusivity that is non-monotonic with Péclet number. Such predictions agree with numerical simulations, but hitherto have not been tested experimentally. Here, to facilitate comparison, we obtain new solutions of the axial dispersion theory accounting both for swimmer negative buoyancy and a local nonlinear response of swimmers to shear, provided by two alternative microscopic stochastic descriptions. We obtain new predictions for suspensions of the model swimming alga Dunaliella salina, whose motility and buoyant mass we parametrise using tracking video microscopy. We then present a new experimental method to measure gyrotactic dispersion using fluorescently stained D. salina and provide a preliminary comparison with predictions of a non-zero drift above the mean flow for each microscopic stochastic description. Finally, we propose further experiments for a full experimental characterisation of gyrotactic dispersion measures and discuss the implications of our results for algal dispersion in industrial photobioreactors.

Styles APA, Harvard, Vancouver, ISO, etc.

35

Choudhury, Spandan, Jaime E. Pineda, Paola Caselli, Stella S. R. Offner, Erik Rosolowsky, Rachel K. Friesen, Elena Redaelli et al. « Transition from coherent cores to surrounding cloud in L1688 ». Astronomy & ; Astrophysics 648 (avril 2021) : A114. http://dx.doi.org/10.1051/0004-6361/202039897.

Texte intégral

Résumé :

Context. Stars form in cold dense cores showing subsonic velocity dispersions. The parental molecular clouds display higher temperatures and supersonic velocity dispersions. The transition from core to cloud has been observed in velocity dispersion, but temperature and abundance variations are unknown. Aims. We aim to measure the temperature and velocity dispersion across cores and ambient cloud in a single tracer to study the transition between the two regions. Methods. We use NH3 (1,1) and (2,2) maps in L1688 from the Green Bank Ammonia Survey, smoothed to 1′, and determine the physical properties by fitting the spectra. We identify the coherent cores and study the changes in temperature and velocity dispersion from the cores to the surrounding cloud. Results. We obtain a kinetic temperature map extending beyond dense cores and tracing the cloud, improving from previous maps tracing mostly the cores. The cloud is 4–6 K warmer than the cores, and shows a larger velocity dispersion (Δσv = 0.15–0.25 km s−1). Comparing to Herschel-based dust temperatures, we find that cores show kinetic temperatures that are ≈1.8 K lower than the dust temperature, while the gas temperature is higher than the dust temperature in the cloud. We find an average p-NH3 fractional abundance (with respect to H2) of (4.2 ± 0.2) × 10−9 towards the coherent cores, and (1.4 ± 0.1) × 10−9 outside the core boundaries. Using stacked spectra, we detect two components, one narrow and one broad, towards cores and their neighbourhoods. We find the turbulence in the narrow component to be correlated with the size of the structure (Pearson-r = 0.54). With these unresolved regional measurements, we obtain a turbulence–size relation of σv,NT ∝ r0.5, which is similar to previous findings using multiple tracers. Conclusions. We discover that the subsonic component extends up to 0.15 pc beyond the typical coherent boundaries, unveiling larger extents of the coherent cores and showing gradual transition to coherence over ~0.2 pc.

Styles APA, Harvard, Vancouver, ISO, etc.

36

Yan, Sheng, Zhili Zou et Zaijin You. « Eulerian Description of Wave-Induced Stokes Drift Effect on Tracer Transport ». Journal of Marine Science and Engineering 10, n^o 2 (12 février 2022) : 253. http://dx.doi.org/10.3390/jmse10020253.

Texte intégral

Résumé :

The wave-induced Stokes drift plays a significant role on mass/tracer transport in the ocean and the evolution of coastal morphology. The tracer advection diffusion equation needs to be modified for Eulerian ocean models to properly account for the surface wave effects. The Eulerian description of Stokes drift effect on the tracer transport is derived in this study to show that this effect can be accounted for automatically in the wave-averaged advection-diffusion equation. The advection term in this equation is the wave-averaged concentration flux produced by the interaction between fluctuations of linear wave orbital velocity and tracer concentration, and the advection velocity is the same as the Stokes drift velocity. Thus, the effective dispersion of tracers by surface gravity waves is calculated due to the Stokes drift effect and the corresponding dispersion coefficient in the depth-integrated equation is then derived. The Eulerian description of Stokes drift effect of tracer concentration is illustrated by the direct numerical simulation of the advection–diffusion equation under simple linear waves. The equivalence between both the Eulerian and Lagrangian descriptions is also verified by particle tracking method. The theoretical analysis is found to agree well with the wave-induced dye drift velocity observed outside the surf zone in a longshore current experiment.

Styles APA, Harvard, Vancouver, ISO, etc.

37

Massardier, Valérie, Molka Louizi, Elisabeth Maris et Daniel Froelich. « High shear dispersion of tracers in polyolefins for improving their detection ». Polímeros 25, n^o 5 (octobre 2015) : 466–76. http://dx.doi.org/10.1590/0104-1428.1974.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

38

Artale, V., G. Boffetta, A. Celani, M. Cencini et A. Vulpiani. « Dispersion of passive tracers in closed basins : Beyond the diffusion coefficient ». Physics of Fluids 9, n^o 11 (novembre 1997) : 3162–71. http://dx.doi.org/10.1063/1.869433.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

39

Müller, W. C., et A. Busse. « Diffusion and dispersion of passive tracers : Navier-Stokes vs. MHD turbulence ». Europhysics Letters (EPL) 78, n^o 1 (22 mars 2007) : 14003. http://dx.doi.org/10.1209/0295-5075/78/14003.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

40

Jullien, Marie-Caroline. « Dispersion of passive tracers in the direct enstrophy cascade : Experimental observations ». Physics of Fluids 15, n^o 8 (août 2003) : 2228–37. http://dx.doi.org/10.1063/1.1585030.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

41

Faller, Alan J., et Stephen J. Auer. « The Roles of Langmuir Circulations in the Dispersion of Surface Tracers ». Journal of Physical Oceanography 18, n^o 8 (août 1988) : 1108–23. http://dx.doi.org/10.1175/1520-0485(1988)018<1108:trolci>2.0.co;2.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

42

Cheong, Hin-Fatt, M. H. Abdul Khader, Chor-Juen Yang et R. Radhakrishnan. « The dispersion of radioactive tracers along the east coast of Singapore ». Coastal Engineering 17, n^o 1-2 (mai 1992) : 71–92. http://dx.doi.org/10.1016/0378-3839(92)90014-l.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

43

Pietrzak, Damian, Jarosław Kania, Ewa Kmiecik et Katarzyna Wątor. « Identification of transport parameters of chlorides in different soils on the basis of column studies ». Geologos 25, n^o 3 (1 décembre 2019) : 225–29. http://dx.doi.org/10.2478/logos-2019-0024.

Texte intégral

Résumé :

Abstract Knowledge of transport patterns of chemicals in groundwater is essential for environmental assessment of their potential impact. In the present study, the mobility of a chloride tracer injected into three different soils was investigated, using column experiments. The column tests were performed under steady-state conditions to determine parameters of chloride migration through soils. Based on breakthrough curves, pore-water velocity, dispersion coefficient and dispersivity constant were calculated for each soil sample using CXTFIT/STANMOD software. Pore-water velocity was in the range of 0.31 cm/min for fine sand, to 0.35 cm/min for silty sand and to 0.40 cm/min for vari-grained sand. The highest values of dispersion coefficient and dispersivity constant were observed for silty sand (0.55 cm2/min and 1.55 cm, respectively), while the lowest value was found for fine sand (0.059 cm2/min and 0.19 cm, respectively). Column experiments for chlorides (conservative tracer) are a preliminary stage for further research which will be undertaken to investigate migration parameters of selected neonicotinoids (reactive tracers) through different soils.

Styles APA, Harvard, Vancouver, ISO, etc.

44

Imhoff, Kurt S., et Andrew C. Wilcox. « Coarse bedload routing and dispersion through tributary confluences ». Earth Surface Dynamics 4, n^o 3 (26 juillet 2016) : 591–605. http://dx.doi.org/10.5194/esurf-4-591-2016.

Texte intégral

Résumé :

Abstract. Sediment routing fundamentally influences channel morphology and the propagation of disturbances such as debris flows. The transport and storage of bedload particles across headwater channel confluences, which may be significant nodes of the channel network in terms of sediment routing, morphology, and habitat, are poorly understood, however. We investigated patterns and processes of sediment routing through headwater confluences by comparing them to published results from lower-gradient confluences and by comparing the dispersive behavior of coarse bedload particles between headwater confluence and non-confluence reaches. We addressed these questions with a field tracer experiment using passive-integrated transponder and radio-frequency identification technology in the East Fork Bitterroot River basin, Montana, USA. Within the confluence zone, tracers tended to be deposited towards scour-hole and channel margins, suggesting narrow, efficient transport corridors that mirror those observed in prior studies, many of which are from finer-grained systems. Coarse particles in some confluence reaches experienced reduced depositional probabilities within the confluence relative to upstream and downstream of the confluence. Analysis of particle transport data suggests that variation in the spatial distribution of coarse-sediment particles may be enhanced by passing through confluences, though further study is needed to evaluate confluence effects on dispersive regimes and sediment routing on broader spatial and temporal scales.

Styles APA, Harvard, Vancouver, ISO, etc.

45

Pecly, J. O. G., et J. S. F. Roldão. « Dye tracers as a tool for outfall studies : dilution measurement approach ». Water Science and Technology 67, n^o 7 (1 avril 2013) : 1564–73. http://dx.doi.org/10.2166/wst.2013.027.

Texte intégral

Résumé :

Dye tracer technique is well established and of wide application for assessment of outfalls and for delineation of near field and far field extensions. Common goals of a tracer study include the measurement of the dilution factor, estimation of the dispersion coefficients, measurement of the effluent discharge and calibration of a contaminant transport model. This paper presents a brief review of the methods involving the use of dye tracer for outfall assessment and illustrates the methods of slug release and continuous injection based on two real cases of campaigns carried out on Brazilian coastal waters. Slug injection on the surface of the water body was used for preliminary dispersion studies aiming at outfall positioning. During the operational phase of an outfall, the continuous injection of dye tracer was used to determine effluent dilution in different seasons. In coastal waters of Rio de Janeiro city, sea current pattern, tidal modulation and thermal stratification explained the main features of the dilution field.

Styles APA, Harvard, Vancouver, ISO, etc.

46

BEC, J., L. BIFERALE, A. S. LANOTTE, A. SCAGLIARINI et F. TOSCHI. « Turbulent pair dispersion of inertial particles ». Journal of Fluid Mechanics 645 (9 février 2010) : 497–528. http://dx.doi.org/10.1017/s0022112009992783.

Texte intégral

Résumé :

The relative dispersion of pairs of inertial point particles in incompressible, homogeneous and isotropic three-dimensional turbulence is studied by means of direct numerical simulations at two values of the Taylor-scale Reynolds number Reλ ~ 200 and Reλ ~ 400, corresponding to resolutions of 5123 and 20483 grid points, respectively. The evolution of both heavy and light particle pairs is analysed by varying the particle Stokes number and the fluid-to-particle density ratio. For particles much heavier than the fluid, the range of available Stokes numbers is St ∈ [0.1 : 70], while for light particles the Stokes numbers span the range St ∈ [0.1 : 3] and the density ratio is varied up to the limit of vanishing particle density. For heavy particles, it is found that turbulent dispersion is schematically governed by two temporal regimes. The first is dominated by the presence, at large Stokes numbers, of small-scale caustics in the particle velocity statistics, and it lasts until heavy particle velocities have relaxed towards the underlying flow velocities. At such large scales, a second regime starts where heavy particles separate as tracers' particles would do. As a consequence, at increasing inertia, a larger transient stage is observed, and the Richardson diffusion of simple tracers is recovered only at large times and large scales. These features also arise from a statistical closure of the equation of motion for heavy particle separation that is proposed and is supported by the numerical results. In the case of light particles with high density ratio, strong small-scale clustering leads to a considerable fraction of pairs that do not separate at all, although the mean separation increases with time. This effect strongly alters the shape of the probability density function of light particle separations.

Styles APA, Harvard, Vancouver, ISO, etc.

47

Olsson, Åsa, et Peter Grathwohl. « Transverse dispersion of non-reactive tracers in porous media : A new nonlinear relationship to predict dispersion coefficients ». Journal of Contaminant Hydrology 92, n^o 3-4 (juillet 2007) : 149–61. http://dx.doi.org/10.1016/j.jconhyd.2006.09.008.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

48

Jung, Minji, Woong June Chung, Minki Sung, Seongmin Jo et Jinkwan Hong. « Analysis of Infection Transmission Routes through Exhaled Breath and Cough Particle Dispersion in a General Hospital ». International Journal of Environmental Research and Public Health 19, n^o 5 (22 février 2022) : 2512. http://dx.doi.org/10.3390/ijerph19052512.

Texte intégral

Résumé :

Identifying infection transmission routes in hospitals may prevent the spread of respiratory viruses and mass infections. Most previous related research focused on the air movement of passive tracers, which typically represent breathing. In this study, particle evaporation and dispersions with various particle sizes were applied to evaluate particle movement because of breathing and coughing using computational fluid dynamics (CFD) simulations. Pyeongtaek St. Mary Hospital, where a Middle East respiratory syndrome (MERS) index patient infected several patients on the same floor, was used for a case study. We compared the dispersion characteristics of various particle sizes and validated results by comparing infection rates in different ward. Results indicated that droplets spread across the corridor and dispersed to wards that were more than 17 m apart from the index patient by natural ventilation. Droplets from exhaled breath under steady-state simulation showed a wider range of dispersion than cough droplets under transient simulation, but cough droplet dispersion was more consistent with the actual infection rate in each ward. Cough droplets sized under 75 µm evaporated to 26% of the initial size and started to disperse into the corridor within one minute; in nine minutes, droplets dispersed throughout every ward. This study may increase awareness on the dispersion characteristics of infectious particles.

Styles APA, Harvard, Vancouver, ISO, etc.

49

Purdy, R. N., B. N. Dancer, M. J. Day et D. J. Stickler. « Bacillus Phages as Tracers of Water Movement ». Water Science and Technology 18, n^o 4-5 (1 avril 1986) : 149–53. http://dx.doi.org/10.2166/wst.1986.0190.

Texte intégral

Résumé :

Vegetative bacterial cells, bacterial spores and bacterial viruses have all been used to trace the dispersion of sewage from coastal outfalls. The phages are considered to have advantages over other mlcroblal tracers in that they can be prepared in high titres (lO13 particles ml−1) and can be assayed In less than 8h. Furthermore, because these viruses only Infect bacteria they represent no health risk. In this paper we describe the selection of five Bacillus phages. from our collection, for use as tracers. They each have distinct, clear plaque morphologies and specific bacterial host ranges. The stability of these phages in sea water was compared to that of E. coll and some other phages. We report on the use of a membrane filtration technique to assay phages in 100ml volumes of sea water and present data from a field study where one of these tracers was released from a coastal outfall.

Styles APA, Harvard, Vancouver, ISO, etc.

50

Crawford, Alice. « The Use of Gaussian Mixture Models with Atmospheric Lagrangian Particle Dispersion Models for Density Estimation and Feature Identification ». Atmosphere 11, n^o 12 (17 décembre 2020) : 1369. http://dx.doi.org/10.3390/atmos11121369.

Texte intégral

Résumé :

Atmospheric Lagrangian particle dispersion models, LPDM, simulate the dispersion of passive tracers in the atmosphere. At the most basic level, model output consists of the position of computational particles and the amount of mass they represent. In order to obtain concentration values, this information is then converted to a mass distribution via density estimation. To date, density estimation is performed with a nonparametric method so that output consists of gridded concentration data. Here we introduce the use of Gaussian mixture models, GMM, for density estimation. We compare to the histogram or bin counting method for a tracer experiment and simulation of a large volcanic ash cloud. We also demonstrate the use of the mixture model for automatic identification of features in a complex plume such as is produced by a large volcanic eruption. We conclude that use of a mixture model for density estimation and feature identification has potential to be very useful.

Styles APA, Harvard, Vancouver, ISO, etc.

Articles de revues sur le sujet « Tracers dispersion »

Créez une référence correcte selon les styles APA, MLA, Chicago, Harvard et plusieurs autres