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Dispersion et mélange turbulents de particules solides et de gouttelettes par une simulation des grandes échelles et une modélisation stochastique lagrangienne. Application à la pollution de l'atmosphère.

Abstract : In order to study atmospheric pollution and the dispersion of industrial stack emissions, a large
eddy simulation with the dynamic Smagorinsky-Germano subgrid-scale model is coupled with Lagrangian tracking of fluid particles containing scalar, solid particles and droplets.
The movement of fluid particles at a subgrid level is given by a three-dimensional Langevin model.
The stochastic model is written in terms of subgrid-scale statistics at a mesh level. By introducing a diffusion model, the coupling between the large-eddy simulation and the modified three-dimensional
Langevin model is applied to passive scalar dispersion. The results are validated by comparison with
the wind-tunnel experiments of Fackrell & Robins (1982).
The equation of motion of a small rigid sphere in a turbulent flow is introduced. Solid particles and droplets are tracked in a Lagrangian way. The velocity of solid particles and droplets is considered to have a large scale component (directly computed by the large-eddy simulation) and a subgrid scale part. Because of inertia and gravity effects, solid particles and droplets, deviate from the trajectories of the surrounding fluid particles. Therefore, a modified Lagrangian correlation timescale is introduced into the Langevin model previously developed for the subgrid velocity of fluid particles. Two-way coupling and collisions are taken into account. The results of the large-eddy simulation with solid
particles are compared with the wind-tunnel experiments of Nalpanis et al. (1993) and of Tanière
et al. (1997) on sand particles in saltation and in modified saltation, respectively.
A model for droplet coalescence and breakup is implemented which allows to predict droplet
interactions under turbulent flow conditions in the frame of the Euler/Lagrange approach. Coalescence
and breakup are considered as a stochastic process with simple scaling symmetry assumption for the
droplet radius, initially proposed by Kolmogorov (1941). At high-frequency of breakup/coalescence phenomena, this stochastic process is equivalent to the evolution of the probability density function of droplet radii, which is governed by a Fokker-Planck equation. The parameters of the model are obtained dynamically by relating them to the local resolved properties of the dispersed phase
compared to the main fluid. Within each grid cell, mass conservation is applied. The model is validated by comparison with the agglomeration model of Ho & Sommerfeld (2002), the stochastic model for secondary breakup of Apte et al. (2003) and the experimental results on secondary breakup in a coaxial jet of Lasheras et al. (1998). The large-eddy simulation coupled with Lagrangian particle tracking and the model for droplet coalescence and breakup is applied to the study of the atmospheric dispersion of wet cooling tower plumes. The simulations are done for different droplet size distributions and volume fractions. We focused on the influence of these parameters on mean concentration, concentration variance and mass
flux profiles.
In order to gain insight into the transport of solid particles and droplets in a turbulent boundary
layer flow, the evolution of particles that were initially distributed in an uniform way in the flow, is analysed. This simple test case represents a first approach for understanding the phenomena that take place within large clouds of pollution, sand storms or when fog disappears under the influence of a rising wind. The period and the size of regions of preferential concentration are determined. This regions are of particular interest in the study of atmospheric dispersion of particles because they can lead to pollution peaks in an otherwise, not polluted atmosphere.
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Submitted on : Tuesday, May 30, 2006 - 4:04:11 PM
Last modification on : Wednesday, July 8, 2020 - 12:42:06 PM
Long-term archiving on: : Monday, April 5, 2010 - 9:47:29 PM

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  • HAL Id : tel-00077281, version 1

Citation

Ivana Vinkovic. Dispersion et mélange turbulents de particules solides et de gouttelettes par une simulation des grandes échelles et une modélisation stochastique lagrangienne. Application à la pollution de l'atmosphère.. Océan, Atmosphère. Ecole Centrale de Lyon, 2005. Français. ⟨tel-00077281⟩

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