An interaction by exchange with the conditional mean (IECM) micromixing model is coupled to a three-dimensional single-particle Lagrangian stochastic (LS) model to estimate concentration fluctuations in plumes of a passive (i.e., non-buoyant), non-reactive (i.e., no chemistry) tracer dispersing from a variety of source configurations in four neutrally stratified flows: a horizontally-homogeneous wall shear layer flow; a horizontally-homogeneous representation of the Tombstone canopy flow; a three-dimensional inhomogeneous representation of the Tombstone canopy flow; and a three-dimensional inhomogeneous representation of the Mock Urban Setting Trials (MUST) canopy flow. The IECM micromixing model incorporates the combined effects of turbulent and molecular mixing on particle concentration. This allows the numerical estimation of all moments of the scalar concentration field, which is a significant advance over traditional LS models given that concentration fluctuations are a ubiquitous feature of a dispersing plume. The single-particle implementation of the LS-IECM model is based upon a previously reported implementation that used simultaneously computed particle trajectories to estimate the conditional mean concentration field [Cassiani, M. A., Franzese, P. A. and Giostra, U. A.: 2005, A PDF micromixing model of dispersion for atmospheric flow. Part I: development of model, application to homogeneous turbulence and to a neutral boundary layer, Atmospheric Environment 39, 1457-1469]. The model used in this thesis pre-calculates the conditional mean concentration field with an LS model for use with the IECM model, which runs as a separate simulation. The principal advantage of this single-particle approach is the performance increase on parallel computer architecture, which scales directly with the number of processors. The simulations presented in this thesis go beyond those performed with the previous model by considering three-dimensional inhomogeneous flows, as well as one-dimensional horizontally-homogeneous flows. The accuracy of the LS-IECM model was good for the flows with horizontal-homogeneity, and comparable to the results of previous simulations from older models. Rogue velocities in the simulations utilising inhomogeneous flow statistics resulted in acceptable to poor accuracy in these simulations. Suggestions for improvements to the model are made.

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This study analyzes a turbulent passive scalar field using two-point and three-point correlations of the fluctuating scalar field. Multipoint correlation functions are investigated because they retain scaling property information and simultaneously probe the concentration field for the spatial structure of the scalar filaments. Thus, multipoint correlation functions provide unique information about the spatial properties of the concentration filaments. The concentration field is created by the iso-kinetic release of a high Schmidt number dye into a fully developed turbulent boundary layer of an open channel flow. The concentration fields were previously measured using the planar laser-induced fluorescence technique. The two-point correlations of the fluctuating scalar field indicate that as the scalar field evolves downstream, the anisotropic influence of the tracer injection method diminishes, and the scalar field becomes dominated by the mean velocity shear. As the scalar filaments align with the mean velocity gradient, the elliptical shape associated with the contours of the correlation function tilts in the direction of the mean velocity gradient. As a result, the two-point correlation contours of the concentration fluctuations indicate that anisotropic conditions (i.e. the tilted, asymmetric, elliptical shape) develop as a consequence of the mean velocity shear. Three-point correlations of the fluctuating scalar field are calculated based on configuration geometries defined by previous researchers. The first configuration follows Mydlarski and Warhaft (1998), which employs two cold-wire measurements and Taylor's frozen turbulence hypothesis. The three-point correlation contours of the concentration fluctuations associated with the cold-wire measurements exhibit a symmetric characteristic V-shape. Similar symmetric properties are observed in the current study. The second set of configurations follows on recent theoretical predictions, which indicate that the three-point correlation of the fluctuating scalar field is dependent on the size, shape, and orientation of the triangle created by the three points. The current study analyzes two geometric configurations (isosceles and collinear). The geometric configurations are defined to ensure that the influence of the shape remains constant as the configuration is rotated, translated, and dilated. Additionally, the scaling exponent in the inertial-convective regime is calculated to determine the dependence of the correlation function on the size of the triangle pattern.

This experimental study analyses the dynamics of concentration fluctuations in a passive plume emitted by a point source within the turbulent boundary layer. We aim to extend the popular study of Fackrell and Robins (1982) about concentration fluctuations and fluxes from point sources by including third and fourth moments of concentration. We also further inquire into the influence of source conditions, such as the source size, source elevation and emission velocity, on higher order concentration moments. The data set is completed by a detailed description of the velocity statistics within the TBL, with exhaustive information on both the temporal and spatial structure of the flow. The experimental data-set has been used to test two different modeling ap- proaches: an analytical meandering plume model (in one and in three dimen- sions) and a Lagrangian stochastic micro-mixing model.

Cette étude, menée principalement par des moyens expérimentaux, analyse la dynamique des fluctuations de concentration produites au sein d’un panache d’un scalaire passif émis par une source ponctuelle dans une couche limite turbulente. En étendant le travail de Fackrell and Robins (1982) sur les fluctuations de concentration, on décrit l’évolution des quatre premiers moments de la concentration et on étudie l’influence des conditions à la source (telles que la taille et la hauteur de la source et la vitesse de sortie) sur les moments mêmes. L’étude est complétée par une description détaillée des statistiques du champ de vitesse dans la couche limite, comprenant des informations sur la structure spatiale et temporelle de la turbulence. Cette base de données expérimentale a été utilisée pour valider deux différentes approches à la modélisation de la dispersion: un modèle analytique de meandering et un modèle Lagrangien stochastique avec un schéma de micro mélange. / This experimental study analyses the dynamics of concentration fluctuations in a passive plume emitted by a point source within the turbulent boundary layer. We aim to extend the popular study of Fackrell and Robins (1982) about concentration fluctuations and fluxes from point sources by including third and fourth moments of concentration. We also further inquire into the influence of source conditions, such as the source size, source elevation and emission velocity, on higher order concentration moments. The data set is completed by a detailed description of the velocity statistics within the TBL, with exhaustive information on both the temporal and spatial structure of the flow. The experimental data-set has been used to test two different modeling ap- proaches: an analytical meandering plume model (in one and in three dimen- sions) and a Lagrangian stochastic micro-mixing model.