The relative dispersion of one fluid particle with respect to another is fundamentally related to the transport and mixing of contaminant species in turbulent flows. The most basic consequence of Kolmogorov's 1941 similarity hypotheses for relative dispersion, the Richardson-Obukhov law that mean-square pair separation distance grows with the cube of time at intermediate times in the inertial subrange, is notoriously difficult to observe in the environment, laboratory, and direct numerical simulations (DNS). Inertial subrange scaling in size parameters like the mean-square pair separation requires careful adjustment for the initial conditions of the dispersion process as well as a very wide range of scales (high Reynolds number) in the flow being studied. However, the statistical evolution of the shapes of clusters of more than two particles has already exhibited statistical invariance at intermediate times in existing DNS. This invariance is identified with inertial-subrange scaling and is more readily observed than inertial-subrange scaling for seemingly simpler quantities such as the mean-square pair separation Results from dispersion of clusters of four particles (called tetrads) in large-scale DNS at grid resolutions up to 4096 points in each of three directions and Taylor-scale Reynolds numbers from 140 to 1000 are used to explore the question of statistical universality in measures of the size and shape of tetrahedra in homogeneous isotropic turbulence in distinct scaling regimes at very small times (ballistic), intermediate times (inertial) and very late times (diffusive). Derivatives of fractional powers of the mean-square pair separation with respect to time normalized by the characteristic time scale at the initial tetrad size constitute a powerful technique in isolating cubic time scaling in the mean-square pair separation. This technique is applied to the eigenvalues of a moment-of-inertia-like tensor formed from the separation vectors between particles in the tetrad. Estimates of the proportionality constant "g" in the Richardson-Obukhov law from DNS at a Taylor-scale Reynolds number of 1000 converge towards the value g=0.56 reported in previous studies. The exit time taken by a particle pair to first reach successively larger thresholds of fixed separation distance is also briefly discussed and found to have unexplained dependence on initial separation distance for negative moments, but good inertial range scaling for positive moments. The use of diffusion models of relative dispersion in the inertial subrange to connect mean exit time to "g" is also tested and briefly discussed in these simulations. Mean values and probability density functions of shape parameters including the triangle aspect ratio "w," tetrahedron volume-to-gyration radius ratio, and normalized moment-of-inertia eigenvalues are all found to approach invariant forms in the inertial subrange for a wider range of initial separations than size parameters such as mean-square gyration radius. These results constitute the clearest evidence to date that turbulence has a tendency to distort and elongate multiparticle configurations more severely in the inertial subrange than it does in the diffusive regime at asymptotically late time. Triangle statistics are found to be independent of initial shape for all time beyond the ballistic regime. The development and testing of different schemes for parallelizing the cubic spline interpolation procedure for particle velocities needed to track particles in DNS is also covered. A "pipeline" method of moving batches of particles from processor to processor is adopted due to its low memory overhead, but there are challenges in achieving good performance scaling.

La turbulence est connue pour sa capacité à disperser efficacement de la matière, que ce soit des polluantes dans les océans ou du carburant dans les moteurs à combustion. Deux considérations essentielles s’imposent lorsqu’on considère de telles situations. Primo, l’écoulement sous-jacente pourrait avoir une influence non-négligeable sur le comportement des particules. Secundo, la concentration locale de la matière pourrait empêcher le transport ou l’augmenter. Pour répondre à ces deux problématiques distinctes, deux dispositifs expérimentaux ont été étudiés au cours de cette thèse. Un premier dispositif a été mis en place pour étudier l’écoulement de von Kàrmàn, qui consiste en une enceinte fermé avec de l’eau forcé par deux disques en contra-rotation. Cette écoulement est connu pour être très turbulent, inhomogène, et anisotrope. Deux caméras rapides ont facilité le suivi Lagrangien des particules isodenses avec l’eau et petites par rapport aux échelles de la turbulence. Ceci a permis une étude du bilan d’énergie cinétique turbulente qui est directement relié aux propriétés de transport. Des particules plus lourdes que l’eau ont aussi été étudiées et montrent le rôle de l’anisotropie de l’écoulement dans la dispersion des particules inertielles. Un deuxième dispositif, un écoulement de soufflerie ensemencé avec des gouttelettes d’eau micrométriques a permis une étude de l’effet de la concentration locale de l’eau sur la vitesse de chute des gouttelettes grâce à une montage préexistant. Un modèle basé sur des méthodes théorique d'écoulements multiphasiques a été élaboré enfin de prendre en compte les effets collectifs de ces particules sedimentant dans un écoulement turbulent. Les résultats théoriques et expérimentaux mettent en évidence le rôle de la polydispersité et du couplage entre les deux phases dans l’augmentation de la sédimentation des gouttelettes. / Turbulence is well known for its ability to efficiently disperse matter, whether it be atmospheric pollutants or gasoline in combustion motors. Two considerations are fundamental when considering such situations. First, the underlying flow may have a strong influence of the behavior of the dispersed particles. Second, the local concentration of particles may enhance or impede the transport properties of turbulence. This dissertation addresses these points separately through the experimental study of two different turbulent flows. The first experimental device used is the so-called von K\'arm\'an flow which consists of an enclosed vessel filled with water that is forced by two counter rotating disks creating a strongly inhomogeneous and anisotropic turbulence. Two high-speed cameras permitted the creation a trajectory data base particles that were both isodense and heavier than water but were smaller than the smallest turbulent scales. The trajectories of this data base permitted a study of the turbulent kinetic energy budget which was shown to directly related to the transport properties of the turbulent flow. The heavy particles illustrate the role of flow anisotropy in the dispersive dynamics of particles dominated by effects related to their inertia. The second flow studied was a wind tunnel seeded with micrometer sized water droplets which was used to study the effects of local concentration of the settling velocities of these particles. A model based on theoretical multi-phase methods was developed in order to take into account the role of collective effects on sedimentation in a turbulent flow. The theoretical results emphasize the role of coupling between the underlying flow and the dispersed phase.