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Direct numerical simulations of turbulent convection in a differentially heated vertical channel flow with Prandtl number P r = 0.71 are conducted with a fourth-order accurate finite volume method for a bulk Reynolds number R e b = 4328 and three Grashof numbers Gr ∈ {0, 6.4 · 105, 9.5 · 105}. The discussion of instantaneous flow field snapshots, first- and second-order moments, budget equations, power density spectra and quadrant analyses shows that the turbulent velocity fluctuations are attenuated in the aiding flow and enhanced in the opposing flow. In contrast, temperature fluctuations are attenuated in the opposing flow and enhanced in the aiding flow. The analyses further reveal that the low-momentum flow structures in the aiding flow are warmer than the high-momentum flow structures and vice versa in the opposing flow. Due to their different temperatures, buoyancy accelerates and decelerates the flow structures differently, which leads to reduced and increased pressure and shear fluctuations in the aiding and opposing flow. Thus, the redistribution of turbulent velocity fluctuations is lower in the aiding flow and higher in the opposing flow. The Reynolds shear stresses are consequently decreased in the former and increased in the latter, influencing the production of streamwise velocity fluctuations accordingly. In summary, the discussed physical mechanisms underline the indirect effect of buoyancy on the turbulent velocity fluctuations.

The evolution of plankton in the ocean, the mixing and deposition of sediments in rivers or the manufacture of some food products imply the transport of finite-size, neutrally buoyant material particles by a turbulent flow. Despite the numerous natural and industrial processes where they appear, the dynamics of such particles is still not fully understood. In particular, there is no acceptable model either for their motion or for the manner with which they perturb the carrier flow. The numerical and experimental work of this thesis contribute to a better understanding of such aspects. A first part is dedicated to study numerically the dynamics of isolated spherical particles. I have developed analysis tools that allowed me to quantify their relative motion to the fluid and their local influence on the turbulent flow. A second part concerns the collective effects of a large number of particles onto turbulence. For that, I have performed experimental particle-tracking measurements in a von Kármán flow seeded with various particles volume fractions. The main result is a tendency for the particles to attenuate the amplitude of turbulent fluctuations. At variance, the particles have no influence on the fine statistical properties of the flow. Also, these measurements suggest the large scales of the flow undergo a phase transition at volume fractions above a critical threshold.; L’évolution du plancton dans les océans, le mélange et la déposition de sédiments dans les cours d’eau ou encore la fabrication de certains produits agroalimentaires impliquent le transport de particules matérielles de taille finie et de flottabilité nulle par un écoulement turbulent. Malgré les nombreux processus naturels et industriels où elles interviennent, la dynamique de telles particules reste à ce jour mal comprise. En particulier, il n’existe pas de modèle adéquat pour leur mouvement ou la façon dont elles perturbent le fluide porteur. Les travaux numériques et expérimentaux de cette thèse contribuent à une meilleure compréhension de tels aspects. Un premier volet est consacré à l’étude numérique de la dynamique de particules sphériques isolées. J’ai notamment développé des outils d’analyse qui m’ont permis de quantifier leur mouvement relatif au fluide ainsi que leur influence locale sur l’écoulement turbulent. Un second volet porte sur les effets collectifs d’un grand nombre de grosses particules sur la turbulence. Pour cela j’ai effectué des mesures expérimentales par suivi lagrangien dans un écoulement de von Kármán ensemencé avec différentes fractions volumiques de particules. Le résultat principal est que les particules ont tendance à atténuer l’amplitude des fluctuations turbulentes. En revanche, celles-ci n’ont pas d’influence sur les propriétés statistiques fines de l’écoulement. Aussi, ces mesures suggèrent l’existence d’une transition de phase dans les grandes échelles de l’écoulement pour des fractions volumiques de particules au delà d’un seuil critique.

This paper is focused on the effect of turbulence in atmospheric transmission media. A short introduction of the origin of turbulent atmosphere is given. Two classical methods for the calculation of turbulence attenuation are mentioned and briefly described in the paper – the Rytov approximation and Andrews’s method, and a new technique designed by the authors of this paper – the method of available power – is presented in short as well. We have run simulations in Matlab to compare the results of turbulence attenuation given by the classical methods and by the new technique - the method of available power. These calculations were performed for communication wavelengths of 850 nm and 1550 nm. The distance between the optical transmitter and receiver of horizontal links was set to values ranging from 0 m to 2500 m. We have taken into account the homogenous turbulence with low (10^-16 m^-2/3), medium (10^-15 m^-2/3) and high (10^-14 m^-2/3) structure parameter of refractive index Cn2.