SUMMARYA Reynolds stress model for the numerical simulation of uniform 3D turbulent open-channel ows is described. The ÿnite volume method is used for the numerical solution of the ow equations and transport equations of the Reynolds stress components. The overall solution strategy is the SIMPLER algorithm, and the power-law scheme is used to discretize the convection and di usion terms in the governing equations. The developed model is applied to a ow at a Reynolds number of 77 000 in a rectangular channel with a width to depth ratio of 2. The simulated mean ow and turbulence structures are compared with measured and computed data from the literature. The computed ow vectors in the plane normal to the streamwise direction show a small vortex, called inner secondary currents, located at the juncture of the sidewall and the free surface as well as the free surface and bottom vortices. This small vortex causes a signiÿcant increase in the wall shear stress in the vicinity of the free surface. A budget analysis of the streamwise vorticity is carried out. It is found that both production terms by anisotropy of Reynolds normal stress and by Reynolds shear stress contribute to the generation of secondary currents.
Numerical investigations of mean flow and turbulence structures of partly-vegetated open-channel flows using the Reynolds stress model Investigations Numériques sur l'écoulement moyen et les structures turbulentes en canaux à surface libre partiellement revêtus de végétaux en utilisant le modèle de Reynolds SUNG-UK CHOI, Professor, ABSTRACT A Reynolds stress model for the numerical simulation of partly-vegetated flows is presented. The model uses Speziale, Sarkar, and Gatski's model for the pressure-strain correlation, Mellor and Herring's model and Rotta's model for the diffusion and the dissipation rate of the Reynolds stress, respectively. The model is applied to partly-vegetated rectangular open-channel flows, and simulated results are compared with experimental data. The model satisfactorily predicts mean flow and turbulence statistics. Through numerical experiments, the evolution of secondary current patterns and mean flow structure are presented for different densities of vegetation. A budget analysis of the streamwise vorticity equation is also performed to investigate the mechanism by which secondary currents in a partly-vegetated open-channel flow are generated. In the vegetated zone, the production by anisotropy is important in generating secondary currents over the entire depth, except for regions close to the free surface and the bottom where Reynolds shear stress plays a key role. This is different from the vortical structure of plain open-channel flows.
RÉSUMÉOn présente un modèle de contrainte de Reynolds pour la simulation numérique des écoulements en milieu partiellement végétal. On utilise le modèle de Speziale, Sarkar, et Gatski pour la corrélation pression-contrainte, le modèle de Mellor et Herring et le modèle de Rotta pour la diffusion et le taux de dissipation de la contrainte de Reynolds, respectivement. Le modèle est appliqué à des écoulements en canaux ouverts rectangulaires partiellement revêtus de végétaux, et les résultats sont comparés à des données expérimentales. Le modèle prévoit d'une manière satisfaisante l'écoulement moyen et les statistiques de turbulence. Au moyen d'expériences numériques, on présente l'évolution du courant secondaire et de la structure de l'écoulement moyen pour différentes densités de végétation. Une analyse du bilan de l'équation de vorticité dans le sens du courant est également exécutée pour étudier le mécanisme par lequel les courants secondaires sont générés dans ce type d'écoulement. Dans la zone végétale, la production par l'anisotropie est importante en générant des courants secondaires sur toute la profondeur, excepté près de la surface libre et du fond où l'effort de cisaillement de Reynolds joue un rôle clé. Cela est différent de la structure verticale des écoulements simples en canaux à surface libre.
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