Mixing layer and coherent structures in compound channel flows: effects of transverse flow, velocity ratio and vertical confinement
Couche de mélange et structures cohérentes dans des écoulements en lit composé: effets de courants transverses, du ratio de vitesse et du confinement vertical
Proust, S. ; Fernandes, J.N. ; Leal, J.B. ; Riviere, N. ; Peltier, Y.
Type de document
Article de revue scientifique à comité de lecture
Affiliation de l'auteur
IRSTEA LYON UR HHLY FRA ; HYDRAULICS AND ENVIRONMENT DEPARTMENT NATIONAL LABORATORY FOR CIVIL ENGINEERING LISBOA PRT ; FACULTY OF ENGINEERING AND SCIENCE UNIVERSITETET I AGDER GRIMSTAD NOR ; UNIVERSITE DE LYON INSA LYON CNRS LMFA UMR 5509 LYON FRA ; LHSV ECOLE DES PONTS CEREMA EDF UPE CHATOU FRA
Résumé / Abstract
Turbulent mixing layers associated with streamwise uniform and non-uniform flows in compound channels (main channel with adjacent floodplains) are experimentally investigated. The experiments start with uniform flow conditions. The streamwise non-uniformity is then generated by imposing an imbalance in the upstream discharge distribution between main channel (MC) and floodplains (FPs), keeping the total discharge constant, which results in a transverse depth-averaged mean flow. This study firstly aims at assessing the effect of a transverse flow on the mixing layer and coherent structures that form at the MC/FP interfaces. A wide range of initial velocity ratio or dimensionless shear between MC and FP is tested. The study secondly aims at assessing the effect of this velocity ratio on the mixing layer, for a fixed vertical confinement of flow. The total discharge was then varied to quantify the confinement effect. The results show that, far from the inlet section, Reynolds-stresses increase with local velocity ratio for a fixed confinement, and decrease with confinement for a fixed velocity ratio. It is also shown that, irrespective of confinement, the existence of quasi-two-dimensional coherent structures is driven by velocity ratio and the direction and magnitude of transverse flow. These structures cannot develop if velocity ratio is lower than 0.3 and if a strong transverse flow towards the MC occurs. In the latter case, the transverse flow is the predominant contribution to momentum exchange (compared with turbulent mixing and secondary currents), convex mean velocity profiles are observed, preventing the formation of quasi-two-dimensional structures.
Water Resources Research, vol. 53, num. 4, p. 3387 - 3406