Identification of turbulent structures of inclined negatively buoyant jets with bed effects

Abstract

Present study aims at exploring turbulent structures of inclined negatively buoyant jets (INBJs) using large eddy simulation (LES). Four types of turbulent structures are observed using the Q criterion and vorticity; namely, shear layer vortices, buoyancy-induced vortices, counter-rotating vortex pair (CRVP), and gravity current structures. The shear layer vortices emerge and develop in the momentum-driven regime of INBJ, where the jet core concentration reduces by nearly 20% due to the small amount of entrainment. Further downstream, where the negative buoyancy dominates the INBJ, the buoyancy-induced vortices form, which are larger and weaker than the momentum-driven structures. The velocity magnitude of the entrained flow also increases by a factor of 4, and the relative concentration decay rises to 70% in the buoyancy-driven region. A CRVP is also detected in the jet cross-sections. The vortex pair may be induced by a secondary flow inside the jet sections due to the jet deflection by gravity. The size of CRVP constantly grows from upstream to downstream, while the vorticity magnitude of it has an increasing-decreasing trend due of the changes in the jet trajectory curvature. A density current forms with big rollers on its edges when the plume-like flow impacts the bottom. Similar to previous studies of density currents, the streaky structure is also observed on the bed. Furthermore, it is found that increasing the bed slope results in bigger and more dispersed turbulent structures, especially in the plume-like regime after the maximum height point.