Mass transport mechanisms and turbulence structures of intrusive gravity currents at channel confluences: Insights from large eddy simulations

Abstract

This study investigates the hydrodynamics of intrusive bottom-propagating gravity currents at channel confluences using large-eddy simulations, emphasizing interactions between buoyancy-driven flows and the ambient momentum of the main channel. The analysis systematically examines how the velocity ratio (β) between confluent flows regulates transport processes and turbulence dynamics during the transition from quasi-planar to fully three-dimensional gravity currents. Three distinct flow regimes are identified: buoyancy-dominated (β ≤ 0.47), transitional (0.47 < β ≤ 1.42), and momentum-dominated (β > 1.42) cases. In buoyancy-dominated cases, gravity currents form nearly axisymmetric, radially spreading fronts characterized by lobe–cleft patterns and coherent vortex rings. A distinct transition occurs beyond β = 1.42, where momentum dominance suppresses lateral spreading and promotes rapid downstream reorientation. Within this regime, vortical structures at the mixing interface evolve from laminar sheet-like layers to tube-shaped and arch-like vortices, eventually breaking down into smaller-scale turbulence. Streamwise-oriented vortices emerge as the dominant coherent structures within the mixing layer, resembling those found at natural river confluences, where momentum contrasts between converging flows sustain longitudinal vorticity. Despite geometric and dynamic differences between density-driven intrusions and homogeneous confluent flows, the interfacial vortical evolution exhibits notable similarity, suggesting that shear-induced instabilities primarily govern mixing dynamics.