Abstract
Large eddy simulation (combined with the mixture model) and laboratory experiment were used to investigate the impact of emergent and rigid vegetation on the dynamics of downslope gravity currents in stratified environments. The reliability of the numerical model was assessed with the corresponding laboratory measurements. The results show that the vegetation cylinders lead to severe lateral non-uniformity of the current front, causing more evident lobe and cleft structures. In stratified environments, the smaller driving force leads to less propagating velocity until the current separates from the slope. The transition point (from acceleration to deceleration phases) of current velocity appears earlier as the vegetation becomes denser. The peak value of the bulk entrainment coefficient Ebuik is inversely proportional to the vegetation density, while the final converged value of Ebuik is proportional to the vegetation density. Vegetation patches make the degree of fluctuation of the instantaneous entrainment coefficient Einst more intense, and even negative values appear locally, indicating that the gravity current is detrained into the ambient fluid. The velocity profiles of gravity current develop multi-peak patterns in stratified environments due to fingering intrusive patterns. Our analysis reveals that as the vegetation density increases, the generated wakes behind vegetation cylinders increase local entrainment and mixing, causing the density of current flow from vegetation to decrease and reach the neutral buoyancy layer of ambient fluids earlier, finally leading to a smaller separation depth.
Highlights
Gravity currents, called density currents, are driven by the density difference between two fluids, mainly in the horizontal direction [1]
Downslope gravity currents could horizontally separate from the slope at the location where the current density and density of ambient fluids are equal; multiple intrusions of current appear, and the separation depth decreases with the increasing stratification level of the ambient water [5,9]
This is because gravity currents in stratified environments tend to horizontally intrude into the neutral buoyancy layer at the beginning stage and enlarge the mixing regions with ambient fluids, which is favorable to entrainment processes
Summary
Called density currents, are driven by the density difference between two fluids, mainly in the horizontal direction [1]. Downslope gravity currents could horizontally separate from the slope at the location where the current density and density of ambient fluids are equal ( called the neutral buoyancy layer); multiple intrusions (fingering) of current appear, and the separation depth decreases with the increasing stratification level of the ambient water [5,9]. These studies revealed that the stratified environments generally reduce turbulent mixing and current head velocity [5]. The conclusions drawn can provide a scientific basis for the propagation and separation of gravity currents in nearshore stratified environments
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