Abstract
The structure and propagation of lock-release bottom gravity currents in a linearly stratified ambient with the presence of a submerged canopy are investigated for the first time using large-eddy simulations. The canopy density (i.e. the solid volume fraction), the strength of ambient stratification and the canopy height are varied to study their respective effects on the gravity current. Both denser canopies and stronger ambient stratification tend to switch the horizontal boundary along which the current propagates from the channel bed towards the canopy top (i.e. the through-to-over flow transition). It is found that the dilution of the current density is enhanced by denser canopies but is weakened by stronger ambient stratification. The non-monotonic relationship between front velocity and canopy density proposed by Zhou et al. (J. Fluid Mech., vol. 831, 2017, pp. 394–417) in homogeneous environments is also observed in stratified environments. However, as the ambient stratification is strengthened, the present study shows a shift of the turning point (beyond which increasing canopy density leads to faster current propagation) towards sparser canopies, accompanied by a more pronounced recovery of the front velocity. This is the combined action of three stratification-induced mechanisms: the promotion of through-to-over flow transition (less canopy drag), the upward displacement of current nose in a stably stratified water column (more buoyancy gain) and the weakening of current dilution (less buoyancy loss). Under stronger ambient stratification, the propagation of gravity currents shows a lower sensitivity to the retarding effect of the submerged canopy.
Highlights
Gravity currents are flows driven by horizontal density variations
As the ambient stratification is strengthened, the present study shows a shift of the turning point towards sparser canopies, accompanied by a more pronounced recovery of the front velocity
The over-nose is thinner and subject to stronger vertical convection inside the canopy compared with the case of a short canopy; there is a larger amount of ambient fluid beneath the over-nose, and the unstable stratification across the canopy’s top boundary is stronger since the dense current is propagating at a higher level in the water column
Summary
Gravity currents are flows driven by horizontal density variations. There are many examples of gravity currents, both naturally occurring and man-made (Simpson 1997). A handful of previous studies have dealt with gravity currents interacting with a submerged canopy in a homogeneous ambient. We investigate for the first time the dynamics of bottom gravity currents propagating over a long canopy submerged in a linearly stratified ambient using highly resolved three-dimensional. Not immediately (and directly) represent the forcing and geometry associated with the various real systems, the insights gained have broad practical relevance, e.g. buoyant river plumes interacting with kelp forests in the stratified coastal ocean, salt wedges propagating over bottom roughness in estuaries with tidal variations in stratification, and atmospheric gravity fronts (e.g. haboobs and sea breezes) advancing through urban canopies under stable stratification.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
