Colliding gravity currents in a stratified environment

This study examines the collision of two counterflowing gravity currents (GCs) in a stratified environment using large-eddy simulation. A half-depth lock-exchange configuration with linearly stratified ambient fluid is used to study symmetric collision (i.e., GCs with the same densities and heights). Numerical experiments on collision of both subcritical and supercritical GCs are performed for weak and strong stratification. The focus is on the investigation of both collision and post-collision phases, with emphasis on the height and the vertical velocity of the collided fluid, the bore velocity during the post-collision phase, and the mixing and its efficiency. The results indicate that stronger stratification causes the GCs to become subcritical and decelerate before collision. In contrast, for weaker stratification, GCs become supercritical and the collision resembles that in a non-stratified environment. Subcritical collision is characterized by oscillatory vertical motion of the collided fluid, which can suppress bore formation. Supercritical collision is distinguished by clearly defined phases and bore development during the post-collision phase, similar to that in a non-stratified environment. The qualitative difference between the collision of subcritical and supercritical GCs is due to the occurrence of internal waves, which emerge during GC propagation before collision and interfere in the middle of the tank, determining collision dynamics. Mixing during collision is also affected by stratification strength. While stronger stratification suppresses turbulence and mixing, weaker stratification enhances turbulence and broadens the mixing region, leading to higher mixing with reduced mixing efficiency.