Head-on collision of gravity currents of unequal strengths: Large-eddy simulations and laboratory experiments

The collision of two counterflowing gravity currents with unequal strengths was investigated through large-eddy simulations and laboratory experiments. The collisions were initiated by releasing currents from two partial-depth locks at identical heights but with different densities, characterized by the reduced gravity ratio, γg. By varying γg, we elucidate the transport processes of colliding gravity currents, spanning from comparable driving strengths (γg=1.0) to markedly disparate driving strengths (γg≪1). Three distinct regimes of colliding gravity currents were identified based on kinematic features derived from integrated measures. For γg≥0.92, the collisions are driven by counterflowing gravity currents with comparable driving strengths, leading to nearly symmetrical collisions with negligible impact on evolved flow patterns. In the intermediate regime when 0.4<γg<0.92, the collisions are weakly asymmetric, characterized by differing contact surface steepness and insensitive maximum vertical displacement of ascending motions to γg. For γg≤0.4, strongly asymmetric collisions dominate, featuring minimal vertical convective fluxes in the collision region rather than typical colliding currents. In this flow regime, the denser fluid mass intrudes beneath the less dense one, akin to the propagation of intrusive lock-exchange gravity currents. Additionally, mixing rates over the entire flow domain were quantified using background potential energy calculations. The results reveal that intense diapycnal mixing is predominantly driven by stirring processes before collision, with the mixing rate increasing as γg decreases. From the collision stage onward, currents with the gain of inertia converge within the collided region and move upward with the distinct opposite effect of negative buoyancy. Notably, the mixing rate stabilizes regardless of convective process variations and decreases consistently as the currents slump away from the collision region.