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Hydrodynamics of weakly and strongly stratified two-layer lock-release gravity currents

A series of lock-release experiments are carried out to investigate the hydrodynamics of weakly and strongly stratified two-layer gravity currents (GC) on a flat bottom. A particle image velocimetry (PIV) system combined with an optical method is used to obtain the mixing process, front velocity, and velocity profiles of the two-layer GCs, focusing on the effect of the initial height ratio and lock aspect ratio. For a weakly stratified two-layer GC, the two layers mix thoroughly earlier with a higher initial height ratio and lock aspect ratio. A theoretical model is improved to predict the time that is necessary for the thorough mixing during the propagation of weakly stratified two-layer GCs. For a strongly stratified two-layer GC, a higher initial height ratio leads to a larger velocity of the upper layer because of the larger initial density difference, which results in a shorter time for the upper layer to outrun the lower layer. After the upper layer outruns the lower layer, the current has a constant-velocity phase and then a deceleration phase. The present investigation indicates that the maximum velocity in a vertical velocity profile of a strongly stratified two-layer GC is located at the position near to, or higher than the upper limit for single-layer GCs due to the smaller drag resistance at the upper boundary.