Propagation and interaction of gravity currents induced by successive release of two lock fluids

This study investigates the dynamics of sequentially released gravity currents in a lock-exchange configuration consisting of two lock fluids using high-resolution numerical simulations and compares them with the classical single-lock exchange. The results demonstrate that, in a two-lock-fluid configuration, the overtaking of the lighter lock current by the heavier lock current alters the current’s front dynamics, leading to complex velocity transitions. Before overtaking occurs, the front propagation is slower than in the classical single-lock-fluid case because of lower density contrast, but after overtaking, the heavier lock fluid accumulates at the head region of the current and enhances its speed. The head of the current is primarily dominated by streamwise velocity vectors, which directly influence the front propagation speed. The body of the current exhibits significant components of streamwise and wall-normal velocities, characterised by large eddies and Kelvin–Helmholtz billows at the interface, enhancing entrainment and mixing. In contrast, the tail region consists of small-scale eddies, which contribute to viscous dissipation, gradually reducing the current’s momentum. A parametric study of the two-lock-fluid configuration, conducted to investigate the effects of non-dimensionalised time of delay in the release of heavier lock fluid, $t_R^*$ , the ratio of densities of ambient fluid to heavier lock fluid, $\gamma _2=\rho _a/\rho _2$ , and the non-dimensionalised time of overtaking by the heavier lock fluid, $t_O^*$ , revealed a nonlinear relationship. As local Reynolds number decreases $(Re_l\lesssim 10{\,}000)$ , the relationship becomes nonlinear due to weaker buoyancy forcing of the heavier current travelling in the wake of the lighter current. However, for $Re_l\gtrsim 10{\,}000$ , this relationship becomes linear.