A numerical study of the triggering mechanism of a lock-release density current

Abstract

A numerical study on the effects induced by the impulsive vertical removal of a lock-gate at the interface between two fluids of different densities is presented. This configuration represents the typical setup of those experiments commonly employed for investigating density currents in the laboratory. Experimentally induced effects resulting from opening the lock-gate are expected to occur, but the evaluation of these dynamics and their impact on the evolution of the laboratory density current produced in such a manner are not easy to estimate. Despite the fact that numerical studies are often concerned with lock-release density currents, the triggering mechanism which occurs in the early stages of the evolution of the fluid flow is commonly neglected. Here a comparison is established between the case when the triggering mechanism is completely neglected and a series of cases where, in contrast, this effect is taken into account. The withdrawal of the lock-gate is modeled either by employing a zero-thickness lock-gate or by accounting for the volumetric nature of the lock-gate. Subsequently the influence of speed on the withdrawal of the lock-gate is assessed. The numerical results suggest that the density current is mainly affected by the constraining effect of the lock-gate on the flow and by the responses of the submerged fluid and the free surface to the displacement of the lock-gate. These differences lead to improved physical modeling and numerical simulation validation in the case where the physics of the lock-gate is accounted for. Such differences can be very important particularly in particulate-laden flows, where small changes in initial conditions may lead to longer-term divergence as a result of positive feedback effects. The work has significant implications for physical modeling of density currents and a series of recommendations are made for the standardization of experimental protocols. Finally, the approach adopted here for the moving gate is applicable to civil and environmental engineering problems including dam-break flows and sluice gate modeling.