Gravity currents : laboratory experiments and numerical simulations

Abstract

Gravity currents are buoyancy-driven flows due to a density gradient between two fluids and frequently occur in both natural and industrial flows. In this work two-dimensional and three-dimensional gravity current’s dynamics was investigated by laboratory experiments and numerical simulations. A widely used experimental technique, called lock exchange release was applied to carry out laboratory experiments. In this configuration the tank is divided by a vertical gate into two parts, filled with salty and fresh water, respectively. As soon as the gate is removed, a non-equilibrium condition occurs and the heavier fluid flows under the lighter one, producing the gravity current, whose evolution is recorded by a CCD camera. An image analysis technique, based on the threshold method is then applied in order to measure the space-time evolution of the current’s profile. Experimental 2D gravity currents were realized in order to study the effect of the density difference between the two fluids and both the roughness and the slope’s angle of the bed on the current’s dynamics. In particular, one of the innovative aspects of this paper is to be focused on gravity currents on upsloping bed, while to the author’s knowledge most of the previous studies deals with currents flowing on downsloping beds. Moreover, regarding 2D configuration, instantaneous velocity measurements were performed by PIV technique (Particle Image Velocimetry). Numerical simulations of 2D gravity currents were performed by a 1D, two-layer, shallow-water model developed by Adduce et al. (2012). The model takes into account the space-time evolution of free-surface and the mixing between the two layers. Entrainment at the interface between the gravity current and the ambient fluid is modeled by a modified Ellison & Turner’s formula (1959). Several tests were run to calibrate an entrainment parameter in order to reproduce gravity currents moving on both smooth flat and upsloping beds. Experimental 3D gravity currents were carried out in order to test different values of initial density and height of the current and the length of the gate. A single layer, 2D, shallow-water model was used to perform numerical simulations for 3D currents. As for the 1D model, the entrainment is taken into account in the flow’s dynamics. Experimental results and the comparison between experimental data and numerical prediction for both 2D and 3D configuration are presented, showing that the used models are valid tools to reproduce gravity currents’ dynamics.