Influence of Regular Surface Waves on the Propagation of Gravity Currents: Experimental and Numerical Modeling

Abstract

The propagation of gravity currents is analyzed in the presence of regular surface waves, both experimentally and numerically, by using a full-depth lock-exchange configuration. Full-depth lock-exchange releases have been reproduced in a wave flume, both in the absence and in the presence of regular waves, considering two fluids having densities ρ0 and ρ1, with ρ0<ρ1. Boussinesq gravity currents have been considered here (ρ0/ρ1∼1), with values of the reduced gravity g′ in the range 0.01–0.1 m/s2, while monochromatic waves have been generated in intermediate water depth. The experimental results show that the hydrodynamics of the density current are significantly affected by the presence of the wave motion. In particular, the front shows a pulsating behavior and the shape of the front itself is less steep than in the absence of waves, while turbulence at the interface between the two fluids is damped out. In the present test conditions, the average velocity of the advancing front may be decreased in the presence of the combined flow as a function of the relative importance of buoyancy compared with wave-induced Stokes drift. Moreover, a new numerical model is proposed, aiming at obtaining a simple, efficient, and accurate tool to simulate the combined motion of gravity currents and surface waves. The model is derived by assuming that surface waves are not affected by gravity current propagation at the leading order and that the total velocity field is the sum of velocities forced by the orbital motion and those forced by buoyancy. A Boussinesq-type wave model for nonstratified fluids is solved, and its results are used as input of a gravity current model for stratified flows. Comparisons of the numerical results with the present experimental data demonstrate the capability of the model to predict the main features of the analyzed phenomena concerning propagation of the density current (e.g., averaged velocities, front height), the increase of entrainment of the ambient fluid into the density current in the presence of the waves, and the intrawave pulsating movement of the heavy front.