High resolution numerical simulations of lock-exchange gravity-driven flows

This dissertation focuses on the analysis of lock-exchange gravity-driven flows at high Grashof numbers using highly resolved numerical simulations and Large Eddy Simulation (LES) techniques. The present method uses a non-dissipative Navier-Stokes solver in which the sub-grid scale (SGS) viscosity and diffusivity are calculated dynamically. The use of LES allowed the study of these gravity current flows at Grashof numbers (Gr=109-1012) much higher than those previously achieved using Direct Numerical Simulation (DNS). This is important because most practical applications of gravity current flows in river, coastal and ocean engineering occur at Grashof numbers much higher than the ones considered in previous DNS simulations and even in experimental laboratory studies. Three different Boussinesq compositional gravity-current configurations are examined in detail. The first configuration corresponds to the case of a lock exchange flow in an infinite channel in which the volume of the heavier lock fluid is infinite. The second configuration is the case in which the heavier lock fluid is finite (bottom propagating current). The third configuration corresponds to an intrusion current in which a certain amount of lock fluid is released into a two-layer ambient fluid. For all the configurations, it is found that the three-dimensional (3D) simulations correctly predict the main quantitative (e.g., front velocity at the different phases of the evolution of the current, bore velocity, etc.) and qualitative (e.g., formation of interfacial Kelvin-Helmholtz billows, their stretching and eventual break up into 3D turbulence) aspects of these flows observed in experimental investigations. The spatial and temporal bed shear stress distributions induced by the passage of bottom propagating currents are analyzed in detail, as accurate prediction of this quantity is essential in estimating the amount of sediment entrained by a compositional current propagating over a loose bed in river and coastal applications. Additionally, the energy budget, the distribution of the spanwise-averaged local dissipation rate and the streamwise distributions of the local dissipation rate integrated over vertical planes are analyzed at different stages of the evolution of the current. All these quantities are practically impossible to be determined experimentally but essential in understanding the physics of the flow at the different stages of the evolution of the gravity current.