Boundaries behaviour of sediment gravity current

Gravity currents are buoyancy-driven flows generated by horizontal density gradients, governing the transport of mass, momentum, and scalars in both natural and engineered systems. Examples include brine disposal, reservoir operations, large-scale ocean overturning circulation, industrial pollutant releases, river inflows into lakes and coastal regions, seabed-propagating turbidity currents, and sediment-laden hyperpycnal plumes generated during flood events. Despite their ubiquity and practical importance, gravity currents remain challenging to characterize and predict. The objective of this doctoral thesis is to improve the understanding of the phenomenon in specific contexts through a combined use of numerical and experimental approaches. First, gravity currents induced by salinity differences are analyzed numerically using Large Eddy Simulations to understand the behavior near the bottom wall. Second, the impact of currents on submerged structures, like pipes, is analyzed in terms of the acting forces and the associated erosion and deposition processes. In particular, the case in which density variations are due to the presence of sediments (turbidity currents) is examined. In this context, the analyses are carried out using a two-phase numerical model in order to more accurately represent the dispersed granular phase. Third, the surface dynamics of a hyperpycnal plume, representative of a river–basin system, are analyzed using laboratory data obtained from the Coriolis platform (LEGI, Grenoble), with the aim of assessing the influence of the inlet dimensions. Simulations reveal a well-defined boundary layer beneath the current head, with velocity profiles approximately following a logarithmic law. The use of a parameterized wall shear stress enables an adequate representation of the flow dynamics while employing coarse computational grids, thereby reducing the computational cost by two orders of magnitude. The presence of a bottom cylindrical obstacle strongly modifies flow dynamics, mixing, and near-bed shear, enhancing erosive capacity in the rear and generating significant drag and lift forces, capable of mobilizing both fine and coarse sediments. Finally, it is observed that for hyperpycnal flows the plunging location shifts linearly downstream with increasing inlet width, and Kelvin–Helmholtz instabilities dominate the mixing and entrainment dynamics.