Large Eddy Simulations of mixing in coastal areas

The study of mixing in coastal area is a key issue in environmental fluid mechanics. In most applications it requires a three-dimensional approach in order to take into account phenomena related to buoyancy effects and to complex geometry. In a coastal shallow-water environment the wind-driven upper boundary layer may encroach upon the bottom boundary layer creating a complex turbulent field, making difficult proper modelization with simple RANS-like methodologies. A LES methodology is presented aimed at studying turbulent mixing in coastal areas under general forcing and geometric configuration. The strong grid anisotropy, between the horizontal length scale of kilometers and the vertical one of about 10–100 m, is handled expressing the subgrid stress by means of directional eddy viscosities. Geometry is modeled using a combination of curvilinear grid and the Immersed Boundary Method. Proper validation of the model has been carried out and the superiority with respect to the standard one-coefficient version clearly shown. The model has been applied to the investigation of mixing within the Muggia bay under wind forcing. Overall the model has shown to be able to reproduce the peculiarities of coastal dynamics. The model proved to be successful for the study of shallow-water in-shore fundamental processes.