Large eddy simulation tool for environmental and industrial processes

Computational Fluid Dynamics (CFD) is an established tool for consulting and for basic research in fluid mechanics. CFD is required to provide information where analytical approaches or experiments would be impossible or too expensive. Most of the flows of engineering interest are turbulent. Turbulence is an unresolved problem of classical physics, because of the non linearity of the fluid motion equations. At the moment the only way to face them is numerically. Turbulence is composed of eddies in a broad range of size. To solve numerically the Navier-Stokes equations, the equations set that governs the fluid motion, a very fine grid is necessary in order to catch also the smallest eddies. The computational cost increases as Re3 (Re = ul/ is the Reynolds number with u and l an inertial velocity and length scales and the kinematic viscosity). Real life problems are characterized by very large Reynolds numbers and the consequent computational cost is enormous. So the direct solutions of Navier-Stokes equations (DNS) is not feasible. In many applications it is not necessary to solve all the eddies, it can be sufficient to supply the effects of unresolved scale to the flow. In Large Eddy Simulation (LES) most of the scales of motion are directly solved, in particular all the large energy carrying scales. These scales are influenced by the boundaries and they are strongly anisotropic. The smaller and dissipative scales must be modeled, but these scales loosing memory of the boundary conditions are more isotropic and hence formulating a general model that accounts for their effect is relatively easier. Large Eddy Simulation is a prospective tool for investigation in real life problems, in particular when high detailed analysis is required. This is the case for many industrial and environmental processes. For example, acoustic problems due to hydrodynamic noise are governed over a range of large scales which are easily reproduced by LES solution. However in these types of flows many difficulties arise also for LES. In general these flows are characterized by high Reynolds number. Wall-bounded flow at high Re requires high computational cost because LES is constrained to be DNS-like. Besides complex geometries are often involved. Structured or Unstructured body-fitted grid can be very hard to made, moreover unstructured grid can be expensive and not suited for LES. Scope of this thesis is to develop tools to apply LES to such configurations in order to make numerical simulation more adaptable to real life problems. In particular to deal with complex geometry an Immersed Boundary Methodology has been developed for curvilinear coordinates. The method has been applied to several test cases with good results. Then this methodology has been extended to high Reynolds number flows through the use of a wall model. In order to work on anisotropic grid, typical in sea coastal domain, a modified Smagorisky model has been proposed. Finally particle dispersion has been considered in stratified environmental flow. These tools has been applied to an industrial and to an environmental problem with good results.