ETD

• CFD
• large-eddy simulations
• mixing layer
• turbulence
• VMS-LES

This thesis is the result of a research activity carried out in collaboration
between the Aerospace Engineering Department of the University of Pisa
and INRIA (France), aimed at developing numerical tools and methodologies for the
numerical prediction of complex flows, characterized by massively separated
unsteady wakes and complex geometry, as encountered in many applications
of engineering and industrial interest.
In this work, a systematic investigation on the effects of some of the factors
that affect the results of variational-multiscale and classical LES is started
in the simulation of time-evolving shear-layers. These are a rather classical
flow configuration for direct numerical simulation (DNS) and LES and, thus,
detailed reference data have been available in the literature. Thanks to periodic boundary conditions and the consequent limited streamwise width of
the computational domain, the simulations are significantly less demanding
than those of spatially evolving flows, as for instance bluff-body flows, and
this allows simulations to be carried out for a significant number of parameter values. It has also been possible to obtain, at reasonable computational
costs, a well-resolved Direct Numerical Simulation, useful for making a comparison with the successive simulations.
In a first time, the sensitivity of the results to the amount of numerical viscosity introduced has been tested in the so-called implicit LES : the analysis
highlighted the capabilities of these simulations when progressively coarsening the grid, and, moreover, the characteristics of the numerical methods
adopted. In a second part, LES and VMS-LES simulations have been carried
out using Smagorinsky and WALE subgrid scale models. An analysis of the
sensitivity to the grid refinement has been carried out. As it regards all the
other parameters involved in LES and VMS-LES, no particular adaptation
to the considered test-case has been made.