Matteo Montecchia

In this thesis effects of plasma actuators based on Dielectric Barrier Discharge (DBD) technology over a NACA 0015 bidimensional airfoil have been analyzed in an experimental way, at low Reynolds number. Work developed on thesis has been carried on in partnership with the Department of Electrical Engineering of Università di Bologna, inside Wind Tunnel of the Applied Aerodynamic Laboratory of Aerospace Engineering faculty. In order to verify the effectiveness of these active control devices, the analysis has shown how actuators succeed in prevent boundary layer separation only in certain conditions af angle of attack and Reynolds numbers. Moreover, in this thesis actuators’ chordwise position effect has been also analyzed, together with the influence of steady and unsteady operations

Anisotropy-resolving Reynolds stress turbulence modelling in a RANS context has been shown to be superior to standard eddy-viscosity models (EVM) in flows that are more complex than simple plane shear flows. Differential Reynolds stress models (DRSM) as well as their algebraic counterpart (EARSM) have been shown to be able to capture effects of rotation, curvature, swirl, secondary flows, non-equilibrium and separation onset somewhat better than EVMs. Still massive separation and flows around blunt bodies cannot be well captured by RANS models mainly because of non-local effects, which is the primary motivation for hybrid RANS LES approaches. EARSM has been applied also in hybrid approaches for the attached RANS part, see e.g. Jaffrézic & Breuer [1], but the LES part is modelled using more standard EVM SGS modelling. It is commonly believed that the SGS turbulence is quite isotropic, which is true when the small scales are purely driven by an energy cascade in equilibrium. However, ...

Sub-grid scale (SGS) models are required in order to model the influence of the unresolved small scales on the resolved scales in large-eddy simulations (LES), the flow at the smallest scales of turbulence. In the following work two SGS models are presented and deeply analyzed in terms of accuracy through several LESs with different spatial resolutions, i.e. grid spacings. The first part of this thesis focuses on the basic theory of turbulence, the governing equations of fluid dynamics and their adaptation to LES. Furthermore, two important SGS models are presented: one is the Dynamic eddy-viscosity model (DEVM), developed by \cite{germano1991dynamic}, while the other is the Explicit Algebraic SGS model (EASSM), by \cite{marstorp2009explicit}. In addition, some details about the implementation of the EASSM in a Pseudo-Spectral Navier-Stokes code \cite{chevalier2007simson} are presented. The performance of the two aforementioned models will be investigated in the following chapters, ...