• streaks
• strato limite
• boundary layer
• transition
• transizione
• Tollmien Schlichting
• TS
• wave
• onda
• Orr Sommerfeld
• flat plate
• lastra piana
• turbulence
• turbolenza
• local
• locale
• stability
• stabilità
• analysis
• analisi
• temporal
• temporale
• spatial
• spaziale
• biglobal
• biglobale
• Reynolds
• disturbance
• disturbi
• neutral
• neutra

It has been shown in literature that high velocity streaks in a boundary layer can delay transition to turbulence. This thesis focuses on the effects of a row of Miniature Vortex Generators (MVGs), which are installed on a flat plate with the aim of introducing the streaks in the boundary layer. Local stability analyses are carried out on the flow in order to study the effects of the MVGs on the Tollmien-Schlichting (TS) waves. Amplification of TS-waves is in fact the main physical mechanism which cause transition when external perturbations, as for instance free-stream turbulence, is low enough.
Spatial and temporal local analyses are carried out on experimental flow fields, provided by Prof. J. Fransson (KTH mechanisms, Stockholm) and on fields computed by direct numerical simulation.
A further analysis is also performed employing the Parabolic Stability Equations.
The results show how the mechanisms of growth of disturbances are modified in presence of the MVGs. In particular the neutral stability curve is computed and compared with the Blasius one, showing that branch II of the neutral curve in the controlled case is sensibly anticipated with respect to the streamwise direction.