• blasius
• delay
• DNS
• drag
• flat
• generators
• KTH
• miniature
• MVG
• plate
• reduction
• Schlichting
• simulations
• Tollmien
• transition
• vortex

This thesis focuses on the effects of a row of Miniature Vortex Generators (MVGs) on a flat-plate; the MVGs are a passive mean to delay the transition to turbulent, which is obviously desired to reduce the global friction drag, that increments drastically when the flow switches from laminar to turbulent.

In case of reduced intensity of external disturbances, transition is caused by the amplification of Tollmien-Schlichting (TS) waves, that are stream-wise instabilities which arises in a viscous boundary layer and which can be predicted by the linear stability theory. In this thesis, we analyze by Direct Numerical Simulation (DNS) the evolution of the TS waves on a flat plate both with and without the MVGs. In particular, we investigate how the presence of the MVGs affects the evolution of the TS waves, and a quantification of the impact of the MVGs on the total drag acting on the plate is reported. The solver used is Nek5000, implementing a spectral element code to solve the Navier-Stokes equations for incompressible flow.

Although the DNS simulations are limited to the sub-critical part of the flat-plate, where the flow is laminar both with and without MVGs, the effectiveness of the mean in delaying transition is proved by the experimental evidence.

The results reported here are in excellent agreement with all the experimental data provided by KTH. At the same time, the simulations provide very detailed information on the flow, which can be used in synergy with the experimental data for the investigation of the flow at issue. The methods developed in this thesis can be used for future works on this topic.