In this paper, an experimental study on flow control with mean of vortex generators is presented. A turbulent boundary layer separation, induced by an adverse pressure gradient is generated in a wind tunnel. Pressure and PIV measurements are performed so as so investigate VGs efficiency and their interactions with the separation. The most efficient VGs pattern in term of separation reduction considered. Shear stresses distributions reveal that wall effects dominate the flow kinematics, through cooperative instabilities mechanisms and vortex interaction. The hypothesis is that this mechanism fixes the vortices lifetime, and may be a key to optimize vortex generators. A model based on this assumption is presented, and compared with experimental results.