Experimental Evolution of Local and Global Variables in the Subcritical Transition in Channel Flow

We perform experiments on the subcritical transition to the turbulence in a water channel with plane Poiseuille flow, perturbed by controlled injection of water normally to the wall. For different values of Reynolds number $Re$ and different amplitude of the perturbation $u_{jet} /u_{cl} $, we observed different states from laminar to turbulent. Using Particle Image Velocimetry, we study the dynamics of a local variable of the velocity field as the transverse magnitude and simultaneously we follows a global one, as the deformation of the mean velocity profile $\tilde {u}=\bar {u}_{cl} /\bar {u}_{cl,unperturbed} $. We discuss the evolution in the phase space of those variables as a function of the strength of the perturbation, and compare it with predictions made from low dimensional models.