  Flow field measurements from a Couette-Poiseuille experiment are used to
examine quantitatively certain steps of the self-sustained process (SSP) of
wall-bounded transition to turbulence. Although the different parts of the SSP
have been discussed at large in the literature, direct measurements from
experiment are scarce and, to our knowledge, the present results are the first
to show, using a local analysis of the turbulent patterns, that: (1) the
amplitude of streamwise rolls is related to streak waviness, bringing a
quantitative picture to one of the main physical mechanisms of Waleffe's model
of SSP ; and (2), at low waviness, direct measurements of the correlation
between the streak and roll amplitudes, respectively probed by the streamwise
and wall-normal velocity perturbations, quantify the lift-up effect.
