Experimental results concerning the influence of transition processes on the initial structure of a turbulent boundary layer

How the initial structure of the turbulent boundary layer can be influenced by the transition mechanism is examined. In the case of zero pressure gradient the evolution from the laminar to the turbulent regime is characterized by the intermittency phenomenon; the conditional sampling of the hot wire signal shows that the turbulent spots exhibit a structure similar to the classical turbulent boundary layer structure. With a mild positive pressure gradient, it becomes difficult to describe the intermittency. When the transition is beginning downstream from the laminar separation point, the shape parameter decreases but the intermittency appears no longer. Only a progressive deformation of the instability waves is observed; at the end of the transition, the spectrum of the turbulent energy is composed of a series of peaks corresponding to harmonics of the instability waves.