Abstract
The randomization process of fluctuations is examined experimentally for the case of the ribbon-induced transition initially governed by a periodic T-S wave in plane Poiseuille flow with weak residual turbulence. The effects of the residual turbulence become appreciable as the T-S wave undergoes three-dimensional distortions leading to the formation of the high-shear layer. In fact it causes the fundamental and harmonic waves to modulate in both amplitude and phase at low frequencies, in particular at the position of the high-shear layer. This in turn changes the low-frequency spectrum from the original. These modulations manifest themselves as side-band spectral components around each harmonics. The randomization process is characterized by the growth of these side-hand components. At the stage of the high-frequency secondary instability, the side-band components around the higher harmonics are highly amplified and thus contribute to the breakdown of the high-shear layer into small scale eddies.
