Velocity fluctuation enhancement in the transition to turbulence in a pipe

A laser-Doppler velocimetry study of the features of a duct flow around the laminar-turbulent transition under the influence of a finite perturbation is reported. Through the spectral analysis of the light scattered by particles crossing interference fringes, the time-average velocity and the velocity fluctuation profiles are studied. It is shown that the ratio between the magnitude of the velocity fluctuations and the axial time-average velocity exhibits a sharp peak in the transition region. It is pointed out how that feature is independent of the discontinuities inducing the perturbation at the entrance as well as of the position downstream.