Intermittency in the Transition to Turbulence for a Shear-Thinning Fluid in Hagen-Poiseuille Flow

Since Reynolds original experiments in 1883, very few studies have been carried out on the transition to turbulence for shear thinning fluid in Hagen-Poiseuille. Compared to the Newtonian fluids, the results showed two interesting phenomenon: a delay in transition to turbulence and the appearance of an asymmetry in the mean axial velocity profiles. In this study, we measured experimentally the fraction factor and axial velocity profiles using Laser Doppler Velocimetry system (LDV) at different Reynolds numbers. It is shown an axisymmetric in the mean axial velocity profiles for laminar and turbulent regimes and an increasing asymmetry profile with increasing Rew for transition regime. Two different stages are clearly identified during transition to turbulence. The first stage which we called it pre-transition, took place only in the non Newtonian fluids while the second stage correspond to the appearance of turbulent puffs. Another objective of this study is the statistical analysis of the axial velocity fluctuation. In the transition regime, power spectra identified the existence of a weak turbulence. It is generated by the shear-thinning character of the fluid, which follows a power law with an exponent close to -3 (f -3).