Transition to turbulence in pipe flow of Newtonian and Non Newtonian fluids

Abstract

Experimental investigations were carried out in pipe flow of Newtonian and Non Newtonian fluids. The investigations of Newtonian flow were focused on the origin of laminar turbulent intermittency in the flow. It was found that upon reduction of the Reynolds number starting from fully turbulent flows laminar regions appear randomly. Unlike reported for other shear flows there was no wavelength induced instability in pipe flow. The development of intermittent patterns and in particular the minimum spacing between turbulent puffs is shown as a consequence of an interaction between neighboring pus which has been identied. The pu interaction distance is found to decrease with increase in Re and it is in quantitative agreement with minimum spacing of plane Couette and Taylor Couette flow. In the second part of this thesis the transition to dilute polymer solutions has been investigated. Compared to the Newtonian case transition is delayed. At higher concentrations however Newtonian like transition disappears. Instead disordered motion sets in globally and onset of transition is independent of additional perturbation to the flow. This new type of instability , a combined effect of elastic and inertial eects, termed as elasto inertial instability sets in and ultimately results in the asymptotic state of maximum drag reduction (MDR). Most strikingly disordered motion is observed at much lower Re than Newtonian turbulence.