Transition From Laminar to Turbulent Flow in Non-Newtonian Channel Flows: The Impact of Drag Reducing Polymer Additives

Abstract

This thesis investigates the effect of a drag-reducing polymer on laminar-turbulent transition in a gravity-driven channel flow. The drag-reducing polymer is polyacrylamide (PAM) dissolved in water at concentrations of 50, 75, 100, and 150 parts per million (ppm). An extensive experimental campaign utilizing particle image velocimetry is conducted to measure the velocity field in streamwise-wall-normal and streamwise-spanwise planes across a range of Reynolds numbers (Re). The transition to turbulence is delayed by the introduction of PAM, with the delay increasing with the relaxation time of the polymer solution. For low PAM concentrations of 50 and 75 ppm, the skin friction coefficient (Cf) increases with increasing Re before the onset of drag reduction around Re = 2500. After the drag reduction onset, Cf values will decline with further increase in Re. The transitional flow structures in the streamwise-wall-normal plane resemble Newtonian turbulence, containing recognizable turbulent spots. In the streamwise-spanwise plane, the turbulent core appears connected to chaotic streaks attached to the leading edge of the spot. In contrast, the higher PAM concentrations of 100 and 150 ppm display substantially different transitional physics. The Cf values depart the laminar trend at Re ≈ 3000, approaching the maximum drag reduction asymptote without demonstrating an onset point. The solutions undergo a remarkably smoother transition spanning over a Re range of approximately 1000 units. Turbulent spots are minimal, and elongated streamwise streaks dominate the flows instead. Analysis of turbulence statistics reveals concentration-dependent modifications. At 50 and 75 ppm, turbulence intensity matches the Newtonian value of around 1% in laminar regions between turbulent spots. However, at 100 and 150 ppm, the intensity grows to 1.7% in laminar regions, indicating enhanced fluctuations. Within turbulent regions, intensity declines from 5% in Newtonian cases to 3% at 150 ppm, suggesting substantial turbulence attenuation. In summary, this thesis demonstrates the impact of polymer additives on transitional channel flows. The additives expand the transitional regime and transform the emergence of coherent structures in a concentration-dependent manner, as evidenced by elongated streaks dominating at higher concentrations.