Time for polymers: Onset of drag reduction in the Reynolds–Weissenberg-plane

The flow of dilute aqueous polyacrylamide PAAm solutions with varying polymer molecular weight was studied in turbulent pipe flow. Scans in Reynolds number Re were performed in Newtonian turbulent flow [pre drag reduction (DR) onset] and intermediate DR flow. Polymer degradation shifted the onset of DR to higher Re. Polymer relaxation times derived from molecular weight and viscosity contribution of the polymer combined with the shear rate of the flow were used to calculate the Weissenberg number Wi. Fanning friction factors f derived from experimental data were fitted as a function of Re and flow distance to obtain flow properties for any state of polymer degradation and at any Re. From this fit function the onset of DR for varying polymer molecular weight was derived in the Re–Wi plane and the flow regimes accessed with increasing Re at constant molecular weight were identified. Trends in DR onset strongly depend on the shear rate used to calculate Wi. The elasticity number El was only independent of Re when Wi was determined from volume shear rate instead of the wall shear rate. Our observations indicate that DR is dominated by processes in the volume rather than those occurring at the wall.