Experimental investigation of intermittency in transitional channel flows with drag-reducing polymers

The unforced transition from laminar to turbulent flow with increasing Reynolds number (Re) is compared between water and various aqueous solutions of drag-reducing flexible polymers in a channel flow. Time-resolved and double-frame particle image velocimetry are used to measure the velocity field and compare the transition process of the Newtonian and polymeric channel flows. For the channel flows of water, the transition to turbulence occurs between Re of 1500 and 1965. Between these Re, the transitional Newtonian channel flows exhibit intermittent zones of laminar and turbulent flow, and the fraction of time for which the flows are turbulent varies between 40% to 80%. For all polymeric flows, the transition to turbulence is delayed and the critical Re at which the flow turns chaotic increases as the polymer concentration grows. Moreover, the range of Re where flows exhibit intermittent laminar-turbulent patches increases with increasing polymer concentration. For flows with a polymer concentration of 150 ppm, transitional flow begins at Re of 2810 and persists until Re of 5370. At this large polymer concentration, transitional flows exhibit laminar zones with larger fluctuations due to growing instabilities.