(532h) Intermittency Measurements during Laminar-to-Turbulent Transition of Drag-Reducing Polyethyleneoxide Solutions

Intermittency measurements were used to explore triggered laminar-to-turbulent transition in the flow of 0 (solvent), 1, 3, 10, 31 and 100 wppm deionized water solutions of a polyethyleneoxide, called W301, of molecular weight 5.2 x 10⁶ in a seamless 316 stainless steel pipe of i.d. 4.57 mm and length 200 diameters which was probed at its exit by a hot-film anemometer located at the pipe centerline. The choice of pipe and polymer was such that the critical wall shear stress Twc at the onset of turbulence exceeded the onset wall shear stress Tw* for drag reduction by polyox W301, so all the polymer solutions began drag reduction immediately upon attaining turbulent flow. The solvent data exhibited a transition region 1950 < Re < 2650 over which the intermittency factor, defined as the fraction of time that the flow is turbulent, increased 0 < γ < 1, in accord with numerous earlier results for Newtonian pipe flow. The W301 solution results fell into three groups: The 1, 3 and 10 wppm intermittency data resembled those of the solvent but their intermittency curves of γ vs Re were shifted to higher intermittencies at any fixed Re. Two 31 wppm W301 solutions behaved curiously: in one the transition occurred over an extended range, with 0 < γ < 1 for 1950 < Re < 3100, while for the second, turbulent slugs occurred and could be clearly detected in the range 2000 < Re < 3200 but the intermittency factor was very much less than in solvent, the highest value being γ ~ 0.35 at Re ~ 3200. For Re > 3200 turbulent slugs could not be seen and instead a continuous turbulence appeared, yielding γ ~ 1 at Re ~ 3500; turbulent slugs reappeared upon reducing Re < 3200. Finally the 100 wppm W301 solution exhibited no critical Re and no intermittency; instead, velocity fluctuations appeared at Re ~ 1500 and increased an order of magnitude by Re ~ 4000. These results can be partially understood by noting that three types of laminar-to-turbulent transitions are possible during Type A drag reduction by random-coiling polymers, namely L → N, into a regime without drag reduction, prior to onset; L → P, into the polymeric regime of drag reduction, after onset; and L → M, into the regime of asymptotic maximum drag reduction. The attached figure shows representative raw and processed intermittency traces for Solvent C = 0 and W301 C = 31 wppm.