Polymer-Induced Fluctuations in High-Strain-Rate Laminar Flows

Two-dimensional, laminar, free-surface water-table flows of water and dilute polyethylene oxide) (PEO) solutions have been investigated at high strain rates. Velocity measurements were obtained using both a hot film probe and a laser—Doppler anemometer (LDA) for water flows, and using the LDA for polymer flows. Measurements of water flows agreed very well with calculations. Provided that the wall strain rate was below 750 sec^-1, flows of polymer solutions were essentially the same as those of water. Above this value velocity fluctuations coherent over a few cm in the flow direction were observed in the high-strain-rate portions of the flow. Velocity spectra showed a pronounced frequency distinct from turbulence which was lowest near the wall where the strain rate is a maximum. A modified dumbbell model of an isolated PEO molecule is proposed which appears to account for many of the observed results. The model allows for partial expansion of the polymer molecule due to the strain field while retaining rotation.