Injection of Drag Reducing Additives Into Turbulent Water Flows: Two-Component Velocity Measurements and Mixing Length Model Predictions

摘要

The basic goals are: To determine the mechanism(s) by which a drag-reducing additive modifies the turbulent momentum transport near a wall; and to develop optimal methods for injecting these additives into wall flows. The structural characteristics of channel flows well downstream of the additive injectors were deduced from two-component laser velocimeter measurements while a mixing length model was developed to predict the wall shear stress of water flows with additive injection. A three beam, two-color laser velocimeter was used to measure simultaneously the velocity components parallel and normal to the wall between the channel centerline and y+ approx. 11 for three channel flows. The baseline case was a fully developed water flow; the second a well mixed drag-reducing flow whose Reynolds number matched that of the water flow; a third was a well mixed drag-reducing flow whose wall shear stress matched that of the water flow. Profiles of the mean and root-mean-square velocities confirmed that the additives modify the buffer region of the flow. The major influence occurred through damping of the fluctuations of the normal velocity component. Eulerian 'burst' detectors were developed by extensions of the quadrant two, VITA and u-level detection methods. When properly extended all three methods yielded an average time between bursts that agreed with previous flow visualization results. In these very dilute drag-reducing flows, the average time between burst increased the same amount as the streak spacing.