THE EFFECT OF DRAG-REDUCING ADDITIVES ON THE TURBULENT STRUCTURE IN CHANNEL FLOWS (BURSTS, EJECTIONS, COHERENT)
Abstract
It is well known that small amounts of long-chain high molecular weight polymer molecules reduce viscous drag in wall bounded turbulent liquid flows. However, the mechanisms through which drag reduction is achieved are not understood largely because the rheology of the drag-reducing solutions is not well defined. In the present study, two fully developed, low concentration (less than 3 ppm) drag-reducing flows with 20 to 30% drag reduction were investigated and compared to a water flow. At these very low concentrations, the difference in the shear viscosity of the drag-reducing solutions and the solvent are minimal. A two-component laser Doppler velocimeter was used to make simultaneous measurements of the velocity components parallel and normal to the wall. Measurements of mean and root-mean-square (RMS) velocities as well as the uv turbulent shear stress confirmed that the additives modify the buffer region of the flow. The major influence occurred through damping of fluctuations normal to the wall. Eulerian "burst" detection techniques were developed through extensions of the quadrant two, VITA, u-level and modified u-level detection techniques. When properly used all of these techniques gave an accurate estimate of the average time between bursts for 8700 (LESSTHEQ) Re(,h) (LESSTHEQ) 17800 in water flows and in the two drag-reducing flows. In these very dilute drag-reducing flows, the average time between bursts increased about the same amount as the streak spacing. The modified u-level technique was shown to do the best job of detecting the entire ejection and burst event. Conditional velocity samples using the modified u-level technique showed that the ejection and burst event are closely correlated with the occurrence of slower than average fluid moving both away from the wall and toward the wall.
Degree
Ph.D.
Subject Area
Mechanical engineering
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