Drag-reducing polymers and turbulent bursts

Abstract

THE flow in a pipe is said to become well developed at the point where the axial pressure gradient ceases to depend on the distance along the tube. At such a point, the injection of drag-reducing polymer solutions produces a developing situation in which the magnitude of the observed drag reduction increases in the streamwise direction. This variation is due to the polymers spreading out radially as they are carried downstream. It continues until the polymer is uniformly distributed over the cross-section of the pipe. We have found that when the injection is at the pipe wall, the resulting streamwise variation of wall friction can have an oscillatory character, and we report here that this may be related to the phenomenon of turbulent bursts^1–4.