Injection of a Drag-Reducing Fluid into Turbulent Pipe Flow of a Newtonian Fluid

Recent experiments with additives which reduce turbulent skin friction in turbulent shear flow of liquids have suggested that the significant changes in the shear flow occur in the flow very near the wall, in the region of the viscous sublayer. A series of experiments, in which dilute solutions of drag-reducing fluids were injected into turbulent pipe flow of a Newtonian fluid, were performed in order to determine whether the presence of the additive only in the wall region could produce significant local shear stress reduction. It was found that the local pressure gradient could be reduced by an amount comparable to the flow of a uniform concentration when the fluid was injected in the wall region. Conversely, when the fluid was injected into the turbulent core no reduction in local pressure gradient occurred until the fluid diffused into the wall region. The effects of the injection flow process and the injection apparatus were evaluated and found to be small compared to the results of injection of the drag-reducing fluid.