AN INVESTIGATION OF POSSIBLE MECHANISMS OF HETEROGENEOUS DRAG REDUCTION IN PIPE AND CHANNEL FLOWS

When concentrated polymer solutions are injected into the core-region of a turbulent pipe or channel flow, the injected polymer solution for ms a thread which preserves its identity far beyond the injection poin t. The resulting drag reduction is called heterogeneous drag reduction . This study presents experimental results on the mechanism of this ty pe of drag reduction. The experiments were carried out to find out whe ther this drag reduction is caused by small amounts of polymer removed from the thread and dissolved in the near-wall region of the flow or by an interaction of the polymer thread with the turbulence. The frict ion behavior of this type of drag reduction was measured for different concentrations in pipes of different cross-sections, but of identical hydraulic diameter. The parameters of the injection, i.e. injector ge ometry as well as the ratio of the injection to the bulk velocity, wer e varied. In one set of experiments the polymer thread was sucked out through an orifice and the friction behavior in the pipe was determine d downstream of the orifice. In another experiment, near-wall fluid wa s led into a bypass in order to measure its drag reducing properties. Furthermore, the influence of a water injection into the near-wall reg ion on the drag reduction was studied. The results provide a strong ev idence that heterogeneous drag reduction is in part caused by small am ount of dissolved polymer in the near-wall region as well as by an int eraction of the polymer thread with the turbulence.