Small-angle-neutron-scattering measurements (SANS) were done with a dilute solution of the surfactant n-tetradecyltrimethylammoniumbromide (TTAB) with sodium salicylate (Na Sal) in pipe and channel flow. The solvent used was heavy water. The anisotropy observed in the SANS-curves can be related to the drag reduction behaviour of the surfactant solution. The breakdown of the drag reducing properties above the critical wall shear stress is accompanied by a strong decrease in the scattering anisotropy. However, in this flow region the scattering curves reveal a very similar micellar structure to that found in the fluid at rest. This shows that the micelles exist above the critical wall shear stress.

Abstract:The results of an experimental study of the injection of concentrated polymer solutions into the near-wall region of a turbulent pipe flow are reported. The injection experiments described here show drag reduction that was significantly larger than that obtained for homogeneous polymer solutions of the same average concentration. Local drag reduction and friction behavior was obtained by measuring pressure differences over a test section of 13 m in length. Furthermore the flow behaviour of the injected polymer solution was in.vestigated by flow visualization experiments. Velocity profile measurements elucidate in case of near-wall injection that the turbulent structure could be altered in the ~aear-wall and also in the core region of the pipe flow, indicating that the polymer lumps and threads created by the near-wall injection are able to influence a much wider spectrum of turbulent eddies in comparison to centreline injection or, all the more, to homogeneous drag reduction.

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