Use of particle image velocimetry to study heterogeneous drag reduction

Large polymer filaments can form when drag reducing polymers are injected through wall slots. The presence of these structures enhances the performance of the drag reducing function by mechanisms which are not understood. This paper shows how particle image velocimetry (PIV) techniques can be used to study changes in the configuration of the injected polymer and in the structure of the velocity field with increasing drag reduction. The filaments are found to behave as solid bodies which break up in high shear regions close to a boundary. The breakup process provides an explanation of why the filaments are not observed close to a wall and offers the possibility of providing a heterogeneous distribution of small aggregates of polymers which could be more effective than uniformly distributed molecules as suggested by Hoyer and Gyr (J Non-Newton Fluid Mech 65:221-240, 1996; J Fluids Eng 120:818-823, 1998), Dunlop and Cox (Phys Fluids 20:203-213, 1977) and Vlachogiannis et al. (Phys Fluid 15:3786-3794, 2004). PIV measurements show dramatic qualitative changes in the velocity patterns at maximum drag reduction.