A Study of the Diffusion of Slot-Injected Drag-Reducing Polymer Solution in a Turbulent Boundary Layer Modified by Large-Eddy Breakup Devices

The effects of a tandem set of large-eddy breakup (LEBU) devices on the diffusion of drag-reducing polymer solution and of water injected into a turbulent boundary layer (TBL) flow have been studied. Laser-Doppler velocimeter measurements were taken in the LEBU modified TBL with and without polymer injection. A laser-induced fluorescence technique was used to examine the development of concentration profiles of the injected fluids with increasing distance from the injection slot for a range of injected rates. The diffusion rate of water, a passive contaminant, was diminished by the LEBU devices over a distance of only 10 to 15 boundary thicknesses before returning to the case of an unmodified flow. It has been found that the initial diffusion zone of the passive contaminant has been extended in the LEBU modified TBL. The LEBU devices did have a major effect on the diffusion of polymer over the entire steamwise distance studied, compared to the case of an unmodified flow. Large reductions of turbulent normal and shear stresses were observed downstream of the devices, especially with polymer injection.