Experimental investigation of polymer diffusion in the drag-reduced turbulent channel flow of inhomogeneous solution

Abstract

Spatial polymer diffusion in the drag-reduced turbulent channel flow of an inhomogeneous polymer solution was investigated by simultaneously measuring velocity and concentration fields using particle imaging velocimetry and planar laser-induced fluorescence techniques. The polymer solution was dosed into the turbulent channel flow from the surface of one-side of the channel wall. The Reynolds number (based on channel height, bulk velocity and solvent viscosity) was set as 4.0×104 and the weight concentrations of dosed polymer solution were set to 25, 50 and 100ppm. The measurements were obtained in the streamwise wall-normal (x–y) plane at three streamwise positions along the dosing wall. The detailed statistical analyses consisting of concentration distribution, turbulence modification, turbulent mass flux, and eddy diffusivities of momentum and of mass are presented. The results show that the polymer diffusion, which has a close relationship with the local polymer concentration and drag reduction in the drag-reduced turbulent channel flow, is suppressed due to the inhibited turbulence other than the diffusion of passive scalar in ordinary turbulence. Two characteristic regions exist in the near-wall region according to the diffusion characteristics and altered motions in the wall-normal direction. The wall-normal turbulent fluxes that control the transport of mass are reduced significantly in the near-wall region for the drag-reduced flow when compared with the case of dosing water. With the increase of local polymer concentration in the “effective position”, the corresponding drag reduction rate (DR) increases. The turbulent Schmidt number (ScT), which represents the relative intensities of the eddy diffusivities of momentum and of mass, is also found to increase with increasing DR. The mean value of ScT for the drag-reduced flow can rise to 2.9, while it is 1.2 for the case of dosing water in the present measurements.