Effect of polymer additives on the small-scale structure of grid-generated turbulence

The effect of polyethyleneoxide (Polyox grade WSR 301) on grid-generated turbulence was investigated using laser anemometry and flow-visualization techniques. It was found that the polymer additive reduced both the turbulent intensity and the rate of decay behind the grid. At typical drag-reducing concentrations, turbulent energy spectra were qualitatively the same as those in water, in agreement with the results of other investigations. However, at higher additive concentrations, the dissipation-range spectra showed noticeable attenuation. This seemed to be a threshold effect with onset at a polymer concentration between 100 and 250 ppm. This result was supported by photographs of dye-injection tracer but in this case the onset concentration for small-eddy suppression was between 50 and 100 ppm.