Influence of Polymer Additives on Turbulence in a Mixing Layer

Summary

Mixing layers provide an opportunity for investigating the influence of polymer additives on the turbulent momentum transfer processes that are important in all turbulent flows. In the investigations described here both flow visualization and laser-Doppler anemometry were employed to study the differences between a plane mixing layer in water and in a 50 ppm aqueous polyacrylamide (Separan AP30) solution. The polymer additive was found to enhance the large-scale structure in the mixing layer whilst suppressing the fine-scale turbulence. Furthermore, the ratio of the intensity of the longitudinal to the transversal fluctuations as well as the magnitude of the Reynolds shear stresses were increased; the latter increase was associated with a more rapid growth in the width of the mixing layer in the polymer solution. These changes can be explained in terms of the rheological properties of the polymer additives.