Grid-generated turbulence in dilute polymer solutions

Abstract

Measurements were made of some of the properties of grid-generated, turbulence for several concentrations of a drag-reducing polymer additive in water. Reference measurements of the grid pressure drops, streamwise intensities and one-dimensional spectra in pure water agreed with previous measurements obtained in Newtonian fluids. Corresponding results for the polymer solutions showed that the grid pressure drops were generally lowered, the turbulence intensity levels were increased and the one-dimensional energy spectra were unchanged compared to the results in water. A dimensionless rate of decay correlation was introduced which removed the dependence of the rate of decay on the initial conditions of grid-generated turbulence for Newtonian fluids: the polymer solution decay results did not follow this correlation, indicating that the decay process is different in these fluids. The one-dimensional energy spectra of the turbulence in the polymer solutions were of the same shape as those in Newtonian fluids, and were normalized with modified Kolmogoroff variables using an effective viscosity (Lumley 1964) and the viscous dissipation in order to provide a quantitative comparison to Newtonian spectra. All of the normalized spectra in the polymer solutions collapsed to a single curve which coincided with the normalized Newtonian spectral curve. It was also found that the rate of decay was less than the viscous dissipation in the polymer solutions, in accordance with recent theoretical predictions.