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

Abstract

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.

Drag reduction by polymer additives in decaying turbulence

“…However, significant differences show up in the dissipation range: the spectrum for the FENE-P shell model falls much more slowly than its GOY-model counterpart indicating greatly reduced dissipation at large wavenumbers. Experimental [4,5] energy spectra do not cover as large a range of spatial scales as we can cover in our shell-model study, and thus, to the best of our knowledge, these dissipation-range discrepancies of the energy spectra, with and without polymer additives, have not been noticed earlier. We note that our results in FIG.…”

“…Experimental studies of decaying, homogeneous turbulence behind a grid indicate, for such dilute polymer solutions, a turbulent energy spectrum similar to that found without polymers [4,5]. However, flow visualization via die-injection tracers [5] and particle image velocimetry [6] show an inhibition of small-scale structures in the presence of polymer additives. To the best of our knowledge decaying turbulence in such polymer solutions has not been studied numerically.…”

“…By contrast, there is an extreme scarcity of results concerning the effects of polymer additives on decaying turbulence [3]. Experimental studies of decaying, homogeneous turbulence behind a grid indicate, for such dilute polymer solutions, a turbulent energy spectrum similar to that found without polymers [4,5]. However, flow visualization via die-injection tracers[5] and particle image velocimetry [6] show an inhibition of small-scale structures in the presence of polymer additives.…”

“…The qualitative agreement with a laboratory experiment (for statistically steady turbulence) supports our definition of drag reduction for decaying turbulence. Laboratory experiments in both statistically steady [17] and decaying [5,6] turbulent flows of dilute polymer solutions show an inhibition of small-scale structures and narrower probability distribution functions of velocity differences [17]. We plot in FIG.…”

“…Our results are consistent, therefore, with laboratory experiments which show a suppression of small structures that would imply reduced intermittency in the dissipation range of our shell model. Laboratory experiments [5,6] of decaying turbulence behind grids indicate a reduced decay rate of the kinetic energy in a dilute polymer solution, relative to the pure solvent. In the initial period of decay, before the integral scale of turbulence becomes of the order of the size of the system (the minimum wavenumber, in the case of shell models), we observe a decay rate of −1.80 ± 0.01 for the FENE-P shell model and a decay rate of −2.01 ± 0.02 for the GOY shell model (a result consistent with Ref.…”