Abstract

Both numerical and experimental data confirm that the introduction of dilute polymers in a turbulent flow changes some mean characteristics of the flow: the wall drag can decrease up to 40% while the average spacing among streaks can even double. Actually a link exists between streaks spacing and mean velocity gradient at the wall.

In such viscoelastic flows the wall turbulence regeneration is still influenced by the mean shear and by the interaction of the coherent structures even if they appear more ordered and larger with respect to Newtonian flows. These phenomena, which have been repeatedly observed since the findings of Toms, would require a better understanding of the mechanisms that locally induce this behaviour. The analysis of data from direct numerical simulation with FENE-P model for the polymers, seems to suggest, as the main effect of the viscoelastic reaction, a stabilizing action on the low speed streaks and a related decrease in the population of the wall layer coherent structures. The decreased level of activity induces relevant changes in the scales of the turbulence and originates the observed drag reduction.

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