DNS of wall turbulence: dilute polymers and self-sustaining mechanisms

Several studies, both numerical and experimental, have shown that the introduction of a small amount of long chain polymers in a turbulent flow alters dramatically the length and time scales which are typical of Newtonian fluids, even though turbulence self-sustaining mechanisms remain approximately the same. In such viscoelastic flows wall turbulence regeneration is still influenced by the mean shear and by the interaction of the coherent structures which still generate low and high speed streaks, but more ordered and larger with respect to Newtonian flows. In order to gain a deeper understanding of the regeneration mechanisms and the modifications induced by the presence of the polymers, we analyze the data obtained from direct numerical simulation with a micro-rheological model for the polymers. Thus, the velocity fields have been studied together with the coupling terms in the momentum equations, i.e. the divergence of the extra-stress terms due to the polymers. The analysis seems to suggest, as main effect of the viscoelastic reaction, a quite concentrated action on bursting phenomena and a stabilization of the streaks with a related decrease in the population of the wall-layer coherent structures. In addition the correlations between velocity fluctuations and viscoelastic responses have been considered with the aim to single out the passive or the active role of the polymers in different flow locations.