The role played by the flexible polymer polyacrylamide (PAM) and the rigid polymer xanthan gum (XG) on drag in Taylor-Couette geometry: from Taylor's vortexes to fully turbulent flow

We study how the flexible polymer polyacrylamide (PAM) and the rigid polymer Xanthan gum (XG) affect drag in a Taylor-Couette geometry, mainly for small Reynolds numbers, in which the Newtonian flow is characterized by laminar instabilities. Our study is focused on the range 145 <= Re <= 1200. The role played by concentration is very complex for Re < 600. We suppose that two distinct polymeric effects occur all together. One of them is the so-called elasto-inertial turbulence, small elasto-inertial instabilities that work to increase drag, and the other effect is related to changes in the large structures, which contributes to reduce drag. It seems that elasto-inertial turbulence is important at very small Re. In such regime, the drag reduction is not so clear, but for Re >= 600, the drag is clearly reduced and it is possibly related to changes in the large structures.