The reorganisation of turbulent pipe flow by a drag-reducing polymer additive

The effect of a drag-reducing polymer additive on the organisational states of turbulent pipe flow is investigated by performing stereoscopic particle image velocimetry measurements in a large-scale pipe flow experiment at $Re_D=10000$ using both water and a visco-elastic, shear-thinning, semi-dilute aqueous polymer solution. The effect of the polymer is to decrease the drag (by 62\%) whilst significantly increasing the probability that the flow exists in a flow state with a low azimuthal wavenumber ($k_\theta=2$). This result indicates that the $k_\theta=2$ state is potentially a favourable (i.e.~low drag) flow state