Polymer induced drag reduction in turbulent pipe flow was
investigated using a non-intrusive laser based diagnostic
technique, namely Particle Image Velocimetry (PIV). The drag
reduction was measured in a pressure-driven flow facility, in a
horizontal pipe of inner diameter 25.3 mm at Reynolds
numbers ranging from 35 000 to 210 000. Three high-
molecular-weight polymers (polyethylene oxide 2x10^6 –
8x10^6 Da) at concentrations in the range of 5 – 250 wppm were
used. The results, obtained from the PIV measurements, show
that the drag reduction scales with the magnitude of the
normalized streamwise and spanwise rms velocity fluctuations
in the flow. This scaling seems to universal, and is independent
of the Reynolds number and in some cases also independent of
the distance from the wall where the velocity fluctuations are
considered. Furthermore, the instantaneous PIV observations
indicate that as the level of drag reduction increases, the flow in
the pipe is separated into a low-momentum flow region near the
pipe wall and a high-momentum flow region in the turbulent
core. Based on these findings a new mechanism of polymeric
drag reduction is proposed in this paper.