Structure of the turbulent boundary in drag-reducing pipe flow

Abstract

The effect of a drag-reducing additive on the structure of wall turbulence in pipe flow was investigated experimentally. Real-time hologram interferometry was used for flow visualization and turbulence measurements. The real-time modulation of interference fringes by a refractive-index enhancer infused into the near-wall flow was recorded by medium-speed motion photography. The spanwise direction and the direction normal to the wall were studied to investigate the ‘streaks’ and ‘bursts’ that originate in the sublayer. A region of the flow was sampled for spatial and temporal correlations of concentration fluctuations to detect the scales of eddy interaction.The addition of 50 p.p.m. by weight of Separan AP30 to water significantly altered the Newtonian wall-flow structure. The drag-reducing additive suppressed the formation of streaks and the eruption of bursts. When compared at the same wall shear, the sublayer period increased over the Newtonian value by a factor almost equal to the ratio of the corresponding non-dimensional streak spacings.These results suggest a stabilized wall layer in the drag-reducing solution as compared with that of the Newtonian solvent, resulting in less turbulence production and reduced frictional drag. The role of the extensional viscosity of the dilute polymer solution is discussed as a possible mechanism for explaining the visualized and measured phenomena.