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Structure of pipe wall turbulence in Newtonian and drag-reducing flow : a hologram-interferometric study Achia, B. Umesh

Abstract

The effect of a drag-reducing long-chain polymeric additive on the structure of wall turbulence in pipe flow was investigated experimentally. The system studied was a 50 wppm polyacrylamide-water in a 2.63 cm diameter pipe. Due to the anomalous behaviour of conventional turbulence probes in drag-reducing flows, a nondisturbing optical technique was developed. Real-time holographic interferometry was used for flow visualization and turbulence measurements. An interference fringe pattern generated by the holographic method was superimposed on a region of the pipe wall. The real-time modulation of these fringes by a refractive index enhancer infused into the flow at the wall was recorded by medium-speed motion photography. Spatial and temporal correlations of concentration fluctuations and intensity measurements during turbulent flow were obtained from motion pictures. Both spanwise and normal directions to the wall were observed to investigate 'streaks' and 'bursts' that originate in the wall-layer flow. Measurements of gross flow, spanwise streak spacing, burst frequency and sublayer period were obtained in addition to visual impressions of the flow. The drag-reducing Separan AP30 solution flows appeared physically different from the water flows when comparison was made at either equal flow rate or equal wall shear velocity. Measurements showed that the drag-reducing additive increased both the physical and non-dimensional spanwise streak spacing. The additive also drastically reduced the rate of bursting. The time interval between bursts, computed using the model of Kim-Kline-Reynolds, was almost equal in both drag-reducing and water flows when compared at equal wall shear. Direct measurements of streak lifetime using wall layer concentration autocorrelations showed the drag-reducing sublayer to have a longer lifetime than that of water at the same wall shear. The drag-reducing sublayer period increased over the Newtonian value by a factor almost equal to the ratio of the non-dimensional streak spacing during drag reduction to the corresponding wall shear Newtonian value. These results suggest a stabilized wall layer in the polymeric solution flow as compared to that of the Newtonian solvent, resulting in lesser turbulence production and reduced frictional drag. The role of elongational viscosity of the dilute polymer solution is discussed as a possible mechanism to explain the visualized and measured phenomena.

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