Experimental study of the characteristics of very-large-scale motions in polymer pipe flows

Polymer drag reduction plays an important role in pipeline transportation. The drag reduction mechanism of polymer solutions is directly related to the coherent structure in pipe flows. High Reynolds number flows are the mainstream in the actual engineering, and very-large-scale structures with the streamwise length scale of 10 times pipe diameter are the most significant coherent structures in high Reynolds number pipe flows. In the present paper, experimental study is carried out on the very-large-scale structures in high Reynolds number polymer pipe flows. Premultiplied spectral analysis is used to examine the scale and strength properties of the very-large-scale structures in 12 sets of TR-PIV experiments with 4 polymer concentrations and 3 Reynolds numbers. The findings demonstrate that the scale and intensity of the very-large-scale structures in the central flow region significantly increase with an increase in the polymer concentration under the same Reynolds number, and the ratio of the intensity of the very-large-scale structures to that of the large-scale structures also significantly increases. The large-scale structure is replaced by the very-large-scale structure, which thereafter dominates as the main energy-containing structures in the outer region at elevated concentrations.