Abstract
The effect of excess addition of counter-ion on the development of a drag reducing surfactant solution flow has been studied by using a laser Doppler velocimetry (LDV) system. Aqueous solution of cationic surfactant, oleylbishydroxyethylmethylammonium chloride, at the concentration of 1,000ppm with counter-ion, sodium salicylate, which molar ratio to surfactant, ξ, was set at 100, was tested. Time-averaged streamwise velocity profiles and fluctuating intensity distributions in a duct were measured in the six cross-sections with the different streamwise distance from the inlet of the test section. The results were compared with those in the previous case when the molar ratio of the counter-ion to the surfactant was set at 1.5. While the high level fluctuation was observed even at the last measuring cross-section in the case of ξ = 1.5, the excess addition of counter-ion causes effective drag reduction as reported in the previous study, but the peak value of fluctuating flow intensity is found to decrease streamwisely as flow develops. This indicates flow laminarization occurs in the present case. From the results of wavelet analysis, the periodic wavelet pattern which was reported in the case of ξ = 1.5 disappears in the downstream region where the significant flow fluctuation is not observed. This is consistent with the previously reported fluid characteristics that the elasticity becomes weaker by excess addition of the counter-ion. From this, it is concluded that the high-level fluctuating intensity existence often reported is not a main cause of the drag reduction occurrence, but flow laminarization is a primary cause.
