Particle image velocimetry (PIV) measurements were performed on drag-reducing channel flow with surfactant additives. In order to clarify the large-scale structures, the measurement field was set to a relatively large size (130 mm in the streamwise direction × 40 mm in the wall-normal direction). The surfactant used in this experiment was a cationic surfactant CTAC, and NaSal was added to the solution with the same weight concentration as counter ions. We examined the effects of Reynolds number ranging from 15000 to 30000 and concentration of CTAC which was set to 25 ppm or 40 ppm in the flow. Based on these measurements, two-dimensional two-point correlations were calculated to illustrate the characteristic structures. In drag-reducing flow, there were large-scale fluid lumps in the near-wall region. These structures extended toward the streamwise direction and were inclined at approximately 10 degrees to the channel wall. These characteristic structures were only observed in the case of high drag reduction rate and the tendency of these structures did not change with varying Reynolds number. It is thus considered that turbulent coherent structures change, and sweep and ejection which produce skin frictional drag are suppressed.
2011 The Society of Rheology, Japan