Experimental Study on "Unstable Layer" Appearing in the Drag Reducing Channel Flow of Viscoelastic Fluid

In this paper, drag reducing flow of dilute surfactant solution in two dimensional channel is investigated experimentally. To grasp the instantaneous velocity u-v in x-y plane, particle image velocimetry (PIV) is employed. The experiments pay attention to Reynolds number ranging from 10000 to 40000 and concentration of additive CTAC (a kind of cationic surfactants) from 25 to 100 ppm on the flow. One of the findings from this study is the appearance of unstable layer in the drag-reducing flow. A train of inclined fluid lumps with high streamwise velocity component at the fixed y-position is appeared in the instantaneous picture of streamwise velocity fluctuation in the flow with surfactant additive. As the turbulent statistics are calculated at identical y-position, u-rms has local maxima at this position. Surprisingly, the position of unstable layer is almost unchanged for number of PIV vector maps. We accumulated five hundred of maps and calculated distribution of turbulent statistics. The profiles of turbulent statistics have multiple peaks corresponding to the position where the unstable layer exists. Moreover, the unstable layer appears in the range of Reynolds number and additive concentration where the drag reduction appears strongly. But the reason of the appearance of "unstable layer" is not much clear nowadays.