Structure of Low-Speed Streaks in Drag Reducing Flow With Polymer Solution Blown From the Channel Wall

For the investigation of turbulent structure in drag reducing flow with polymer solution blown from the channel wall (wall blowing), instantaneous velocity field has been precisely measured in the x-z plane at different locations along the wall-normal direction via Particle Image Velocimetry (PIV). Polymer solutions with 25 ppm and 100 ppm of weight concentration were tested at a blowing ratio of 1.2×10⁻⁴ and at 20000 of Reynolds number. About 5% and 11% of drag reduction (DR) rate was obtained, respectively. As a result of this experiment, turbulent statistic data showed that the Root Mean Square (RMS) of streamwise velocity fluctuation increased and RMS of spanwise velocity fluctuation was suppressed comparing with water flow. We found that these low-speed streaks became relatively regular in the buffer layer, including an increase of both length and width, which indicated a depression of turbulence by polymer diffused in the buffer layer.