Modification of turbulent boundary layer coherent structures with drag reducing polymer solution

Abstract

The modification of dominant coherent structures that extend through the log-region of a drag reduced turbulent boundary layer is studied via examination of two-point correlations from time-resolved particle-image-velocimetry. Measurements were acquired in polymer oceans (uniform concentration) at drag reduction levels corresponding to the low drag reduction regime (<40%) and the high drag reduction (HDR) regime (>40%) and at an intermediate level (46%). The mean velocity profiles and two-point correlations were compared with those of water (Newtonian, DR = 0%). These results show that, with increasing drag reduction, the inclination of these dominant coherent structures decreases, their streamwise extent increases, and the fluctuations in the correlations are suppressed (especially at HDR). These observations are examined in comparison with the coherent structure literature (Newtonian and polymeric).