Modification of Turbulent Boundary Layer Coherent Structures with Drag Reducing Polymer Solution

TL;DR

This study investigates how polymer solutions modify the coherent structures in turbulent boundary layers, revealing that increased drag reduction leads to less inclined, longer structures with suppressed fluctuations, especially at high drag reduction levels.

Contribution

It provides new insights into the changes in turbulent coherent structures across different drag reduction regimes using two-point correlation analysis.

Findings

Inclination of structures decreases with drag reduction

Streamwise extent of structures increases with drag reduction

Fluctuations in correlations are suppressed at high drag reduction

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%), the high drag reduction (HDR) regime (>40%), and at an intermediate level (46%). The mean velocity profiles and two-point correlations were compared with that of water (Newtonian, DR = 0%). These results show that with increasing drag reduction the inclination of these dominant coherent structures decrease, 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).