Microbubble effect on Reynolds shear stress in drag reduction process in turbulent pipe flow

This study experimentally investigates the modulation of Reynolds shear stress (RSS) by low void fraction microbubbles (α ≈ 2 × 10−5) in turbulent pipe flow at ReD ≈ 16 400, based on the pipe diameter and cross-sectional mean velocity, using three-dimensional Lagrangian particle tracking (3 D-LPT). The results show that microbubbles reduced the RSS by approximately 9% ± 3% within the wall-normal range 0.2 ≤ y/R ≤ 0.6. Quadrant analysis indicates that this reduction arises from the suppression of Q2 (ejection) and Q4 (sweep) events. Further examination using hole analysis reveals that this suppression is highly selective: rather than uniformly weakening all Q2 and Q4 events, microbubbles preferentially suppress a portion of the stronger ones, shifting them toward lower intensities and consequently reducing the frequency of such stronger events. These findings suggest that microbubbles may interact with turbulence by selectively weakening stronger coherent events, which could in turn contribute to reduced turbulent momentum transport and frictional drag.