Microbubble drag reduction and effect on small-scale turbulent structures in pipe flow

This study experimentally investigates the modifications of small-scale turbulent structures and drag reduction induced by microbubbles in horizontal turbulent pipe flow at low void fraction (α ≈ 2 × 10−5) and a Reynolds number of approximately 16 400, based on the pipe diameter and cross-sectional mean velocity. The flow field is measured using three-dimensional Lagrangian particle tracking (3D-LPT), and frictional drag is evaluated via pressure drop measurements. The results show that microbubbles can achieve approximately 5% drag reduction, accompanied by a 3%–5% decrease in turbulent intensity in all three directions. It was found that this turbulence intensity reduction relates to the modifications of small-scale turbulent structures identified by the Q-criterion. Both the number and total volume of these structures decrease in the logarithmic and outer layers, with more pronounced reductions observed in the logarithmic region, where the number of structures decreases by about 4.6% and the total volume decreases by about 8.2%, in the time average. The turbulent kinetic energy (TKE) within structures also shows a significant reduction, with the total TKE density of the structures in the logarithmic and outer layers reduced by approximately 4.5% and 7.2%, respectively. These findings suggest microbubbles may weaken overall turbulence intensity by reducing the turbulent kinetic energy contained within small-scale turbulent structures, implying a clear correlation between the modifications of small-scale structures and the observed drag reduction.