Effect of Microbubbles in Uniform Inflow on the Separated Flow over a Flat Plate

Abstract

Microbubbles in liquid flow considerably alter the near-wall flow field structure and flow characteristics compared with single-phase conditions. Understanding the effect of microbubble populations on near-wall flow field characteristics is crucial for controlling multiphase flow and hydrodynamic properties. In this paper, a microbubble image velocimetry (μBIV) system was established using small-sized microbubbles as tracer particles, and the plate separation flow field under single-phase/mixed multiphase inflow was investigated. The distribution characteristics of time-averaged streamwise/normal velocity, vorticity, and vorticity intensity were statistically compared, and the dimensionless velocity profiles near the reattachment point within the wake region of the plate separation and reattachment flow were fitted. The results indicate that the near-wall streamwise velocity and its normal gradient were reduced by the microbubbles entrained in the inflow. This effect gradually increases with an increase in the void fraction (α). At the same time, the thickness of the viscous sublayer increases and the dissipation of large-scale near-wall vortex structures enhances. The dimensionless streamwise velocity profile remains valid in the wake region of the near-wall separation flow field under mixed multiphase inflow. The results demonstrate the potential application of the μBIV method in multiphase flow scenarios, such as for improving the drag reduction of bubble flow at the bottom of a ship or the drag reduction on underwater vehicles in motion. This paper also discusses the limitations of the μBIV system in testing single-phase flows and suggests potential improvements.

Keywords

Microbubbles flow / Near-wall separated flow / Microbubble image velocimetry / Wake region / Dimensionless profile

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