Image analysis applied to study on frictional-drag reduction by electrolytic microbubbles in a turbulent channel flow

We investigate frictional-drag reduction with electrolytic microbubbles based on image measurement of a turbulent flow in a water channel at Re = 4800 (based on the half channel height). Microbubbles with a diameter ranging 30–200 μm can reduce frictional drag by as much as 30% relative to single-phase flow even at low void fractions (α ≈ 3 × 10−4); however, drag reduction is only effective within a limited downstream distance from an electrode array. Arrangement of the optical system allows us to measure the bubble-production rate by water electrolysis from images near the wall and to trace the motion of bubbles. We also measure velocity fields using particle-tracking velocimetry based on a shallow depth-of-field approach by segregating tracer particles from microbubbles. Vertically oscillating microbubbles likely represent interaction with vortical structures near the wall, and bubbles approaching the wall appear to induce negative streamwise velocity relative to the surrounding fluid. We relate the wall friction with the double integral of the Reynolds-stress profile and show that its variation due to microbubbles decreases the drag on the wall. Microbubbles tend to coalesce downstream resulting in a fewer bubbles but with greater size; accordingly, the oscillatory motion diminishes, and the frictional drag rather increases.