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

Intensified and attenuated waves in a microbubble Taylor–Couette flow

“…This suggests that the wall shear stress may be reduced 10% by the addition of microbubbles even at a very small void fraction, α = 1.2 × 10 −4 . Bubbles in the horizontal channels accumulate in the region so-called "sweep" or "ejection" region; they modify the role of the momentum exchange described as the Reynolds shear stress 5,7,15 and displace the streamwise vortical structures away from the wall because of the gravity force. 15 The reduction of the wall shear stress as the gradient in the streamwise velocity is thought to be resulted from the modification flow structure in the direction of gravity.…”

“…The effects of the modification in density, effective viscosity, and extra energy input would be of the same order as the void fraction, and would be negligible in dilute two-phase flows, i.e., α ≤ O(10 −4 ). The drag reduction rate in previous experimental investigations of dilute two-phase flows containing microbubbles, 4,7,8 in contrast, takes significantly high values since nonlinear interactions occur between microbubbles and flow structures in turbulent flows, as mentioned by L'vov et al 12 Investigations of flow fields in the liquid phase indicate that the drag reduction mechanisms can now be attributed to a decrease in the coherence of near wall structure (the source of Reynolds shear stress), which arises from interactions between microbubbles and coherent flow structures. [5][6][7][8] Numerical works as well as experiments exhibited the decrease in the Reynolds shear stress near the wall, due to the displacement of the near-wall structures away from the wall and reducing the frictional drag.…”

“…The drag reduction rate in previous experimental investigations of dilute two-phase flows containing microbubbles, 4,7,8 in contrast, takes significantly high values since nonlinear interactions occur between microbubbles and flow structures in turbulent flows, as mentioned by L'vov et al 12 Investigations of flow fields in the liquid phase indicate that the drag reduction mechanisms can now be attributed to a decrease in the coherence of near wall structure (the source of Reynolds shear stress), which arises from interactions between microbubbles and coherent flow structures. [5][6][7][8] Numerical works as well as experiments exhibited the decrease in the Reynolds shear stress near the wall, due to the displacement of the near-wall structures away from the wall and reducing the frictional drag. [14][15][16] The influence of the addition of microbubbles on the coherent fluid structure has been commonly thought to be the key factor in the modification of the velocity fields and the wall shear stress.…”

“…1 Drag reduction by the injection of O(10 μm) diameter bubbles (hereafter termed as "microbubbles") into turbulent boundary layers has been a subject of intensive research since the first experimental investigation 2 because of its high efficiency, i.e., drag reduction rate versus the input void fraction. The performance of the drag reduction rate has been experimentally investigated in turbulent channel flows [3][4][5][6][7] or in turbulent boundary layers on a flat wall, 8 and reaches ∼40% at the maximum. The remarkable feature of drag reduction by microbubbles is that O(10%) of the drag reductions are achieved by low void fractions of α ∼ O(10 −4 ).…”

Pulsatory rise of microbubble swarm along a vertical wall

“…-3 -The hydrogen microbubbles are generated by water electrolysis (Hara et al, 2011). Salt is added to tap water at 22 ˚C to act as the electrolyte, with mass concentration of about 0.3 wt%.…”