Three different methods have been investigated for generating microbubbles to control the bubble diameter separately from the main flow velocity. The first two methods achieve this by adjusting the local shear stress where bubbles are generated, while the third method uses foaming of dissolved air to generate very small bubbles. The average diameter of bubbles was successfully controlled by the first two method within the range of 0.5–2 mm for the fixed main flow velocity of U = 3 m/s, while the very small bubbles of 20–40 μm were generated by the third method. The influence of the bubble diameter on the frictional drag reduction was found to be insignificant for the diameter range of 0.5–2 mm, while we also obtained experimental results suggesting that smaller bubbles on the order of 10 μm in diameter can be effective for the drag reduction.

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