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 20-40μm 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 10μm in diameter can be effective for the drag reduction.
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机译：已经研究了三种不同的方法来产生微气泡，以独立于主流速度控制气泡直径。前两种方法是通过调节产生气泡的局部剪切应力来实现的，而第三种方法是使用溶解空气的泡沫来产生非常小的气泡。对于固定的主流速U = 3 m / s，前两种方法成功地将气泡的平均直径控制在0.5-2 mm的范围内，而第三种方法则产生了20-40μm的非常小的气泡。发现在0.5-2 mm的直径范围内，气泡直径对减小摩擦阻力的影响微不足道，同时我们还获得了实验结果，表明直径在10μm左右的较小气泡对于减小阻力是有效的。 。
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