Bubble characteristics and trajectories in a microbubble boundary layer

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Abstract
Optical and photographic surveys of microbubble boundary layers are presented. The results show that the outer edge of the bubble cloud diffuses away from the wall as the bubbles are swept downstream. The plate‐on‐bottom orientation contains a bubble‐free region near the wall that cannot be discerned for the plate‐on‐top configuration. Skin friction measurements made when the bubble‐free region extends to y+=200 show there is no longer any Cf reduction present suggesting bubbles are not effective when they are outside the near‐wall region of the boundary layer. Bubble sizes, which increase with airflow and distance from the injection section and decrease with free‐stream velocity, were measured to be between 150 and 1100 μm.