Clouds of bubbles and droplets; formation and distribution under breaking waves

Formation of bubble clouds under breaking waves and whitecaps is a prominent phenomenon in quantifying the air-sea gas exchange process. The size distribution of bubbles will impact the bubble cloud properties such as volume, concentration, and intrusion depth over time and space. The number, size, and shape of the bubbles are highly associated with the intensity of the breaking wave (i.e., wave steepness). While for low-intensity spilling waves, the rolling surface jet and the subsequent secondary (splash-up) jets may generate clouds of bubbles with nearly similar length and time scales, however under high-intensity plunging waves, a more complicated plume-cloud of bubbles may be generated due to the frontal plunging jets. In the case of an oil spill at sea, breaking waves will also lead to breakup and entrainment of a surface slick into smaller submerged droplets.

A series of lab experiments were conducted in a linear wave flume with a piston-type wave maker, investigating bubble clouds and oil droplets and their spatial and temporal properties under 2-D spilling and plunging breakers. In this work, the SINTEF SilCam camera system with a uniform LED backlight was utilized to capture images of the bubbles and droplets at high frequency. The PyOpia open-source image processing library was enhanced and subsequently trained to capture the overlapping bubbles, droplets, and individual bubbles with highly deformed shapes.

In the present poster, results extracted from the image analysis are described. We show depth- and time-resolved size distributions for both bubbles and oil droplets. Some of the size-distribution models and scales presented in the literature, for both air bubbles and oil droplets, are examined to evaluate their performance with current data.