Measurements of bubble size distribution underneath breaking waves

Entrainment of particles by breaking waves are an important process for several applications. For example, entrainment of air bubbles is relevant for air-sea gas exchange, which in turn is relevant for climate modelling. Entrainment of oil droplets in a marine oil spill will have an effect on the fate of the oil, and help determine environmental effects. Hence, being able to measure and model these entrainment effects are important.

We are conducting experiments in a linear wave flume, with piston-type wave maker, looking at entrainment of air bubbles under breaking waves. Using a camera system with a uniform backlight and a telecentric lens, the SINTEF SilCam, we can image bubbles ranging in size from tens of micrometers, to cm scale. By accurately constraining the measurement volume, we can determine concentration of bubbles of different sizes. Taking images at high frequency, and repeating the same breaking wave many times, we are able to measure the time-development of the ensemble-average bubble size distribution.

In this poster, we describe the camera system and the image analysis pipeline, and we present some preliminary results and discuss some of the inherent challenges in measuring bubble size distributions close to the surface underneath breaking waves.