Bubbles entrained by mechanically generated breaking waves

Abstract

A photographic technique was used to make measurements of the size distributions of large bubbles entrained by mechanically generated breaking waves in freshwater and saltwater. Digital video images of the region immediately beneath and behind the breaking wave crest were analyzed. This imaging technique was accurate for bubble radii, r ≥ 0.8 mm. The magnitude and shape of the bubble size distributions were approximately the same in freshwater and saltwater. The bubble size distributions are well represented by an exponential equation of the form N ∼ e −1.5r or by a power law equation of the form N ∼ r −3.7 . There were no significant differences observed between the depth distribution of the bubbles in saltwater and freshwater. In both cases the data can be represented by N = N o e −z/h , where z is the distance below the free surface, h is the entrainment depth, and N o is the total bubble concentration at the free surface. N o and h were found to be 4 × 10 5 m −3 and 1.0 cm, respectively. The fact that significant numbers of large bubbles are entrained by relatively small breaking waves (wavelength ∼ 1.5 m) suggests that gas transfer due to large entrained bubbles may be important even at low wind speeds.