Characteristics of bubble clouds at various wind speeds

We present the results of a laboratory study on bubble clouds characteristics (length, depth of penetration, width, and void fraction) in fresh water for wind speeds from 9 to 16 m s −1 . Temporal and spatial changes of these characteristics and their statistics were extracted from video images from side and top views. Cloud characteristics were scaled with the period T , wavelength L , phase speed c , and significant wave height H s of the dominant wave. The lifetime of the bubble cloud comprises formation, growth and decay stages. The bubble cloud moves forward horizontally with the wave for the initial 1/3 of the wave period at approximately half the wave phase speed (0.5 c ). The subsequent dominant motion of the bubble cloud in the wave trough is vertical with a mean speed half that of rising bubbles. The void fraction is as high as 80–99% in the first quarter (0.25 T ) of the bubble cloud lifetime corresponding to wave phase of up to 90°. The void fraction decreases steadily to about 20–30% at 0.7 T (wave phase of ∼270°). Probability density functions of the bubble cloud characteristics show that the bubble cloud length varies from 0.1 L to 0.7 L ; the bubble cloud thickness ranges from 0.5 H s to 2 H s . Scaled bubble cloud characteristics and the cloud void fraction vary weakly with the wind speed for the growth stage. Residual void fractions can persist for long times O (100 T ).