Characteristics of bubble clouds at various wind speeds

Abstract

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 Hs 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 (0.5c). 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 (0.25T) of the bubble cloud lifetime corresponding to wave phase of up to 90°. The void fraction decreases steadily to about 20–30% at 0.7T (wave phase of ∼270°). Probability density functions of the bubble cloud characteristics show that the bubble cloud length varies from 0.1L to 0.7L; the bubble cloud thickness ranges from 0.5Hs to 2Hs. 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(100T).