Room: 5-314
Speaker Name:
Declan Gaylo
Affiliation:
Abstract:
Air entrainment at a free surface is important to a variety of applications including air-sea gas exchange and vessel acoustics. In these applications, a quantity of interest is the size distribution of bubbles, which is driven by, in addition to entrainment, mechanisms including fragmentation and degassing. When fragmentation is dominant, Garrett et al. (J. Phys. Oceanogr., vol. 30, 2000) predict a -10/3 power law for the bubble size distribution of large Weber number, We, bubbles. This power law is ubiquitous in breaking waves. More representative of the flow in a vessel’s wake or in rivers, we study entrainment by free surface turbulence (FST) where, unlike breaking waves, the energy to entrain bubbles comes directly from turbulence beneath the free surface. We perform direct numerical simulation (DNS) of a canonical FST flow at a range of Froude number, Fr. Using Eulerian label advection, we can directly measure individual mechanisms, allowing quantification of the relative effects of fragmentation versus degassing. Even at We>>1 when fragmentation is strongest, we show degassing is dominant over fragmentation. We develop a theory to predict the power law of the bubble size distribution under degassing dominance, which agrees with DNS results. This distribution is distinct from the fragmentation-dominated distribution in two ways. First, two regimes of bubble degassing in turbulence lead to a split power law, with a critical bubble radius scaling with Fr and Reynolds number. Second, in both regimes the power law is more negative (fewer large bubbles) than -10/3. We discuss what measures can be used to determine if a given air entraining flow is fragmentation or degassing dominated.