On the Transfer of Microbubbles by Surface Waves

Direct numerical simulation (DNS) is performed and the dynamics of a near-surface water layer populated by air bubbles in the presence of a stationary surface wave is investigated. Complete 3D Navier–Stokes equations for the carrier flow are solved in a Eulerian frame simultaneously with the Lagrangian equations of the bubble motion taking into account the impact of the bubbles on the carrier flow. The bubble diameters are considered in the range from 200 to 400 μm, and the effects related to bubble deformation are neglected. Simulation results show that rising bubbles enhance the carrier-flow turbulence due to their vortex wakes. The results also show that bubbles dynamics are governed by buoyancy, the surrounding fluid acceleration force, and the drag force, whereas the impact of the lift force based on the surrounding flow vorticity and bubble relative velocity remains negligible. Parameterizations of the vertical and horizontal fluxes of the void fraction are obtained which show that the horizontal transport of bubbles is controlled by the Stokes drift, whereas the vertical flux is determined by the terminal bubble velocity in an acquiescent water.