Diffusion of spherical bubbles in a turbulent boundary layer

Dilute suspensions of spherical air bubbles in a liquid-turbulent boundary layer were experimentally studied to note the motion and distribution. Bubbles with diameters of 0.37–1.2 mm were injected at various transverse wall positions in upward flowing water of free-stream velocities between 0.4 and 0.9 m/s. Bubble injection frequency was limited to very low mean void fractions, ca. 0.01% in order to avoid bubble coalescing at the wall for certain flow conditions. The experimental diagnostics included bubble tracking for bubble trajectories and planar laser intensity profiles for bubble concentration distribution. Data for the lateral void fraction distributions were collected over a sufficiently long sampling time to allow statistical description of the bubble diffusion. types of bubble motion were observed in the boundary layer: free-dispersing bubbles were drawn to the wall (by transverse fluctuations), bounced along the wall (by coefficients of restitution of order unity), and sliding bubbles along the wall trapped by lift forces. This led to a variety of void profiles: from peaking at the wall to diffusing beyond the mean boundary layer width, depending on the Stokes number and bubble injection location. In general bubbles collected along the wall for high Stokes number conditions (larger bubbles or weaker turbulence). In contrast, the lower Stokes number conditions produced Gaussian-type profiles throughout the boundary layer, which were associated with a tendency for the smaller bubbles to act like tracer particles dispersing throughout the flow. Qualitative measurements of intermediate-size bubbles indicated diffusion rates as a function of drift parameter which were similar to that expected from solid particle diffusion and free shear bubble diffusion.