Spherical bubble motion in a turbulent boundary layer

Monodisperse dilute suspensions of spherical air bubbles in a tap-water tur bulent vertical boundary layer were experimentally studied to note their mo tion and distribution. Bubbles with diameters of 0.37-1.2 mm were injected at various transverse wall-positions for free-stream velocities between 0.4 and 0.9 m/s. The bubbles were released from a single injector at very low frequencies such that two-way coupling and bubble-bubble interaction were n egligible. The experimental diagnostics included ensemble-averaged planar l aser intensity profiles for bubble concentration distribution, as well as C inematic Particle Image Velocimetry with bubble tracking for bubble hydrody namic forces. A variety of void distributions within the boundary layer wer e found. For example, there was a tendency for bubbles to collect along the wall for higher Stokes number conditions, while the lower Stokes number co nditions produced Gaussian-type profiles throughout the boundary layer. In addition, three types of bubble trajectories were observed-sliding bubbles, bouncing bubbles, and free-dispersion bubbles. Instantaneous liquid forces acting on individual bubbles in the turbulent flow were also obtained to p rovide the drag and lift coefficients (with notable experimental uncertaint y). These results indicate that drag coefficient decreases with increasing Reynolds number as is conventionally expected but variations were observed. In general, the instantaneous drag coefficient (for constant bubble Reynol ds number) tended to be reduced as the turbulence intensity increased. The averaged lift coefficient is higher than that given by inviscid theory (and sometimes even that of creeping flow theory) and tends to decrease with in creasing bubble Reynolds number. (C) 2001 American Institute of Physics.