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.