  The complex interactions between turbulence and the free surface, including
air entrainment processes, in boundary layer shear flows created by vertical
surface-piercing plates are considered. A laboratory-scale device was built
that utilizes a surface-piercing stainless steel belt that travels in a loop
around two vertical rollers, with one length of the belt between the rollers
acting as a horizontally-moving flat wall. The belt is operated both as a
suddenly-started plate to reproduce boundary layer flow or at steady state in
the presence of a stationary flat plate positioned parallel to the belt to
create a Couette flow with a free surface. Surface profiles are measured with a
cinematic laser-induced fluorescence system in both experiments and air
entrainment events and bubble motions are observed with stereo underwater
white-light movies in the suddenly started belt experiment. It is found that
the RMS surface height fluctuations, $\eta$, peak near the boundaries of the
flows and increase approximately linearly with belt speed. In the Couette flow
experiments, a dominant peak in the spectrum of $\partial\eta/\partial t$ is
found at the dimensionless frequency $fH/U\approx 0.7$. In the suddenly started
belt experiment, surface fluctuations appear to be strongly influenced by
sub-surface turbulence within the boundary layer, while ripples propagate
freely when farther from the belt. Additionally, some mechanisms for air
entrainment are observed and comparisons are made to predictions of incipient
entrainment conditions in Brocchini and Pregrine (J. Fluid Mech., 499, 225-254,
2001).
