  The complex interaction between turbulence and the free surface in boundary
layer shear flow created by a vertical surface-piercing wall is 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 accelerated suddenly from rest until reaching constant speed in order
to create a temporally-evolving boundary layer analogous to the
spatially-evolving boundary layer that would exist along a surface-piercing
towed flat plate. Surface profiles are measured with a cinematic laser-induced
fluorescence system and sub-surface velocity fields are recorded using a
high-speed planar particle image velocimetry system. It is found that the belt
initially travels through the water without creating any significant waves,
before the free surface bursts with activity close to the belt surface. These
free surface ripples travel away from the belt before appearing to become
freely-propagating waves. From sub-surface velocity measurements, it is found
that close to the surface, transition to turbulence happens sooner than far
from the surface, leading to an overall thicker boundary layer in the vicinity
of the free surface. A secondary peak in streamwise velocity fluctuations
accompanies this transition to turbulence and this peak reaches a maximum value
a short time later before smoothing outward. Using momentum thickness as a
length scale and the streamwise velocity fluctuations at the location of this
outer peak as a velocity scale, free surface bursting and air entrainment onset
are found to depend in some way on Weber number and agreement is found with
scaling arguments for air entrainment presented by Brocchini and Pregrine (J.
Fluid Mech., 499, 225-254, 2001).
