  We investigate ambient fluid entrainment and near-field flow characteristics
of a free surface plunging jet for five Reynolds numbers ranging from 3000 to
10000 using time-resolved stereo particle image velocimetry (SPIV). We present
time-averaged velocities, RMS velocity fluctuations, mean entrainment and
unsteady flow features and compare them with previous studies on free jets. We
find that plunging jets have a smaller potential core length, and earlier decay
of the mean centerline velocity. The peak RMS velocity fluctuations occur at a
location significantly upstream compared to the free jets reported in the
literature. Near-field ambient fluid entrainment of plunging jets is measured
for the first time and is found to be considerably higher than free jets in the
low Reynolds number range. For the plunging jet case at Re = 3000, faster jet
decay, higher levels of turbulent intensity in the near-field, and augmented
mass entrainment result from strong primary vortices that give the
turbulent/non-turbulent interface (TNTI) its convoluted shape which facilitates
both bulk entrapment of ambient fluid and small scale nibbling because of
larger surface area. These primary vortices occur right below the free surface
and disintegrate into secondary structures at axial locations that are upstream
compared to those of free jets. At higher Reynolds numbers, primary vortices
are smaller in size, weak in swirling strength, and disintegrate prematurely,
resulting in suppressed mixing and reduced entrainment efficiency.
