  This study reports on experimentally observed near-wall reverse flow events
in a fully developed flat plate boundary layer at zero pressure gradient with
Reynolds numbers between $Re_\tau = 1000$ and $Re_\tau = 2700$. The reverse
flow events are captured using high magnification particle image velocimetry
sequences with record lengths varying from 50,000 to 126,000 samples. Time
resolved particle image sequences allow singular reverse flow events to be
followed over several time steps whereas long records of nearly statistically
independent samples provide a variety of single snapshots at a higher spatial
resolution. The probability of occurrence lies in the range of 0.01% to 0.1%
which matches predictions made with direct numerical simulations (DNS). The
self-similar size of the reverse flow bubble is about 30-50 wall units in
length and 5 wall units in height which also agrees well to DNS data provided
by Lenaers et al. (ETC13, Journal of Physics: Conference Series 318 (2011)
022013).
