  We present new experimental results on the development of turbulent spots in
channel flow. The internal structure of a turbulent spot is measured, with Time
Resolved Stereoscopic Particle Image Velocimetry. We report the observation of
travelling-wave-like structures at the trailing edge of the turbulent spot.
Special attention is paid to the large-scale flow surrounding the spot. We show
that this large-scale flow is an asymmetric quadrupole centred on the spot. We
measure the time evolution of the turbulent fluctuations and the mean flow
distortions and compare these with the predictions of a nonlinear reduced order
model predicting the main features of subcritical transition to turbulence.
