  This fluid dynamics video submitted to the Gallery of Fluid motion shows a
turbulent boundary layer developing under a 5 metre-long flat plate towed
through water. A stationary imaging system provides a unique view of the
developing boundary layer as it would form over the hull of a ship or fuselage
of an aircraft. The towed plate permits visualisation of the
zero-pressure-gradient turbulent boundary layer as it develops from the trip to
a high Reynolds number state ($Re_\tau \approx 3000$). An evolving large-scale
coherent structure will appear almost stationary in this frame of reference.
The visualisations provide an unique view of the evolution of fundamental
processes in the boundary layer (such as interfacial bulging, entrainment,
vortical motions, etc.). In the more traditional laboratory frame of reference,
in which fluid passes over a stationary body, it is difficult to observe the
full evolution and lifetime of turbulent coherent structures. An equivalent
experiment in a wind/water-tunnel would require a camera and laser that moves
with the flow, effectively `chasing' eddies as they advect downstream.
