  We study experimentally the vortex streets produced by a flapping foil in a
hydrodynamic tunnel, using 2D Particle Image Velocimetry (PIV). An analysis in
terms of a flapping frequency-amplitude phase space allows to identify: 1) the
transition from the well-known B\'enard-von K\'arm\'an (BvK) wake to the
reverse BvK vortex street that characterizes propulsive wakes, and 2) the
symmetry breaking of this reverse BvK pattern giving rise to an asymmetric
wake. We also show that the transition from a BvK wake to a reverse BvK wake
precedes the actual drag-thrust transition and we discuss the significance of
the present results in the analysis of flapping systems in nature.
