  A boundary layer interacting with a cavity is a benchmark case that is
present in environmental applications, aeronautics, automobile aerodynamics or
industrial applications, where the velocity over a rectangular cavity is
relatively low. In the present investigation, the interaction between a laminar
boundary layer with external velocity Ue and an open cavity is investigated
experimentally for medium range Reynolds numbers (between 4600 and 18500) with
flow diagnostics optical methods, for a cavity aspect ratio (length over depth)
of two. A shear layer induces a main vortex and a counter-rotating secondary
vortex inside the cavity. The shear layer also develops instabilities
interacting with the downstream cavity edge. Particle image velocimetry (PIV)
and laser Doppler velocimetry (LDV) measurements are synchronized in order to
get global information of both spatial velocity fields and time-resolved
signals in a particular position situated downstream of the development of the
shear layer above the cavity. The PIV velocity fields are phase-averaged with a
singular value decomposition of the LDV signal.
