The interaction between a laminar boundary layer and a rectangular cross-sectional cavity is studied with tracer injection visualizations and particle image velocimetry (PIV) in different planes of observation. Flow dynamics inside the cavity is driven by the shear layer, developing above the cavity, and a main vortex, developing along the whole cavity span. Secondary vortices are also present for some cavity geometries. 3D flow morphology is confirmed by tracer injection and PIV velocity streamlines. Transverse flows show low-velocity loops from the cavity centerline towards the lateral sides. For some Reynolds numbers and geometries, pairs of counter-rotating vortices are appearing, superimposed to the transverse flow. They are identified on both tracer injection visualizations and PIV measurements. The investigation of a wide range of parameters gives the thresholds for the appearance and disappearance of such centrifugal instabilities which are resulting from the imbalance between centrifugal forces and radial pressure gradient inside the main cavity vortex. The PIV field leads to the measurement of the momentum thickness and advection velocity inside the cavity, allowing a representation in a stability diagram.