In this work, we present an experimental and numerical study in a horizontal duct with a rectangular cross-section (300x30mm2). In the middle of this cross-section, a rectangular obstacle (20x10mm2) then a squared obstacle (10x10mm2) were placed in order to study experimentally and numerically the vortex shedding and their interactions in the flow. Upstream obstacles, the studied flows are laminar. In our experimental study, the PIV technique was used in order to obtain instantaneous velocity fields downstream the used obstacles. From these measurement results, a post-processing was used (especially the Γ2 criterion) in order to well extract instantaneous vortices contained in the flow downstream obstacles. In parallel with this experimental study, a 2D numerical simulation was achieved in order to be validated by the experimental results. Other complementary PIV measurements were carried out in the duct top by visualizing the flow downstream obstacles in order to study the 3D effects of the flow.