The study is an experimental investigations using PIV. The measurements were obtained by PIV for an unsteady laminar flow across a rectangular channel with a cross-section 300 × 30mm2, in the middle of which is located a cylindrical or a square obstacle. In the case of the cylindrical configuration and due to the confinement, PIV measurements in the range of 40 < Re < 200 clearly show that the von Karman vortex shedding appears at a critical Reynolds number which is about 66. A post-processing of these PIV measurements using the Proper Orthogonal Decomposition (POD) technique is by keeping only the first most energetic six modes, can be used as a filtering process to remove noise from instantaneous velocity signals. In the case of the square obstacle, PIV measurements obtained in the range of 30 < Re < 350 show the absence of vortex detachments and the chaotic behavior of the wake behind the obstacle beyond a certain Reynolds number. By examining the POD post-possessing results, the existence of a dynamic detachments’ regime (instantaneous breaking and coalescence of vortices), can be clearly observed. Given the chaotic behavior of the wake behind the obstacle, the application of the POD filtering process to only the first most energetic modes, cannot lead to good results.

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