Here we used modified PIV technique completed with long distance microscope probe for experimental investigation of the flow velocity profile in a rectangular duct. We came from the analytical and numerical prediction of the entrance length for fully developed velocity profile. The results of measurement completed knowledge about the flow stability and velocity profile shape in the channel of 0.00375 hydraulic diameters. There was marked a range of entrance length constant for the transient flow area. The presents of the fluctuating velocities in the transition flow is explained with POD snapshot and modes projection. The minimal entrance length for the laminar, transition, and turbulent flow is set.

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