Vortex macrostructures, which are formed downstream a step axially-symmetrical pipe constriction, within the range where flow changes from the laminar flow to a turbulent one are described. Understanding the nature of these structures and determining their dimensions as well as the range of recirculation zones downstream the constriction makes it possible to take advantage of this phenomenon in a mixing process. Experimental study of the structures originating downstream a pipe constriction was performed in the case of an unsteady flow exposed to low-frequency pulsation (2,5-5 Hz). It was proved that in a pulsating jet the mixing zone changes downstream the constriction.