The effect of the diameter of an orifice plate when a bluff body is placed upstream of it is numerically studied. A circular cylinder used as a bluff body usually generates a wake of vortices known as Von Karman vortices. It was found in this study that it depends on the opening of an orifice plate behind the bluff body there is a transition from the classical Von Karman system of vortices to a pulsation type of flow. The numerical calculation performed by a Lattice Boltzmann approach reveal the nature of the vortices each time that the diameter of the orifice is changed as well as the frequencies related to the shedding of the vortices behind the obstacles.

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