The flow passing through a sharp-edged orifice is studied using two experimental techniques over a pipe Reynolds number range of 4000 to 27000. The flow separates at the orifice inlet and is accelerated through its hole in the form of a confined jet. For a given orifice, the mean reattachment length is found to remain fairly independent of the inflow Reynolds number. Velocity and pressure fluctuations attain peak values in regions lying upstream of the mean reattachment point. Under the conditions tested, the orifice jet shows a low frequency flapping motion which was observed to occur at a Strouhal number 0:02 based on the orifice jet velocity and the difference in internal diameters of the pipe and orifice.@en

Orifice plates or flow restrictions are key components used in industry for flow measurement and control. They find application in gas and liquid circuits of, e.g., lithography machines, nuclear power plants and aerospace propulsion systems [1]. They are used typically either for measuring flow-rate or to introduce a pressure drop for purposes of flow balancing. It is widely acknowledged in literature that the turbulent, unsteady nature of the flow through an orifice can be a source of structural vibration [@en