Orifice

An orifice is a localized opening through which a fluid passes, often from a vessel into a region of lower pressure. A simple inviscid incompressible estimate for an opening of area $A$ iswhen the reservoir velocity and elevation change are negligible and $\Delta p$ is the driving pressure difference. The relation follows from a Bernoulli equation estimate of exit speed.

Real jets contract and dissipate energy, so a discharge coefficient $C_d$ is often introduced through $Q=C_d{Q}_{\mathrm{ideal}}$. The vena contracta can have a smaller area than the opening itself. Measuring collected fluid per unit time and the driving head tests the coefficient for a specified geometry and regime. For substantial gas compression or choked flow, the incompressible formula must be replaced by an appropriate compressible-flow model.

The vena contracta is the narrowest jet section after an opening. Its area may be smaller than the opening area, so the jet speed and volumetric flow rate require separate comparisons with ideal flow.