ESDU 18005 provides a method of predicting the mass flow rate of a leak from an external stream into a plenum located within an air vehicle. The leak mass flow rate is dependent on the pressure difference across the leak orifice, the geometry, size, and orientation of the leak orifice and the external flow Mach number.

The method requires knowledge of the leak orifice area together with values of the maximum orifice dimensions in the streamwise direction and normal to it. Boundary-layer thickness is not observed to affect the leak mass flow rate from an external stream into a plenum.

The method is derived using test data for flows into a plenum from an external stream through various rectangular and circular orifice geometries in thin plates. The method is applicable to rectangular, square, elliptical and circular orifice geometries; external flow Mach numbers up to 1.3 and unit Reynolds numbers greater than 1×106 m–1. Leak mass flow ratio is a useful parameter and is the ratio of the mass flow rate through the leak orifice to the mass flow rate in the external stream. Sketches comparing predicted data with test data show that in most cases predictions lie within ±0.05 for leak mass flow ratio.