Through a set of small scale experiments at varying pressures, Birch et al. formulated an equation that allowed the estimation of a virtual surface source, considering the conservation of mass between the exit plane of the orifice and the virtual surface. This approach allows to simulate a compressible, under-expanded jet as an incompressible, fully-expanded jet. As a consequence, a simpler CFD model can be simulated by using the following diameter (named pseudo-diameter) as the new exit plane: