Physical and numerical experiments on an under-expanded jet

The axial characteristics of axisymmetric under-expanded sonic jets are dea lt with, both experimentally and numerically. Experiments were carried out in a test rig where a sonic nozzle discharged into an ambient which could b e kept at different pressures. The density measurements were taken by means of a differential interferometer. The numerical simulation was obtained by adapting a code by Bird (1994) which is based on the MonteCarlo method. Th e comparison between experimental data and numerical results was carried ou t by considering the density distribution along the axis of the jet. The di rect simulation was then applied to explore the jet characteristics in a ra ther wide range of the dimensionless products upon which the phenomenology depends. New expressions are presented for the density distribution along t he jet axis and for the location of the Mach disk.