An under-expanded jet is one that manifests when the pressure at downstream conditions (at the end of a nozzle or orifice) is greater that the pressure of the environment where the gas is being released in. It is said to be under-expanded since the gas will expand, trying to reach the same pressure of its surroundings. When under-expanded, the jet will have characteristics of a compressible flow, a condition in which pressure variations are significant enough to have a strong effect on the velocity (where it can exceed the speed of sound of the gas), density and temperature. It is important to note that as the jet expands and incorporates gases from the surrounding medium, the jet will behave more and more like an incompressible fluid, allowing for a general definition of the structure of a jet to be the following:
*Nearfield zone: this zone is composed of a core layer that is isolated from the surrounding medium, with its behavior being mostly dominated by compressible effects, and an outer layer that is in contact with the surrounding medium fluid. Due to turbulent effects, the outer layer, nominated as mixing layer, permits gas entrainment as it is facilitated, diluting the jet. In this shearing zone, a subsonic and supersonic section may be distinguished, where temperature, density and pressure vary wildly in a few centimeters of distance from the source. This zone has the characteristics of a compressible fluid.
*Transition zone: the beginning of this zone represents the ending of the nearfield zone, where variations (longitudinally and radially to the axis of the jet) are small compared to the previous one. Density and temperature variations are mostly because of mixing with the surrounding fluid.
*Farfield zone: this final zone is one of a fully expanded and incompressible jet. Longitudinal velocity and temperature are now inversely proportional to the distance from the source and radial evolution can be described by a gaussian dispersion model. It is important to note that this zone can be further split into inertial (dominated by initial acceleration), buoyant (dominated by internal buoyancy forces) and turbulent zones (dominated by ambient turbulence).