Self-similar jets.

General arguments and numerical calculations are used to show that the flow caused by a supersonic gas jet is self-similar under certain conditions. If we assume that the jet has a high initial Mach number and is generated in a region small compared to its length, then the type of similarity solution depends on the density distribution of the gas through which the jet propagates. If this density decreases faster than 1/R^2^, where R is the distance from the source, then the length of the jet increases linearly with time and it may evolve into a classical double if it subsequently encounters a region of higher density. In a more slowly varying external density, the jet is reconfined and the similarity exponent is the same as for an isotropic wind with a constant rate of energy input. At intermediate times this looks like a classical double, but at large times it has many of the characteristics of FRI sources.