On the hydrodynamic expansion of a relativistic gas
An exact solution to the one-dimensional relativistic Euler equations is presented which is not of a self-similar nature and which satisfies certain initial conditions found to be appropriate in describing high-energy cosmic-ray collisions and models of radio galaxies. A full numerical analysis is performed to demonstrate the temporal behavior of all relevant thermodynamic quantities at any given point in the expansion. The results indicate that the motion is self-similar at the very beginning of free expansion, that this description ceases to be valid when the entire bulk of fluid is in motion, and that with the passage of time, the nonsimilar solution prevails over more and more of the fluid except for a small region near the leading edge where the fluid borders on the vacuum. The very bulk of the expansion is shown to become nonsimilar because the leading edge of the self-similar solution moves into the vacuum at the speed of light while that of the nonsimilar solution approaches c only asymptotically. The relativistic expansion of a gaseous disk is analyzed as an example.