In the Ranque-Hilsch vortex tube a compressible fluid (e.g., air) experiences an expansion process leading to separation into a cold and hot gas stream. The compressible flow and energy separation phenomena are simulated numerically by using the code system CFX. In order to calculate successfully the energy separation, the numerical model had to be extended by integrating relevant terms for the shear-stress-induced mechanical work. An axisymmetric model was developed. allowing a successful post-calculation of experimental results.