The study examines an enigmatic physical phenomenon of energy separation in a vortex tube manifested by the fluid temperature. The effect is observed in a motionless device, which uses only a compressed medium as a driving force to split the stream into two: hotter and colder than the initial state. The effect was discovered by George Ranque in 1933 and later investigated by Rudolph Hilsch. Throughout the years many theories explaining the mechanism have been proposed including: the conservation of the angular momentum theory, Görtler-vortices, acoustic streaming or micro refrigerant circuits. The ongoing discussion on the root cause of this phenomenon can be unraveled with a proper approach that includes advanced 3D computational fluid dynamics modeling validated by the experimental data. This study is conducted with the use of LES turbulence model in a transient condition. The mesh sensivity analysis is provided and the obtained numerical results are compared with the data gathered during the experimental part of the investigation, obtaining a relatively small error for such a highly turbulent, unstable and swirling flow. In addition, the numerical model is validated with the visualisation study conducted with the use of Particle Image Velocimetry (PIV) technique. Various working media, numerical schemes, and the use of the latest GPU resources are discussed. The obtained results, allow for a more in-depth examination of the Ranque-Hilsch phenomenon and resolution of disputes concerning the separation mechanism in the vortex tube. Additionally, the numerical model can reproduce the cavitation process recently discovered in the vortex tube to investigate it non-invasively.