Flow dynamics in a pipe with an abrupt change in diameter was experimentally and numerically analyzed. Two-dimensional stationary Reynolds-averaged Navier–Stokes (RANS) k– ε epsilon model was used to describe the development of axial and radial velocity and turbulent kinetic energy in two cases. The theoretical results were compared with experimental findings gained in a transparent pipe test rig. Particle image velocimetry (PIV) technique was used to analyze the development of flow in a pipe with complex geometry. The measured and modeled velocities and turbulent kinetic energy were found to be in good agreement. The two-dimensional stationary RANS k– ε model is suitable for the analysis of the flow dynamics in real old rough pipes where the pipe wall build-up leads to changes in the actual diameter of the pipe but the flow can still be considered axially symmetrical.