Calculation and visualization of the unsteady flow in an axial turbine stage

This paper studies several aspects of the analysis and visualization of the unsteady flow field in the complex three-dimensional geometry of a turbomachine stage. The results of an unsteady viscous flow simulation in a highly loaded last stage of an axial low pressure turbine are used as the data set to be analyzed. After a short description of the numerical method used for the simulation, the test-case with its specific challenges to CFD analysis is introduced. The time-averaged flow-field and the unsteady fluctuations are calculated from the instantaneous solutions to find regions of strong unsteadiness. Some of the flow phenomena that are caused by the stator/rotor interaction are visualized. Examples of these phenomena are complex pressure wave motion in the rotor domain and vortex-like shedding of the wake downstream of the rotor blade trailing edge. The unsteady loading on the rotor blades is analyzed by means of a harmonic analysis of the axial and tangential forces.