The area between the stationary and rotating hub side endwall segments in axial turbines is featured with a specifically shaped annular clearance, termed as the rim seal. For the safe and reliable operation of a gas turbine, this seal depicts a pivotal component. The main design intent is to avoid the ingress of hot main flow to the vulnerable wheel-spaces in the inside of the turbine. To guarantee sealing, the internal cavities are permanently supplied with pressurized air. Termed as rim seal purge flow, a specific fraction is continuously passing the seal, before it is mixed with the main flow. It is known, that rotating large scale instabilities in the rim seal region can effect or even dominate the spatio-temporal character of the flow field in the adjacent blade rows. Being superimposed by the unsteady stator-rotor interaction, appropriate test environments need to account for this multi-dimensional interdependency. In a large scale, rotating turbine test facility, the model of a high pressure axial turbine stage is experimentally investigated. The flow characteristic in the seal region is altered by adjusting the purge flow mass fraction. In the scope of this work, experimentally determined flow quantities in the stator-rotor interface, the seal and the rotating blade row are combined with unsteady, three-dimensional computational fluid dynamics targeting the deviation of the spatio-temporal characteristic of the rim seal purge flow. The numerical setup represents a sixty degree sector of the stage including the seal and sub-cavity. The calculations are initiated with experimentally determined flow data at the boundaries. For validation, results are spot checked with selected unsteady experimental flow data. To globally rate the effect of the purge flow, a framework of one-dimensional performance parameters is presented, in which a thermodynamically sound definition of the ideal reference process accounts for the unavoidable entropy of mixing. By increasing the dimensions considered, the spatio-temporal description of the stage is first introduced at mid-span. Visualizing the three-dimensional distribution of vortical structures, the spatio-temporal character of the interaction of main and purge flow is finally introduced.