## 5 Discussion and Conclusions  
This paper has presented and discussed the results of a wide experimental campaign devoted to the unsteady aerodynamics of an axial turbine stage, representative of a high pressure gas turbine. In particular, the effects of the stage expansion ratio and that of the rotor loading have been considered. Figure 18: Total to total efficiency versus rotor incidence angle at midspan, for the two sets of tests, OP and OPL. In the test campaign on the expansion ratio, as a term of comparison the same stage outlet flow angle at midspan was set by regulating the turbine rotational speed. Four operating conditions were considered, performing comprehensive sets of measurements downstream of the stator and of the rotor in order to provide a complete overview of the stage flow configuration and performance. In the campaign on the rotor loading, three test conditions were chosen by keeping the same expansion ratio and by changing the rotational speed. Based on the presented results, some conclusions can be drawn with regard to the impact of the expansion ratio on the stage unsteady aerodynamics and performance. As a general overview, the impact of the expansion ratio, for an assigned stage-exit flow angle, has several features deeply combined one each other, in terms of Mach and Reynolds number effects as well as of rotor aerodynamic loading. First, looking at the stator aerodynamics, the increase in the expansion ratio--and hence in the Mach and Reynolds numbers--reduces the impact of the secondary flow and wake, thus improving the stator efficiency and providing a more uniform flow at the rotor inlet. Such effects have been acknowledged in the paper as \"direct effects\" of the expansion ratio. Since the compressibility affects the relation between the velocity and the expansion ratio, similarity at the rotor inlet cannot be fulfilled once the stage outlet flow direction is preserved. As a result, the rotor incidence changes, and this effect has been acknowledged in the paper as the \"indirect effect\" of the expansion ratio. So far, transonic conditions (OP1) make the rotor operate under negative incidence and, hence, in reduced loading conditions; on the contrary, at low speed (OP4) the rotor experiences high positive incidence. This behavior has some consequences: at first, the lower rotor loading for the transonic conditions reduces the magnitude of the rotor secondary vortices and wake, resulting in lower losses and a more uniform rotor-exit flow field. This occurrence, combined to more efficient stator aerodynamics, results in a higher overall stage efficiency. On the contrary, the raise of rotor loading--as the OP shifts from the transonic to the subsonic condition--increases the rotor losses, reducing the overall efficiency. Furthermore, the lower the stator-exit Mach number, the higher the unsteady fluctuations at the rotor inlet (in the rotating frame): so far, a larger pulsation in the rotor flow structures takes place, amplifying the aerodynamic forcing and lowering the aerodynamic efficiency at low subsonic conditions. Tests performed for variable rotor loading at constant expansion ratio have allowed better quantifying the magnitude of the indirect effects. In fact, in the zero incidence range, the direct effects contribute to an increase in the efficiency of about 2%, which reduce to 1% when the rotor blade is loaded (given an incidence level mirrored with respect to the reference one) and only partially accountable to the Mach/Reynolds number reduction, (see the increase in the stator total pressure losses). To conclude, once the stage-exit flow angle is set, the impact of the expansion ratio leads to a relevant efficiency variation as a combination of direct Reynolds and Mach number effects on the single blade rows and of indirect effects such as the change in the rotor incidence and in the stator-rotor interaction. The increase in stage efficiency passing from low subsonic to transonic condition was found to be about 7%.