## 4 Results
### Stage Performance Data  
To quantify the weight of the direct effect (due to Mach and Reynolds numbers) with respect to the indirect one (related to the rotor incidence), the stator aerodynamic loss and the overall stage performance are now discussed. Stator losses are expresses by the total pressure loss coefficient (\(Y_{\text{stat}}\)) while the stage performance is expressed by resorting the stage work, calculated by means of the velocity triangles, divided by the total-to-total isentropic enthalpy drop. Results are reported in Table 5, alongside the chord-based stator and rotor Reynolds numbers, the Mach numbers downstream of the stator and rotor, and the rotor incidence angle at midspan (the latter being a marker for the rotor loading condition). First considering the OP, the stator total pressure losses (\(Y_{\text{stat}}\)) clearly mark the effects of the expansion ratio according to what commonly found in literature on the Mach number effects on secondary flows [26]. Figure 17a shows their spanwise trend with peaks at tip and hub whose intensities decrease as the Mach number increases. When the profile losses--evaluated at midspan--are of concern (Figure 17b), a decreasing trend is also found; the loss trend is consistent also with typical Reynolds number effect, which foresees a decrease in friction coefficient as the viscous effects reduces their importance with respect to the inertial one. The limit Reynolds number is located around \(10^{6}\), where the function assumes an asymptotical trend to the value of \(\approx 4.35\). In a nutshell, the midspan losses seem to be more related to the effect of Reynolds number than to that of the Mach number on the blade boundary layer. Figure 16: RMS of periodic unsteadiness for OPLL, (**a**) RMS of the relative total pressure coefficient; (**b**) RMS of the deviation angle. Figure 17: (**a**) Spanwise profile of the stator total pressure loss coefficient; and (**b**) stator profile loss coefficient at midspan vs. Reynolds number. OPLL and OPLU being different from OPLR only for the rotor rotational speed, they do not evidence any significant difference in the stator flow and for this on its loss coefficient. When the effect of stage expansion ratio is of concern, data from Table 5 can be more proficiently analyzed by considering the trends reported in Figure 18, where the total-to-total efficiency is plotted against the incidence angle at midspan, which is taken as indicator for the rotor loading. In general, a decreasing trend is found as the incidence angle increases but some differences appear between the two sets of test OP and OPL. In the incidence range where the data from the data-sets are comparable (\(-5<I_{R,MID}<15\)), the two curves have different slopes; OPL tests, that only feature a change in rotor loading with negligible variation in Reynolds and Mach number, exhibit a smaller slope. On the contrary, in OP tests, where both the direct and indirect effects are present, the efficiency is reduced by both the loading effects and by the Mach/Reynolds ones, all of them playing the same role. It is to be noted that the two data sets are quantitatively coherent, as the OP3 and OPLR conditions show a very good repeatability. In order to provide a quantitative perspective on these effects, the stage performance in an incidence angle range within \(\pm 6^{\circ}\) with respect to the reference condition OP3 is now considered. For positive incidence levels with respect to the reference condition, the lower performance of the stage in the OP4 test with respect to the corresponding OPLL (for example: \(\eta_{\text{TT,OP3L}}-\eta_{\text{TT,OP4}}\approx 1\%\) for \(\text{I}_{\text{R,MID}}=1^{\circ}\)) has probably to be ascribed to the lower Reynolds and Mach numbers experienced by the blade row for low expansion ratio.