## 2 Experimental Setup  
The experimental investigation documented in this paper was performed in the High-Speed Closed-Loop Test Rig facility of the Laboratorio di Fluidodinamica delle Macchine (LFM) of Politecnico di Milano (Italy). The facility is equipped with a high-speed centrifugal compressor (internally designed and manufactured) that provides flow rate and pressure ratio to the axial-flow turbine stage; in between the two machines, a cooler is used to set the temperature level of the turbine main flow. The thermal stability of the tests is within \(\pm 0.5\,^{\circ}\)C. The machines are connected to reversible Direct Current engines that allow a continuous and independent control of the rotational speed, in such a way that the angular speed and the expansion ratio of the turbine can be set independently. Figure 1 provides a schematic view of the turbine section. The turbine stage is fed by a centripetal guide vane composed by 30 high-lift profiles specifically designed to release a meridional flow at the turbine inlet, followed by a straightener. Downstream of the honeycomb, the straight annular duct of 50 mm span connects the volute to the turbine stage. Downstream of the stage the channel span enlarges to 60 mm by reducing the hub radius to exploit a diffuser effect. The research turbine, whose geometry is not restricted, is representative of a modern highly loaded, low aspect ratio, single-stage HPT operating in subsonic/transonic conditions. The blade profiles and the main geometrical features of the machine are reported in Table 1; both the stator and rotor blades were designed applying fully 3D design concepts, such as twist, lean and lean compound. Even though the facility allows testing different axial gap configurations, in the present study only one axial gap, equal to the vane axial chord, is considered. Full details on the test rig and the HPT stage can be found in [28]; the time-mean and unsteady aerodynamics as well as the performance of this turbine operated in subsonic conditions were extensively studied in the last decade [10; 11; 12; 13]. Several measurement techniques, whose uncertainties are reported in Table 2, were applied to investigate the flow in the turbine throughout this research. At first, conventional total pressure and total temperature measurements were performed upstream of the stage to monitor the stage inflow boundary conditions. To characterize the inlet endwall boundary layer, the span-wise profile of inlet total pressure was measured with a flattened total pressure probe (probe head dimension 0.5 mm) traversed 1.5 stator axial chord upstream of the stator leading edge: a flat profile of uniform total pressure was found between 20% and 80% of the span. A hot wire probe was also traversed upstream of the stage to determine the inflow turbulence intensity, which resulted 2.5% at midspan and increased to 4% close to the duct endwalls. The three-dimensional time-averaged flow field downstream of the stator was measured with a 5-Hole Probe (5 HP) located 32% of the stator axial chord downstream of the stator trailing edge. The 5 HP was calibrated in a reference nozzle up to Mach number equal to 1. The probe head dimension  
Figure 1: Meridional sketch of the axial turbine section. is 1.4 mm and the sensitive area is overhung 30 mm with respect to the stem to minimize the stem blockage in the measurement section. The measurement grid consists of 32 points over the blade height and 41 points along the stator pitch. The unsteady flow field downstream of the rotor was measured by a cylindrical single-sensor Fast Response Aerodynamic Pressure Probe (FRAPP) at an axial distance of 32% of the rotor axial chord downstream of the rotor trailing edge. The FRAPP is operated as a virtual three-sensor probe, by means of multiple rotations around its own stem, and allows reconstructing the periodic component of the flow phase-locked on the rotor blade passage by means of ensemble averaging. As a result, the phase-resolved components of total and static pressure and of blade-to-blade flow