## 1 Introduction  
Among the complex targets of gas turbine research, the aerodynamics of the high-pressure section of the turbine (HPT) involve several complex phenomena and still offer a margin for the optimization of future gas turbine systems. The high stage loading and the low aspect ratio typical of HPT blading induce transonic flows, complex shock patterns and large secondary vortices. These effects, coupled with the use of small axial gaps for compactness, drive a severe unsteadiness in the flow of HPTs. In this context, the power control of aero-engines as well as of stationary gas turbines changes the HPT operating point, altering the loading level and the flow regime in the blade rows from transonic to subsonic conditions; these variations in operation may lead to penalties in the efficiency on the HPT and on the whole multistage turbomachine as well. To develop a comprehensive analysis of a HPT, a proper characterization of the unsteady and three-dimensional flow is required to highlight the individual flow features of the stator and of the rotor blade rows, as well as to identify the several blade-row interaction mechanisms. The interaction promoted by stator wakes has been widely studied in the literature (see, for example, [1]); the wakes, chopped in segments by the rotor leading edge, are intermittently released downstream of the stage along fixed trajectories, called wake avenues, determined by the turbine-exit(absolute) flow direction [2; 3]. In low aspect ratio stages, where endwall flows are relevant, the stator wake, potential field and secondary flows also promote interaction mechanisms with the rotor flow field, affecting the generation and development of the rotor secondary flows, and hence resulting in relevant unsteady effects at the stage exit. Several studies [4; 5; 6; 7; 8; 9; 10; 11] focused on the transport of the incoming vortices within the rotor channel and their interaction with the rotor secondary flows; the dependence of these effects on the cascade incidence [12] and on the axial gap [13] was also investigated. In the case of transonic turbines, shocks can also promote significant unsteady effects, especially on the rotor aerodynamics. In particular, the sweeping of the vane trailing edge shocks in front of the rotor can induce large fluctuations on the rotor loading [14; 15; 16; 17], triggering the unsteadiness on the rotor wake [18; 19] and on the rotor secondary flows [20], as well as inducing periodic oscillations in the total pressure distribution downstream of the stage [21]. Finally, the aero-thermal behavior of the HPT is further complicated by the film cooling and the interaction with the combustor, which introduces time-dependent gradients in total pressure and total temperature as well as whirling flow at the stage inlet. These features were found to considerably affect the aerodynamics and the heat transfer [22; 23; 24] as well as the aero-acoustic behavior [25] of the HPT. All the aforementioned phenomena are influenced by the Reynolds and Mach number at which the stage operate and are expected to be modified by changes in the HPT operating condition. In principle, these features can be predicted by resorting to Similarity Theory, which introduces non-dimensional quantities to scale the operating and the geometrical parameters. However, for several reasons (mainly for the temperature levels) the full similarity cannot be guaranteed among the different conditions, especially during the gas turbine off-design operation. The prediction of the off-design performance of HPTs is, therefore, a complex as well as crucial target that demands dedicated studies. In this paper, an experimental analysis of the impact of the expansion ratio on an uncooled research HPT is proposed.