The ultra-high bypass ratio geared turbofan is a key concept to reduce the environmental impact of the next generation aircrafts. The increase in rotational speed of the low-pressure turbine (LPT) enabled by the geared architecture offers potential benefits in efficiency and considerable savings in engine weight, overall dimension, and cost. High-speed LPTs operate at transonic exit Mach numbers and low Reynolds numbers. For this combination of operating conditions, where compressibility influences the blade aerodynamics and loss mechanisms, there is a critical lack of openly available detailed experimental data concerning the combined effects of unsteady wakes, purge streams, leakages, and secondary flows for CFD models validation. The current research project focuses on the full characterization of an experimental test case concerning the impact of unsteady wakes and purge flows on the blade aerodynamics and loss mechanisms in a high-speed LPT cascade tested at engine-scaled conditions. The extensive experimental dataset is collected in an open-access database, providing novel validation material for comparison with numerical computations. In this two-part paper, a comprehensive study on the design, commissioning and testing of a transonic low-pressure turbine blade at on and off-design is presented. Part I describes the design of components and accurate instrumentation required to investigate steady quasi-3D flows in a linear cascade environment at low-Reynolds and transonic exit Mach numbers. A novel concept to perform blade radial traverses to obtain spanwise distributions of high-resolution blade measurements is presented. The repeatability and periodicity of the rig flow conditions are demonstrated. This work critically evaluates the methodology used for the design and adaptation of the existing rig for testing of unsteady three-dimensional flows in linear cascade experiments.

The aerodynamics of a high-speed low-pressure turbine (LPT) cascade were investigated under steady and unsteady inlet flows. The tests were performed at outlet Mach (M) and Reynolds numbers ($Re$) of $0.90$ and 70k, respectively. Unsteady wakes were simulated by means of a wake generator equipped with bars. A bar reduced frequency ($f^{+}$) of ∼0.95 was used for the unsteady case. The inlet flow field was characterized in terms of the total pressure profile and incidence. The blade aerodynamics at midspan and the secondary flow region were investigated by means of pneumatic taps and hot-film sensors. The latter provided a novel view into the impact of the secondary flows on the heat transfer topology on the blade suction side (SS). The cascade performance was quantified in terms of the outlet flow angle and losses by means of a directional multi-hole probe. The results report the phase-averaged impact of unsteady wakes on the secondary flow structures in an open test case high-speed LPT geometry.

The time-averaged aerodynamics of a high-speed lowpressure turbine (LPT) cascade were investigated under the impact of unsteady wakes and purge flows. The tests were performed at an outlet Mach (M) and Reynolds numbers (Re) of 0.90 and 70k, respectively. Unsteady wakes were simulated by means of a spoked-wheel type wake generator (WG) and a reduced frequency of 0.95 was achieved. The development of a purge flow system to operate at low pressure levels is presented alongside guidelines for the operation of the circuit. The new purge system was commissioned in terms of its long-term stability and flow uniformity at the cascade inlet. The impact of varying purge flows from a cavity versus a flat endwall was assessed by means of static pressure measurements on the blade pressure side (PS) and suction side (SS) and traverses performed downstream with a miniaturized multi-hole probe. Differences in the secondary flow structures resultant from injecting purge flow into the flowfield are observed, namely the intensification of the trailing shed vortex (TSV) and passage vortex (PV) that is also displaced away from the endwall. An increase of the endwall losses occurs as the flat endwall was replaced with a cavity geometry and the cavity purge flow rate increases.

In this two-part paper, a novel test-case for transonic low-pressure turbines (LPT) is presented. The current study is a comprehensive report on the design, commissioning and testing of a high-speed LPT cascade. Part II reports the characterization of the aerodynamics at on- and off-design flow conditions. A detailed analysis of the steady aerodynamics of the highspeed low-pressure turbine blade is presented for a range of engine representative outlet Mach numbers from 0.70 to 0.95 and Reynolds numbers from 65,000 to 120,000. The blade 3D aerodynamics are characterized using an innovative set of traversable blades, enabling high-resolution radial measurements. A novel method to estimate the location of separation-reattachment based on pneumatic tap measurements is presented. The separation on the blade suction side is strongly influenced by the Reynolds number at the lower Mach numbers, while open separations were observed at transonic exit conditions independently on the Reynolds number. Downstream measurements by means of a five-hole probe, and pressure taps located in the passage endwall are employed to study the secondary flow development and structures at the cascade outlet. The results show that the losses follow different trends at high and low Reynolds numbers. The profile losses at Re = 70k decrease with increasing Mach number, contrary to what is observed for Re = 120k. The minimum secondary flow losses are found for an off-design condition with lower Mach number with respect to the nominal. The off-design comparisons presented in this paper indicate that at low Reynolds, operating at transonic outlet Mach numbers leads to beneficial effects on the performance.

Particle image velocimetry (PIV) measurements in the blade-to-blade (B2B) plane and cascade outlet plane (COP) of a high-speed low-pressure turbine (LPT) cascade were performed at engine-representative outlet Mach number (0.70-0.95), and Reynolds number (70k-120k) under steady flow conditions. The freestream turbulence characteristics were imposed by means of a passive turbulence grid. The PIV results on the B2B plane were compared against five-hole probe (5HP) and Reynolds-averaged Navier-Stokes (RANS) computations to assess the validity of measurement and simulation techniques in the engine-relevant LPT cascade flows. The PIV captured the wake depth and width measured by the 5HP whereas the RANS displayed an overprediction of the wake Mach number deficit. The 5HP was found to impose sinewave fluctuations of the measured flow angle downstream, around three times higher than PIV. Additionally, PIV estimated turbulence intensity (TI) in the cascade, showing TI decay along a streamline. At the highest Mach number, a peak TI occurred past a shock wave. Measurements of the outlet flow field highlighted a high TI in the secondary flow region whereas high degree of anisotropy (DA) was registered in the boundary of the secondary flow and freestream regions. The contribution of the streamwise fluctuation component was found to be less than the crosswise and radial components in the freestream region. Increasing the cascade outlet Mach number, the contribution of streamwise fluctuation to the DA was observed to decrease.

The influence of unsteady wakes incoming from the upstream stages is of high relevance in modern high-speed, low-pressure turbines (LPT) operating at transonic exit Mach numbers and low Reynolds numbers for their potential to trigger transition and influence the separation of the boundary layer on the blade suction side. The aim of this paper is the experimental characterization of the influence of incoming wakes on the 2D aerodynamics of a high-speed LPT cascade operating at a low Reynolds number and transonic exit Mach number. A detailed analysis of the status of the flow along the blade under investigation and its impact on the profile loss are presented for a range of Mach numbers from 0.70 to 0.95 and Reynolds numbers from 70k to 120k under steady and unsteady inflow conditions. Tests were conducted at on- and off-design engine realistic conditions in the VKI S-1/C wind tunnel on the SPLEEN C1 transonic cascade. The wakes incoming from an upstream blade row have been replicated using a set of rotating bars, which shed wakes at an engine-representative reduced frequency ($f^{+}=0.95$) and flow coefficient ($\Phi =0.80$). A set of densely instrumented traversable blades were used to sample the surface pressure distributions. The development of the boundary layers along the blade suction side is examined through quasi-wall shear stress obtained with surface-mounted hot-film sensors. Wake traverses were carried out downstream of the cascade with a miniaturized L-shaped five-hole probe to characterize the blade losses. The introduction of periodic incoming wakes promotes variations in the flow topology over the blade. The effect on the suction side separation bubble is shown to depend on the exit flow conditions. At low Mach numbers, the incoming wakes determine a reduction in the size of the bubble; in contrast, this effect is not registered as the exit Mach number increases. Consistently, a high dependence of the unsteady wake effect on the profile loss on the exit Reynolds and Mach numbers is demonstrated.