Unveiling Boundary Layer Physics in the SPLEEN High-Speed LPT with Realistic Turbulent Inflow: a Wall-Resolved LES Benchmark for RANS Models

For advancing next-generation Geared Turbofan configurations, the High-Speed Low-Pressure Turbine (HS-LPT) has emerged as a key element of the design process. At the elevated rotational speeds, accurately forecasting the onset of boundary-layer transition is crucial, as it has a direct impact on aerodynamic losses and, consequently, overall engine performance. The present study numerically examines the transition mechanisms in the SPLEEN cascade, experimentally investigated at the von Kármán Institute, and representative of a rotor blade in a modern configuration of high-speed LPT. Wall-resolved Large-Eddy Simulations (LES) are conducted to elucidate laminar-turbulent transition mechanisms in both subsonic and transonic flow regimes. The criteria that need to be met to ensure accurate results are discussed and it is demonstrated that the decay of the inlet turbulence observed in the experiments needs to be reproduced to correctly capture the underlying transition mechanisms. The study also compares the LES results with RANS predictions using the Laminar Kinetic Energy transition model (LKE) at the midspan. A detailed analysis of the boundary layer transition mechanisms is conducted through the examination of integral boundary layer parameters, velocity and turbulent kinetic energy profiles normal to the wall. The key findings demonstrate that at low Reynolds numbers typical of cruise operations, separation-induced transition is the dominant loss mechanism. In the transonic regime, the unique topology of the SPLEEN C1 geometry results in a complex shock system that, while influencing boundary layer thickness, does not trigger full transition before the trailing edge. The demonstrated ability of LES to resolve both the re-energization of the boundary layer and the asymmetric wake mixing offers a clear direction for designing more advanced RANS closure models.