Analysis of Separation and Transition on a Novel High-Speed Low Pressure Turbine Cascade

The transonic aerodynamics of a low pressure turbine (LPT) cascade, mim-
icking geared turbofan conditions during cruise, is currently under study using direct
numerical simulation. In the first step of this endeavour, transition, separation and
loss generation were examined at Mach 0.7, 0.9, and 0.95, with a constant Reynolds
number of 70 × 103 based on the true chord, in clean inlet conditions. The aim is
to identify the impact of sonic conditions and choking on self-induced transition,
generated purely by flow features around the blade. The pressure distribution was
slightly overestimated with respect to experimental conditions. Separation occurs
over the suction side (SS) at 58% and 69% of blade length for the first two Mach
numbers. Skin friction is low in the Mach 0.95 case, but unexpectedly separation
does not take place for this case. The choking of the passage was considered as
influencing factor. A supersonic region followed by a shock appears at Mach 0.9 or
higher, and a fully choked passage is found at 0.95. Separation bubbles with turbulent
reattachment are formed on the pressure side (PS) between 0% and 50% of the blade
length. Separation on the SS was identified as laminar separation long bubble, while
reversed transition develops on the PS after reattachment. Wake losses increase with
higher Mach numbers, and turbulence is more prominent at lower Mach numbers.
This investigation provides insights into the aerodynamic characteristics of the lin-
ear low pressure turbine cascade, addressing separation, transition, losses, and wake
behavior in sonic conditions at low Reynolds numbers.