Mechanism of suction-side boundary-layer deterioration near the critical incidence of a high-lift low-pressure turbine cascade

High-lift low-pressure turbine cascades are prone to rapid aerodynamic deterioration under off-design positive-incidence conditions, whereas the suction-side boundary-layer deterioration mechanism near the critical incidence remains insufficiently understood. In this study, a high-lift low-pressure turbine cascade is investigated by combining experimental measurements with transition-sensitive numerical simulations. The experimental data at the design condition are used to validate the numerical method, and the simulations are then employed to systematically analyze the suction-side boundary-layer separation, transition, and unsteady response before and after the critical incidence. The results show that the total-pressure loss begins to increase rapidly near i=14.5^∘, which is therefore defined as the nominal critical incidence in the present study. Before the critical incidence, suction-side separation mainly appears as a localized closed separation bubble in the middle portion of the suction surface. After transition of the separated shear layer, enhanced near-wall momentum exchange allows the flow to reattach downstream, and the rear suction-side boundary layer retains partial recovery capability. Once the incidence exceeds the critical value, the localized mid-chord separation becomes less dominant, while low-momentum fluid increasingly accumulates over the rear suction surface and the downstream recovery capability deteriorates. Meanwhile, the dominant wall-pressure fluctuation response changes from a localized separated-shear-layer response to a broader response involving the rear suction surface and trailing-edge region. The flow eventually develops into trailing-edge open separation. At this stage, the low-velocity wake region expands, accompanied by an increased wake momentum deficit and more pronounced downstream mixing, which contribute to the rapid increase in total-pressure loss.