Numerical and Experimental Investigation of Boundary Layer of a High-Lift Low-Pressure Turbine Cascade Under Periodic Wakes With Different Flow Coefficients

This article investigates the boundary layer on the suction surface of a low-pressure turbine (LPT) subjected to upstream wakes with three different unsteady flow cases, characterized by the variation of flow coefficients. The unsteady behaviors of the boundary layer on the LPT suction surface are studied numerically and experimentally. The Reynolds number in the cascade is constant. Three flow coefficients are achieved by varying the bar speed, while the cascade's outlet velocity remains stable across the three test cases. The wake shapes under the three flow coefficients are captured by numerical simulation, and the instantaneous characteristics of the boundary layer are analyzed by hot-film test data and numerical results. The wake can be divided into a center and a tail, each with different effects on boundary layer multimode transitions. The wake center promotes the separation bubble breakdown, inducing wake-induced transition. Klebanoff streaks, formed by shear sheltering at the blade's leading edge, accelerate wake-induced transition by breaking down vortices in the interaction between the wake tail and the boundary layer. Under the three different flow coefficients, the wake shapes are different, resulting in a difference in the intensity and development of the Klebanoff streaks. The interactions among the Klebanoff streaks and roll-up vortices, the wake shapes, and the timing of Klebanoff streak involvement in the wake-induced transitions are important coupled factors. Additionally, the flow coefficient can change the factors, resulting in different wake-induced transitions, which should be considered in the design of high-lift LPTs.