Hybrid-Grid Simulation of Unsteady Wake-Boundary Layer Interaction on a High Lift Low Pressure Turbine Airfoil

The unsteady wake-boundary layer interaction on a high lift low pressure (LP) turbine airfoil T106C was investigated by applying the hybrid structured-unstructured RANS solver developed at the DLR. The simulation domain was split into two parts: a translational one with moving bars and a stationary one with turbine airfoils, and in between was a sliding mesh interface. An unstructured grid was generated around the moving bars with particular clustering along the wake path to have a sharp resolution of the shedding vortex street, whereas the stationary blade airfoil subject to the incoming wakes was meshed with a block-structured grid to ease the implementation of the laminar-turbulent transition model around the airfoil. The Wilcox two-equation k-ω turbulence model was applied in conjunction with a multi-mode transition model developed by the authors taking into account several modes of transition, namely natural/bypass, separated-flow and wake-induced transition modes. In this paper, the hybrid-grid modeling is first validated against measurements from the VKI, and then the unsteady flow mechanisms associated with the shedding vortices and the multi-mode transition on the blade airfoil are analyzed. Furthermore, the quasi-steady mixing-plane model on the hybrid grids is also assessed by a comparison with the time-mean of the unsteady state solutions. In particular, different chopping to the incoming vortex street at the blade leading edge is found to have different effects on the separation and transition over the blade suction surface. At the end a composite picture of the boundary-layer development over the suction surface is summarized.