Large Eddy Simulations of a Low-Pressure Turbine: Roughness Modeling and the Effects on Boundary Layer Transition and Losses

Large eddy simulations of a linear low-pressure turbine cascade with the T106A profile and different surface roughness patches were carried out. The aim was to investigate the effects on the laminar and turbulent boundary layer on the blade suction surface. Two different approaches were used to represent the roughness patches. Firstly, a forcing model, reducing the computational costs compared to fully resolved roughness surfaces, was incorporated. Secondly, an immersed boundary method representing an as-cast roughness surface was used, for a more detailed analysis of flow mechanisms over roughness. It was found that the roughness model was able to induce boundary layer transition and alter the turbulent boundary layer, with the results in line with findings in the literature. The instantaneous flow data at different time instants of the as-cast roughness case showed the development of streaks due to distinct roughness peaks, resulting in highly uneven transition positions across the spanwise direction.