Roughness effects on flow losses of a high-lift low-pressure turbine cascade

Laminar separation induced flow transition has been regarded as one main source of flow losses of low-pressure turbines (LPTs). The realistic roughness favors the reduction of flow losses as the separation bubble reduces and even disappears. In this paper, a detailed grid-independent study is first presented to verify and validate the numerical solutions by solving Reynolds-averaged Navier–Stokes and shear stress transport γ-Reθ̃ transition model equations for a typical high-lift LPT. The roughness elements with different heights (Ra) are uniformly imposed on the whole suction side. The effects of Ra on the flow separation and transition and, thus, flow losses are studied. The results demonstrate that there is one critical Ra value, below which the flow losses decrease, while above which the flow losses increase as Ra increases. The roughness allocations by imposing roughness elements on different blade portions of the suction side are also studied. The sensitivities of flow loss reduction to roughness allocation are evaluated and illustrated. Moreover, the effects of Reynolds numbers (Re) are investigated. As Re decreases, the critical Ra value increases and a more roughened blade is favorable for flow loss reduction. This study paves the way for finding a practicable flow control method reducing the flow losses by optimally allocating roughness on the blade of high-lift LPTs.