ASSESSMENT OF THE DDES-\gamma MODEL FOR THE SIMULATION OF A HIGHLY LOADED TURBINE CASCADE

Abstract

The study analyzes the flow over the MTU-T161 low-pressure turbine cascade at Re=90,000 using Delayed Detached Eddy Simulations (DDES). At this operating point, the flow encounters a separation bubble and separation-induced transition on the suction side. The applied DDES method is based on a vorticity-based formulation and incorporates the one-equation \gamma-transition model. Several studies such as a systematic grid convergence study and a detailed analysis of the DDES model parameters are performed to assess the accuracy and performance of the DDES model against experiments and LES. The results show that transitional flows demand higher grid resolution than typical turbulent boundary layers, making DDES computationally more expensive than RANS. However, it remains more efficient than wall-resolved LES. The results show that the DDES approach needs to be coupled with a transition model to capture the flow topology over a turbine blade correctly. The benefit of the DDES \gamma-transition is particularly evident in the prediction of the separated shear layer, transition process and the subsequent reattachment. The applied RANS eddy viscosity turbulence and transition models within our study, are not able to accurately predict the aforementioned mechanisms. Particularly for highly loaded turbine blades, the accurate prediction of flow separation and potential reattachment is crucial for aerodynamic design of turbines, since large parts of the total pressure loss are generated in the separated region. Therefore, the DDES \gamma-transition model can be a good compromise in terms of predictive accuracy and computational costs.