Prediction and Analysis of Turbulence Anisotropy in a Low-Pressure Turbine Cascade at Two Reynolds Numbers Based on Transitional DDES

This study analyzes the influence of Reynolds number on the turbulence anisotropy behavior for the transitional flow over the MTU-T161 linear low-pressure turbine (LPT) cascade. Two operating points at Reynolds number Re = 90,000 and Re = 200,000, both at an isentropic exit Mach number of 0.6, are calculated using a transitional Delayed Detached Eddy Simulation (DDES) model. We focus our investigation on the separation-induced transition occurring on the suction side, its sensitivity to the Reynolds number and the capabilities of the transitional DDES approach for capturing the turbulent state. The computational model of the MTU-T161 cascade consists of a single blade passage, including the diverging viscous sidewalls. Synthetic turbulence is generated at the inlet of the domain to mimic realistic turbomachinery flow conditions. We show that the transitional DDES model is able to capture the separation and transition mechanism correctly for both Reynolds numbers, when compared to experimental data. The main part of this paper consists of a detailed analysis of the turbulence anisotropy behavior with particular attention to the separation bubble when changing the Reynolds number. By increasing the Reynolds number from 90,000 to 200,000, the turbulence anisotropy state of the suction side flow changes only slightly. At the higher Reynolds number, the passage flow is mostly governed by two-component turbulence state, while the separation bubble and its influence on the passage flow become weaker. The turbulence anisotropy analysis reveals an almost two-component state very close to the wall region and a one-component turbulence state in the separated shear layer for both Reynolds numbers. Our results show the capability of the transitional DDES model to capture the correct trend of the Reynolds stresses and the anisotropy behavior by comparing the results to a previously published Large eddy simulation (LES). The DDES method is not successful in capturing the Klebanoff modes (streamwise fluctuations) in the pre-transitional region, when compared to LES. These results enhance our overall comprehension of the turbulence state within different separation bubble sizes. Furthermore, the results indicate that the transitional DDES model resolves the essential characteristics of turbulence while keeping the computational cost up to seven times lower than LES.