Modern low-pressure turbines operate at relatively low Reynolds number values. Hence, their efficiency depends greatly on transition. Unfortunately, the flow inside a real component is very complex due to unsteady and three-dimensional phenomenon. For a better understanding of the different effects, simplified flows must be investigated. In particular, flows through cascades allow to focus on the design’s performance of a blade. For a CFD code, it is then very important to be able to simulate accurately such flows.
The flow through the T106-A turbine cascade is a good example of such a study. The cascade has been designed by MTU-Aero Engines and tested at DLR. Of particular interest is the evolution of the flow when the Reynolds number changes.
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Fig.1 : Pressure coefficient distribution over the T106-A cascade.
In order to demonstrate the capability of TRACE to simulate flows over low-pressure turbine profiles, three types of computation have been carried out. The two-equation k-ω turbulence model of Wilcox has been used to simulate fully-turbulent flows. Those results are compared to the DLR’s in-house Multimode transition model with the γ-Reθ Model of Menter and the experiments in Fig 1.
At low Reynolds number (Fig 1.a), both transition models are able to simulate the separation-induced transition on the suction side. On the contrary, the turbulence model alone cannot reproduce the bubble. In Fig. 1(b), the Reynolds number is increased to reach an intermediate value. It can be noticed that a separation bubble is still present on the suction side and its size decreases in comparison with the flow at lower Reynolds number. The transition models produce results in accordance with the measurements, while the turbulence model is unable to do so. The situation at the highest Reynolds number is shown in Fig. 1(c). No separation bubble is reported in the experiment because the flow is fully turbulent. The transition models predict comparable results to the turbulence model, meaning that the transition models can be used with confidence even when the flow is expected to be turbulent.