Effect of the gas to wall temperature ratio on the bypass transition

The study of the boundary layer transition plays a fundamental role in the field of turbomachinery. The main reason is the strong influence of the transition on the flow field local parameters, such as skin friction and heat transfer, this variation is reflected on the global ones such as efficiency and heat load of the blade row. Turbulent transition models are nowadays commonly used tools in both CFD research and design practice. It is then of particular interest to understand if they are able to predict the effect of temperature on bypass transition and, in case of positive answer, the reasons of their behaviour. This becomes even more interesting as the effect of the flow aero-thermal coupling becomes prominent in the analysis of such phenomena and is typically not accounted for in the validation of turbulence models. In this work we focus our attention on two state of the art transition model that use two radically different approaches to describe transition. To isolate the effects of the temperature ratio on the transition the simulations have been performed keeping the same values of Reynolds and Mach numbers and changing the value of the wall to freestream Temperature Ratio (TR). The results of the two transition models have been compared between them as well as with experimental results. They show that both models are sensitive to TR, though a locally based (rather than correlation based) approach for transition modelling should be favoured.