Modeling laminar-to-turbulent transition in a low-pressure turbine flow which is unsteady due to passing wakes: Part I, transition onset

The open literature on modeling transition to turbulence of boundary layer flows in low-pressure turbines is reviewed. Included are the separated flow transition onset models of Mayle and of Davis et al. and the attached flow transition onset models of Mayle, Abu-Ghannam and Shaw and Drela. Their results are applied against data previously taken at the University of Minnesota (UMN) to assess their performance for use on the suction surface of a turbine blade in the presence of passing wakes. The data show measurements of velocity, turbulence level, intermittency and spatial and temporal acceleration resolved in space and phase angle within the wake-passing period. In a "quasi-static" comparison, the input values to the models are values taken from the experiment, resolved in phase angle within the wake-passing period. Predictions from the models (the flow states with regard to transition throughout the wake-passing period) are compared with instantaneous intermittency values taken from the experiment. The Mayle separated and attached flow onset models are shown to be successful for the case investigated when applied in that fashion. The Abu-Ghannam and Shaw and Drela transition onset models predict onset locations which are somewhat downstream of where the data indicate the transition onset to be. Unique characteristics regarding transition observed at different times in a wake passing cycle are discussed. Some reasons are given to explain the differences between experimental results and model predictions. Transition onset modeling is addressed in the present paper and transition path modeling is addressed in a companion paper (part II).