A STUDY OF TRANSITIONAL UNSTEADY BOUNDARY LAYERS USING HYBRID RANS/LES TURBULENCE MODELS

Wind turbines operate in both atmospheric turbulence and wakes from upstream turbines, resulting in blade-flow features described by large transient changes in angle-of-attack, laminar-turbulent transition, and flow separation and reattachment. The use of large-eddy simulations (LES) for full-scale turbines operating in this unsteady environment exceeds the current state-of-art and the widely-used lower-fidelity engineering models are incapable of predicting the relevant flow phenomena. Prediction of boundary-layer transition is an important element in blade aerodynamics and hybrid Reynolds-averaged Navier-Stokes (RANS)/LES models have demonstrated improved accuracy, compared to full RANS models, in predicting blade loads, especially at fully-stalled and yawed conditions. The objectives of this study include: 1) design of a small-scale surrogate wind tunnel model, using a collaborative CFDEFD approach, which captures key flow physics; 2) perform high-fidelity experiments which can provide validationquality data and guide the development of turbulence models; 3) develop and validate a new hybrid RANS/LES model which incorporates the Langtry-Menter 4-equation transition model; and 4) provide analysis of the results to further our understanding of unsteady transitional boundary layers on wind-turbine airfoils.