A generalised implementation of e^N transition method for airfoils and wind turbine blades

Abstract

Prediction of the boundary layer transition from laminar to turbulent is essential for accurately calculating the aerodynamic forces on airfoils. Yet, it is still a challenging task for most RANS turbulence models (Reynolds-averaged Navier–Stokes) which are build on a fundamental assumption that the flow is fully turbulent. Although there are some derivatives of RANS models that promise to predict transition, the classical eN method still holds its place as strong alternative, especially at high Reynolds numbers. The eN method is usually applied in conjunction with boundary-layer codes for flow over 2D airfoils. In wind energy applications of flow over thick airfoils, the high fidelity CFD models play a central role in the analysis and design workflow. This work is an attempt to couple eN method with an unstructured CFD code, namely OpenFOAM. Most of the previous attempts to achieve this coupling was limited to structured CFD codes and often with boundary-layer codes as an intermediate layer in-between. The newly implemented framework is applied on 2D airfoil cases with Reynolds numbers between 3 and 12 million. The method is extended to also allow for simulations using the Multiple Reference Frame (MRF) approach; this enables the user to calculate laminar-turbulent transition for wind turbine blades in axial flow. The 2D results showed very good agreement with XFOIL and wind tunnel data for forces and transition line prediction. The transition prediction of a 3D single blade of the IEA15MW turbine showed a good agreement with sectionwise transition values. The implemented concept proves the abilities of unstructured CFD based models coupled with the eN method to predict transition accurately without compromising the robustness of the numerical setup.