Modeling of 3D Rotational Effects in a 2D Pressure-Based CFD Code for Airfoil Design

In applications requiring very thick airfoils, relative thickness above 35%, flow separation becomes a recurrent issue; and for airfoil design, RANS simulations become a desirable tool due to their balance in accuracy in separated flows and reasonably low computational cost. However, if the application in question involves rotating wings, such as in wind turbines, three-dimensional effects related to Coriolis and centrifugal forces introduce a strong radial flow near the blade's root, and 2D RANS simulations are not accurate enough. To account for the radial flow, 3D RANS could be used, but this would dramatically increase the computational cost. Instead, this work shows how the most relevant three-dimensional effects can be integrated in a 2D RANS simulation, creating what can be called a 2.5D simulation. The modifications to the 2D momentum equations include convection in the radial direction, dissipation terms related to the curvature, and source terms related to Coriolis and centrifugal forces. The new system of equations introduces the radial velocity and the radial momentum equation. The model is evaluated on a 10 MW reference wind turbine rotor, at realistic flow conditions, and on single-airfoil rotors by comparing 3D results (from simulating the entire rotor) to the corresponding 2D and 2.5D simulations of radial sections at the root. The results show that the 2.5D model significantly reduces the gap between 2D and 3D flows, and in some cases accurately predicts crucial characteristics, such as the separation points of the boundary layer.