Implementation of a near-wake model to improve the radial induction distribution of an existing BEM code validated by comparison to CFD results

Abstract

The Blade Element Momentum (BEM) method is a widely used model for rotor performance and aeroelastic calculations but it has assumptions in its formulation which limit its accuracy. The theoretical background of these limitations has been studied and an improvement in the form of the Near Wake Model (NWM) has been identified. The model adds near wake vorticity effects to the BEM along with modeling the interdependence of the different annular elements.
A coupled model based on the NWM has been developed for steady state analysis of turbine rotors. The model performance has been analyzed for the DTU 10MW Reference Wind Turbine (RWT) and compared to Computational
Fluid Dynamics (CFD) simulations. The comparison has been carried out for even and uneven loading distribution. The uneven loading has been modeled by modifying the chord distribution of the blade. Though the aim was to improve
the induction distribution, due to the known difficulties of calculating induction from CFD, the aerodynamic loads were investigated rather than focusing on the induction distribution. For an evenly loaded blade, the loads from the coupled model are improved because of the added flow physics. For uneven loading, the coupled model improves upon the standard BEM, but if modified locally over a large section of the blade to represent the uneven loading, it is seen that the coupled model has a large difference compared to CFD. Finally, for slender blades with uneven loading, the model performs reasonably well showing a significant improvement to the BEM in the in-plane forces and an overall improvement in the out-of-plane forces as well when compared to CFD.