Progress in Validation of Rotor Aerodynamic Codes Using Field Data

Within the framework of the fourth phase of International Energy Agency IEA Wind Task 29, a large comparison exercise between measurements and aero-elastic simulations has been carried out featuring three simulation cases in axial, sheared and yawed inflow conditions. Results were obtained from more than 19 simulation tools originating from 12 institutes ranging in fidelity from Blade Element Momentum (BEM) to Computational Fluid Dynamics CFD and compared to state of the art field measurements from the 2MW DanAero turbine. More than 15 different variable types ranging from lifting line variables to blade surface pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. The result is a unique insight into the current status and accuracy of rotor aerodynamic modeling. For axial flow conditions a good agreement was found between the various code types, where a dedicated grid sensitivity study was necessary for the CFD simulations. However, compared to wind tunnel experiments on rotors featuring controlled conditions, it remains a challenge to achieve good agreement of absolute levels between simulations and measurements in the field. For sheared inflow conditions, uncertainties due to rotational and unsteady effects on airfoil data result in the CFD predictions standing out above the codes that need input of sectional airfoil data. However, it was demonstrated that using CFD synthesized airfoil data is an effective means to bypass this shortcoming. For yawed flow conditions, it was observed that modeling of the skewed wake effect is still problematic for BEM codes where CFD and Free Vortex Wake codes inherently model the underlying physics correctly. The next step is a comparison in turbulent inflow conditions which is featured in IEA Wind Task 47. Doing this analysis in cooperation under the auspices of IEA Wind has led to many mutual benefits for the participants. The large size of the consortium brought ample manpower for the analysis where the learning process by combining several complementary experiences and modeling techniques gave valuable insights that could not be found when the analysis is carried out individually.