IEA Wind Annex XX: HAWT Aerodynamics and Models from Wind Tunnel Measurements; Final Report, December 2008

The IEA Wind agreement, also known as the Implementing Agreement for Cooperation in the Research, Development, and Deployment of Wind Energy Systems, functions within a framework created by the International Energy Agency (IEA). Views, findings, and publications of IEA Wind do not necessarily represent the views or policies of the IEA Secretariat or of all its individual member countries. Experimental uncertainties constrain our ability to understand horizontal axis wind turbine (HAWT) aerodynamics, or to rigorously formalize understanding in accurate, reliable predictive models. Prior to undertaking Annex XX, turbine aerodynamics measurement uncertainties had been driven down substantially over the previous decade. Concurrent advanced the state of the art for performing research-grade measurements in the field environment. These successes in acquiring highly resolved, quantitative aerodynamic and structural measurements were fostered and documented under the auspices of IEA Wind Annex XIV and Annex XVIII. However, improved field measurement systems and enhanced data quality further accentuated a harsh dilemma that had long confronted turbine aerodynamicists. Full-scale geometries could be densely instrumented and subsequently tested, but only in the uncontrollable and sparsely characterized inflows of atmospheric winds. Here, overriding uncertainties were introduced into the aerodynamics data by inflow fluctuations and anomalies. Alternatively, the wind tunnel environment offered controlled and uniform inflows, but at the expense of severe dynamic similarity parameter mismatches. In the wind tunnel, aggregate data uncertainties were acceptably low, but differences between experiment and field similarity parameter levels led to uncertainties regarding the validity of extrapolating otherwise high quality data to potentially dissimilar physical regimes. Thus, in years leading up to Annex XX, uncontrolled and nonuniform inflows, as well as pronounced similarity parameter level disparities had thus been deemed significant impediments to deeper comprehension of turbine aerodynamics and to further advances in turbine aerodynamics modeling capability. In the United States, this quandary led to plans for testing the NREL Unsteady Aerodynamics Experiment (UAE) HAWT in the NASA Ames 80 by 120 ft (24.4 m × 36.6 m) wind tunnel. This test was designed to produce accurate and reliable experimental measurements with high spatial and temporal resolution, for a realistic rotating blade geometry, under closely matched conditions of dynamic similarity, and in the presence of strictly controlled inflow conditions. To maximize test benefits, NREL convened an international scientific advisory panel consisting of wind turbine aerodynamicists and modelers from around the world. This panel provided guidance for developing a prioritized test plan to ensure that …