Researchers and designers use design codes such as FAST_AD to efficiently, safely, and cost-effectively design and analyze wind energy systems. After developing aerodynamic and structural models of the UAE research wind turbine in FAST_AD, simulations are run by exciting the model with the same conditions seen in the NASA-Ames wind tunnel for several UAE test cases. To facilitate validation of FAST_AD, the load predictions are compared with those made by other similar modeling tools and with experimental measurements taken on the actual UAE wind turbine during the NASA-Ames wind tunnel tests. The experimental test data acquired from the NASA-Ames wind tunnel tests represent the finest, most accurate set of wind turbine aerodynamic and induced flow field data available. There are load magnitude
discrepancies—inconsistencies in predicted and measured aerodynamic force coefficients, rotor shaft torque, and out-of-plane bending moments at the blade root—between the modeling tool predictions and experimental measurements over a range of operating conditions. .
Inconsistencies in input file parameters—variations in aerodynamic force coefficient-angle of attack characteristics, blade node positions for data recording and analysis, and pitch angle convention—explain a noteworthy fraction of the load prediction discrepancies of the various modeling tools. Furthermore, other input file parameter inconsistencies such as variations in the time step, the number of blade elements, and the distributed blade structural properties
contributed little to the load prediction discrepancies. Other reasons for load prediction discrepancies include modeler error and code bugs.
Discrepancies between modeling tool load predictions and physically measured load values highlight weaknesses in tip loss, stall delay, post stall, and other frequently used aerodynamic models. A comparison of force coefficient-angle of attack relationships measured in the three-dimensional environment of the NASA-Ames wind tunnel at each pressure tap grouping span station reveals that three-dimensional flow effects prohibit the use of unmodified
two-dimensional airfoil data at all span stations, not only the inboard stations as originally thought.
These effects include spanwise pressure gradients, spanwise flow, flow circulation, and vorticity.
Moreover, the inability to pinpoint angle of attack values, even when using accurate airfoil data that take into account three-dimensional flow behavior, is a fundamental fault in the classic BEM aerodynamic models.
Demonstrating mispredictions in low-speed shaft torque and root flap and edge bending moments even when mispredictions in aerodynamic forces are eliminated at all span stations uncovers flaws and limitations in the structural models employed by FAST_AD and suggests means of model and code improvement. These flaws and limitations include an improper definition of the blade structural pretwist angle and the inability to model (1) radial aerodynamic forces acting on the blades, (2) blade pitching moments, and (3) properly understanding the relationship between the low-speed shaft torque and root bending moments.
These flaws must be pinpointed and corrected for accurate hub loading, rotor torque, and overall performance predictions. This work should be completed before FAST_AD validation
concludes. Nevertheless, the results demonstrate that until the aerodynamics models are refined or redeveloped, eradicating these flaws is less critical. Our limited understanding of the
three-dimensional flow environment and the flow physics involved overwhelms most of these errors and model limitations. Aerodynamic models need to more accurately incorporate
three-dimensional flow effects and predict angle of attack distributions. Refining or redeveloping efficient rotor wing aerodynamics models is essential if design tools, such as FAST_AD, are to accurately predict component loads and significantly reduce wind-generated electricity costs.
When component loads are accurately predicted, safety factors may be relaxed and turbine designs can be optimized without the fear of detrimental component failure. The consistency of the angle of attack mispredictions at every wind speed makes the feasibility of this task
promising.
References
Buhl, M.L. WT_Perf User’s Guide, National Renewable Energy Laboratory, 2000.
Du, Z. and Selig, M.S. “A 3-D Stall-Delay Model for Horizontal Axis Wind Turbine
Performance Prediction,” Proceedings of the 1998 ASME Wind Energy Symposium, Reno, NV:
pp. 9–19, 1998.
Eggleston, D.M. and Stoddard, F.S. Wind Turbine Engineering Design, Van Nostrand Rheinhold, New York, 1987.
Hand, M.M. NASA-Ames Wind Tunnel Test Home Page. Golden, CO: National Renewable Energy Laboratory (accessed November 2000–September 2001),
<http://wind2.nrel.gov/amestest/>.
Hand, M.M.; Simms, D.A.; Fingersh, L.J.; Jager, D.W.; Cotrell, J.R.; Schreck, S.J.; Larwood, S.M. Unsteady Aerodynamics Experiment Phase VI: Wind Tunnel Test Configurations and Available Data Campaigns, NREL/TP-500-29955; Golden, CO: National Renewable Energy Laboratory, 2001.
Hansen, A.C. and Laino, D.J. User’s Guide to the Wind Turbine Dynamics Computer Programs YawDyn and AeroDyn for ADAMS®, University of Utah, 1998.
Kane, T.R. and Levinson, D.A. Dynamics: Theory and Applications, McGraw-Hill Inc., New York, 1985.
Leishman, J.G. and Beddoes, T.S. “A Semi-Empirical Model for Dynamic Stall,” Journal of the American Helicopter Society, Volume 34(3): pp. 3–17, 1989.
McGowan, J. and Manwell, J. Wind Energy Engineering Fundamentals Course: Course Notes – Part 1, University of Massachusetts, Amherst, MA: Renewable Energy Research Laboratory, 2000.
Simms, D.A., Hand, M.M., Fingersh, L.J., and Jager, D.W. Unsteady Aerodynamics Experiment Phases II-IV Test Configurations and Available Data Campaigns, NREL/TP-500-25950;
Golden, CO: National Renewable Energy Laboratory, 1999.
Simms, D.A., Schreck, S.J., Hand, M.M., and Fingersh, L.J. NREL Unsteady Aerodynamics Experiment in the NASA-Ames Wind Tunnel: A Comparison of Predictions to Measurements, NREL/TP-500-29494; Golden, CO: National Renewable Energy Laboratory, 2001.
Suzuki, A. and Hansen, A.C. “Dynamic Inflow Model for YawDyn,” Proceedings of the 1998 AWEA Windpower Conference, Bakersfield, CA: pp. 233–240, 1998.
Thomson, W.T. and Dahleh, M.D. Theory of Vibrations with Applications, 5th Edition, Prentice-Hall, Inc., New Jersey, 1998.
Viterna, L.A. and Corrigan, R.D. “Fixed Pitch Rotor Performance of Large Horizontal Axis Wind Turbines,” Proceedings, Workshop on Large Horizontal Axis Wind Turbines, NASA CP-2230, DOE Publication CONF-810752, NASA Lewis Research Center, Cleveland, OH: pp. 69–
85, 1981.
Wilson, R.E., Lissaman, P.B.S., and Walker, S.N. Aerodynamic Performance of Wind Turbines, Oregon State University, 1976.
Wilson, R.E., Walker, S.N., and Heh, P. Technical and User’s Manual for the FAST_AD Advanced Dynamics Code, Oregon State University, 1999.