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Frank Kelecy, Turbomachinery Application Specialist, Fluent Inc. A recent project funded by the Department of Energy (DOE) and the National Renewable Energy Laboratory (NREL) involved the study of unsteady blade aerodynamics for large, three-bladed wind turbines at the National Wind Technology Center (NWTC) in Colorado. The project was one component of a larger effort, funded by the International Energy Agency (IEA) R&D Wind Executive Committee, where field data was collected and analyzed for wind turbines operated by five organizations in four different countries. Because the incoming wind velocities were not, in general, normal to the plane of the rotors, the data collected from all of the sites is considered far more insightful than that taken from wind tunnel tests. At NWTC, a three-bladed, 10m diameter, 20kW Grumman wind turbine, operating at a constant speed of 72 rpm, was outfitted with 155 surface pressure taps on one of the rotor blades. The taps were used to collect data for incoming wind speed and angle, and for calculations of turbine power production, and aerodynamic and structural modes of the rotor. At Fluent, a simulation has been carried out for one of the NWTC cases, characterized by an inflow wind speed of 7 m/s, using the steady-state, moving reference frame (MRF) model in FLUENT 6. The geometry of the wind turbine was simplified for the calculation, and consisted of the main blade geometry specified for the NREL turbine (an S809 airfoil) along with an idealized cylindrical nacelle and spinner. The simpler nacelle geometry allowed a single blade to be analyzed due to the circumferential periodicity of the flow. An unstructured mesh was used, consisting of 478,664 tetrahedral cells. The computed pressure distribution on the blades was used to determine the shaft power, from which the generator power could be derived using available powertrain efficiency data. The computed generator power and operating efficiency was found to be within 1% of test data from the reported power curve. Additional simulations will be performed in order to validate the present model over a range of wind speeds. These calculations will serve as a benchmark for others who may wish to pursue wind turbine modeling projects with FLUENT 6. |
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