Inclusion of Nonlinear Aerodynamics in the FLAP Code

Horizontal axis wind turbines usually operate with significant portions of the blade in deep stall. This contradicts the assumption in the FLAP code that a linear relation exists between the angle of attack and the lift coefficient. The objective of this paper is to determine the importance of nonlinear aerodynamics in the prediction of loads. The FLAP code has been modified to include thenonlinear relationships between the lift and drag coefficients with the angle of attack. The modification affects the calculation of the induced velocities and the aerodynamic loads. This requires an iterative procedure to determine the induced velocities instead of a closed form solution. A more advanced tower interference model has also been added that accounts for both upwind and downwindtower effects. Results from the modified FLAP code were compared to the linear version of the FLAP code and field test data from the Solar Energy Research Institute Combined Experiment wind turbine. Only deterministic effects were analyzed. Blade loads, such as root bending moment, were examined, as were blade properties, including angle of attack and induced velocity. The modified FLAP codeshowed improvement in predicting deterministic blade loads, especially load harmonics, because of both the nonlinear aerodynamics and the improved tower interference model.