Development of a coupled CFD-CSD method for floating offshore wind turbines

Abstract

In the present study, a numerical framework for predicting the aerodynamic performance and the aeroelastic behavior of floating offshore wind turbine rotor blades involving platform motion was developed. For this purpose, the aerodynamic and structural analyses were conducted simultaneously in a tightly coupled manner for handling transient flows due to platform motion by exchanging the information about the aerodynamic loads and the elastic blade deformations at every time step. The motion of the floating offshore turbine was prescribed by the six degrees-of-freedom platform motions. The elastic behavior of the turbine rotor blades was described by adopting a structural model based on 2nd order Euler-Bernoulli beam. The aerodynamic loads by the rotor blades were evaluated by adopting a Blade Element Momentum Theory (BEMT) and Computational Fluid Dynamics (CFD). For a validation, several analyses were performed for the NREL 5MW reference wind turbine using the CSD solver alone. At first, an aeroelastic analysis was conducted for the wind turbine with fixed foundation, and the aerodynamic loads and the elastic blade deformation were calculated under various operating conditions. The results were compared with the numerical solutions by FAST. Next, the aerodynamic load variation was calculated when the platform moves in a prescribed sinusoidal surge motion for rigid blades. It was observed that the aerodynamic loads produced by the rotor blades significantly change as the platform of the wind turbine is in a movement. Finally, the effects of the platform motion on the aerodynamic performance and the aeroelastic behavior of the floating offshore wind turbine rotor blades were investigated. The numerical simulations were conducted when the platform of the wind turbine independently moves in each of the six degrees-of-freedom directions consisting of heave, sway, surge, roll, pitch, and yaw at a below-rated flow condition. It was observed that flexible blades exhibit complicated vibratory behaviors when they are excited by the aerodynamic, inertia and gravitational forces simultaneously. It was found that the load variation caused by the platform surge or pitch motion has a significant influence on the flapwise and torsional deformations of the rotor blades. The torsional deformation mainly occurs in the nose-down direction, and results in a reduction of the aerodynamic loads. It was also found that the flapwise root bending moment is mainly influenced by the platform surge and pitch motions. On the other hand, the edgewise bending moment is mostly dictated by the gravitational force, but is not affected much by the platform motion. Developed CSD solver in the present study was coupled with CFD solver, which is based on the three-dimensional unstructured mesh technique, for more accurate aerodynamic analysis. The tight coupling between two solvers was achieved by adopting staggered coupling method, which can reduce the computational time and can maintain the accuracy of the numerical solution. Finally, the numerical analysis for the NREL 5MW wind turbine was performed using the coupled CFD-CSD method. In the present study, the platform surge and pitch motions that have the greatest influence on the changes of the aerodynamic loads and the elastic deformations are only considered. In the normal operating condition, similar aerodynamic loads changes and the vibratory behavior of the blade are presented compared to those of the BEM-CSD prediction. Additionally, detailed pressure distribution on the blade surface are shown by using the coupled CFD-CSD method. It was also found that the rotor wake structure are occurred unsymmetrically due to the platform surge and pitch motions.