The blades of large-scale wind turbines can obviously deform during operation, and such a deformation can affect the wind turbine’s output power to a certain extent. In order to shed some light on this phenomenon, for which limited information is available in the literature, a bidirectional fluid-structure interaction (FSI) numerical model is employed in this work. In particular, a 5 MW large-scale wind turbine designed by the National Renewable Energy Laboratory (NREL) of the United States is considered as a testbed. The research results show that blades’ deformation can increase the wind turbine’s output power by 135 kW at rated working conditions. Compared with the outcomes of the simulations conducted using the model with no blade deformation, the results obtained with the FSI model are closer to the experimental data. It is concluded that the bidirectional FSI model can replicate the working conditions of wind turbines with great fidelity, thereby providing an effective method for wind turbine design and optimization.

As a pollution-free renewable energy technology, wind power generation has developed rapidly in recent years. Hence, the wind turbines have gradually developed into large-scale structures themselves, and the diameter of wind turbine blades and output power gradually increased too. For example, the large wind turbine designed by the National Renewable Energy Laboratory (NREL) of the United States can reach a diameter of 126 m, and output power can reach 5 MW [

The numerical simulation method based on Computational Fluid Dynamics (CFD) was widely used to research the aerodynamics characteristic of wind turbines. Frulla et al. [

When the wind turbine is running, the blade can deform under the action of rotational centrifugal force, aerodynamic force, and other loads. Further, the deformations of the blade also can affect the aerodynamic performance and the output power of the wind turbine. So, it is necessary to employ the bidirectional fluid-structure interaction (FSI) model to analyze the operational characteristics of the wind turbine. Pawar et al. [

In this study, the bidirectional FSI model is used to analyze the operational characteristics of the NREL 5 MW large-scale wind turbine. The flow field around the wind turbine and the deformations of the wind turbine blades under different wind speeds are obtained by the FSI model, and the influence of the blade deformation on the output power of the wind turbine is also analyzed. The research results of this paper can provide a reference and basis for the design and safe operation of large-scale wind turbines.

The bidirectional FSI model is used for the numerical simulation. The turbulent model is used to obtain the velocity and pressure distributions of the flow field, and the finite element model (FEM) model is adopted to calculate the stress and deformations of the structure domain, the interaction between structural deformation and the flow field is analyzed simultaneously.

The SST

Continuity equation:

Momentum equation:

In the above equations, _{i}_{t}_{k}_{ω}_{k}_{k}_{ω}_{ω}_{ω}_{ij}

_{ij}_{ij}_{ω}_{ω}_{ω}

_{1} is the mixing function. The SST _{1 }= 1 and becomes the _{1 }= 0. _{1}, _{2}, _{1}, _{2} and _{ω}_{,2} are coefficients used to calculated _{ω}_{ω}_{ω}

The finite element analysis model is adopted to simulate the stress and deformation of the structure. After the mesh is generated, the displacement vector of the node is solved by the linear equations given as:

The blade of the wind turbine is made from carbon fiber, and the main physical parameters of the carbon fiber are listed in

Density (kg/m^{3}) |
Young’s modulus (MPa) | Poisson’s ratio | Tensile strength (Mpa) |
---|---|---|---|

1500 | 1.25 × 10^{5} |
0.22 | 7.9 × 10^{2} |

Using the weak coupling method to finish the numerical simulation of the FSI model,

A NREL 5 MW wind turbine is selected as the research object. The rated power is 5 MW, the rated rotating speed is 12.1 rpm, and the rated wind speed is 11.4 m/s.

Diameter of wind turbine (m) | Height of tower (m) | Diameter of hub (m) | Hub height from the ground (m) | Diameter of tower bottom (m) | Diameter of tower top (m) | Elevation angle of wind turbine |
---|---|---|---|---|---|---|

126 | 87.6 | 3 | 90 | 6 | 3.87 | 5° |

The three-dimensional structure of the blade is shown in

The sliding mesh technology is used to deal with the rotation of the blades in the fluid domain. The fluid domain is divided into two parts: the rotating and the fixed domain. The fixed domain is a half-cylinder with a radius of 300 m and a length of 500 m; the rotating domain is a flat cylinder with a radius of 70 m and a length of 8 m. The length of the fluid domain in front of the wind turbine is 150 m. The rotating domain rotates with the blades, and the flow data are transmitted by interpolation at the interface between the fixed domain and the rotating domain. To improve mesh quality, tetrahedral meshes are used, and the meshes are refined near the blades.

The mesh independence needs to be verified. The flow field around the wind turbine is simulated by the SST ^{+} ≤ 62 when the wind speed is 11.4 m/s, which meets the requirements of the wall function method.

Wind speed (m/s) | Number of meshes (Million) | The calculated output power (MW) |
---|---|---|

11.4 | 10.5 | 4.442 |

13.5 | 4.653 | |

16.7 | 4.805 | |

19.0 | 4.816 |

The blades are selected as the structure domain of the FSI model. Static analysis is used to verify the mesh independence of the structural region; the gravity load and centrifugal force load with 12.1 rpm rotating are applied to the blades of a wind turbine.

Rotating speed (rpm) | Number of meshes (Million) | The calculated maximum stress of blades (MPa) |
---|---|---|

12.1 | 0.75 | 46.93 |

0.97 | 46.16 | |

1.2 | 45.67 |

From

In order to determine the influence of blades deformation on the performance of wind turbine, the CFD model, without considering the structural deformation and FSI model are used to simulate the operation of wind turbine separately. To maintain consistency, the two models use the same turbulence model and initial meshes in the fluid domain. In the fluid domain, the left side is set to the velocity-inlet boundary condition, the right side is set to the pressure-outlet boundary condition, the bottom surface is set to the wall boundary, and the semi-cylindrical surface is set to the symmetrical boundary. The rotational speed of the rotational domain is 12.1 rpm, and the blade surfaces are set as the fluid-structure interaction interface for the FSI model; the Wall function method is adopted near the wall.

ANSYS software is used to complete the numerical simulation. The finite volume method is used to discretize the momentum equation,

The unsteady method is used for numerical simulation and second-order implicit time stepping scheme is used to discretize the temporal term. The implicit time step scheme has excellent time stability, and there is no strict requirement for the setting of time step size [

The deformation of the wind turbine blade is simulated for four wind speeds of 3, 5, 8, and 11.4 m/s by the FSI model. The simulation results show that the maximum deformation is located at the blade tip. Due to the influence of the tower shadow effect, the deformation of the blade fluctuates periodically.

Zhang [

According to simulation results, the blades have a large deformation when the wind turbine is running; the large blade deformation can affect the aerodynamic characteristics and power output of the wind turbine.

Wind speed (m/s) | Power (FSI model) (MW) | Power (CFD model only) (MW) | Measured power [ |
---|---|---|---|

5 | 0.426 | 0.423 | 0.435 |

8 | 1.639 | 1.623 | 1.761 |

10 | 3.027 | 2.985 | 3.209 |

11.4 | 4.951 | 4.816 | 5.0 |

According to the pressure distribution of the blade surface, the force acted on the wind turbine can be obtained.

Wind speed (m/s) | Thrust coefficient (FSI model) | Thrust coefficient (CDF model only) | Thrust coefficient [ |
---|---|---|---|

5 | 1.358 | 1.372 | 1.388 |

8 | 0.847 | 0.853 | 0.862 |

10 | 0.817 | 0.826 | 0.834 |

11.4 | 0.729 | 0.741 | 0.75 |

The bidirectional FSI model is used to study a large NREL 5 MW wind turbine, the blade deformation, output power, speed, and pressure distribution around the wind have been obtained from the numerical simulation. Henceforth, Therefore, the following conclusions are drawn in this paper:

The deformation of the blade can be calculated accurately by employing the bidirectional fluid-structure coupling model. The deformation of the blade is nonlinear, and the maximum deformation occurs at the blade tip. Underrated conditions, the blade has a large deformation, the maximum deformation is 4.67 m, reaching 7.78% of the blade length.

The large deformation of the blade greatly impacts the wind turbine’s aerodynamic characteristics and power output. Under the rated working condition, the deformation of the blade increases the output power by 2.7%. Following the deformation of the blade, the pressure difference between the pressure surface and the suction surface increases, resulting in the increase of torque in the circumferential direction and power output.

The output power of the wind turbine obtained by the FSI model is closer to the experimental value, which establishes that the FSI model can replicate the working characteristics of the wind turbine with more fidelity. FSI analysis is an important method for the large wind turbine optimization design, which is relevant in important engineering applications.