Numerical Simulation of Airflow around Horizontal Axis Wind Turbine

This paper predicts the aerodynamic and aeroacoustic characteristics of a horizontal axis wind turbine via a two-way fluid-structure interaction approach. In this study, STAR-CCM+ is used to model the flow around a rotating wind turbine, whereas ABAQUS is employed to estimate the blade deformation during the rotation motion of rotor. A weak-coupling approach switching between rotating frame calculations and sliding mesh computations is adopted in the studied problem. The Ffowcs Williams-Hawkings analogy is further employed to forecast the noise spectrum of the investigated wind turbine. The full-scale flow simulation under the rated condition is conducted to study the influence of blade deflection on the aerodynamics and aeroacoustics of a horizontal axis wind turbine. The flow computation of wind turbine with deformable blades is believed to be more close to the industrial practice and can deliver more accurate and realistic information of aerodynamics, as well as aeroacoustics, in operation. Keyword:Horizontal Axis Wind Turbine, Aerodynamics, Aeroacoustics, Numerical Simulation. Introduction Horizontal axis wind turbine is a very promising and attractive renewable energy device due to its balance among capacity, efficiency and cost, when compared with other currently available renewable energy devices. For ensuring high efficiency and sufficient structure safety in operation, the aerodynamic characteristics of wind turbine, is, therefore, of great industrial interests. However, the aforementioned issues are typical examples of flow-structure interaction (FSI), especially focused on the impact caused by the blade deformation, arising in the wind turbine operation. This paper is to simulate the airflow around a single wind turbine and to predict the aerodynamic and aeroacoustic performance of a horizontal axis wind turbine, where the numerical predictions of inflexible and flexible blades are compared and discussed. Method The governing equations to describe the airflow around a rotating wind turbine is described by the timeaveraged continuity and Navier-Stokes equations. Because the airflow around the investigated wind turbine has a relatively high Reynolds number, the transport equation of the turbulent kinetic energy, as well as the specific dissipation rate, is adopted to account for the corresponding turbulence effects, where a shear-stresstransport (SST) k- model [1] is employed in this study. The governing equations of flow field are discretized by a finite volume method, where the velocity and pressure are decoupled via a SIMPLE-type algorithm [2]. The aforementioned approach is implemented in the software STAR-CCM+, which is adopted in this study to predict the turbulent flow field around a wind turbine. The rotor blade is considered as a generalized cantilever beam with structure property variation along its axial direction. The structural analysis of wind turbine blades is numerically conducted via a finite element method, where the software ABAQUS is employed in this study to deliver the deflection prediction of wind turbine blades. The blade orientation is described by the azimuthal angle () measured from the blade position at the 12 o’clock location, Fig.1. The blade deflection at several key azimuthal angles is first determined from the co-simulation of STAR-CCM+ and ABAQUS, where a fully two-way FSI calculation is achieved with a rotating frame approach. The sliding mesh approach is used in the subsequent airflow computation, where the mesh block discretizing the computation domain enclosing the rotor of wind turbine is simultaneously updated at prescribed key azimuthal angles using the deformed blade geometry obtained from the previous co-simulation results. This two-step calculation is regarded as a weak coupling approach of FSI, which can substantially reduce the computational cost required by a fully two-way FSI coupling in a sliding mesh approach. In the noise calculation, a SST k- turbulence model considering detached eddy effect is adopted to improve the prediction accuracy of noise spectrum. Results and Discussion The airflow around a full-scale NREL 5MW wind turbine [3] under the rated condition is studied in this paper. The wind turbine is modeled as an inland wind turbine with zero yaw angle, where a logarithm-law velocity profile [4] is adopted for the incoming wind. The material property of blade is referenced from a recent study [5], where the structural characteristics of a wind turbine blade with similar size are disclosed.