Aerodynamic performance and wind-induced effect of large-scale wind turbine system under yaw and wind-rain combination action
Introduction
Large-scale wind turbine is a high-rise structure with ultra-large flexible blades and ultrahigh fine tower. Particularly, in rainstorm conditions, raindrops cooperate with the incoming wind field in the falling process to influence the microturbulence of wind field, and impact on the wind turbine surface at a high speed due to the driving by gravity and horizontal wind force, thus changing the aerodynamic performance of the wind turbine system [1]. Furthermore, the wind turbine is generally in the yaw state as a result of the failure of the impeller shaft aligning at the incoming wind direction timely for the continuous changes of wind direction and low revolving speed of the yaw system [2,3]. Under this circumstance, aerodynamic force of the wind turbine system is significantly asymmetric.
Refs. [4,5] reported that changes of yaw angle increase dynamic wind effect and aeroelastic response of the wind turbine system significantly, and decline the wind-induced stability of the structural system. Refs. [6,7] discovered that stress distribution of the tower under yaw conditions change and intensify the dynamic buckling of the wind turbine system significantly. Refs. [8,9] studied the aerodynamic performance on the horizontal axis wind turbine based on vortex eddy theory and CFD numerical simulation, and concluded that yaw can affect airflow velocity, dynamic pressure and static pressure on wind turbine surface. Refs. [10,11] analyzed flow field and wake of wind turbine under yaw state by the large eddy simulation technology and PIV wind tunnel experiment. They concluded that yaw angle can change the wake flow symmetry of wind turbine, affect stability of the wind turbine, and reduces generated power. Refs. [12,13] compared the flow field and aerodynamic force in the wind turbine system under different rotation states of blades. They found that complete sheltering of tower by blade is the condition most unfavorable to aerodynamic distribution of the system. To sum up, existing studies mainly focus on effects of yaw under wind load, but none has discussed extreme conditions under wind-rain load. There lack of qualitative and quantitative analyses of impacts of raindrops on wind turbine system. Hence, studying impacts of yaw angle on aerodynamic force and comprehensive stress performance of large-scale wind turbine system under wind-rain load has important theoretical significance and engineering values.
With respect to wind-rain load [[14], [15], [16], [17], [18], [19], [20], [21]], reported many studies on wind-driving rains, but they mainly focused on low houses, stay cables of bridges and power transmission tower. Only few studies [4,22,23] simulated flow field characteristics and wind-rain load distribution on the wind turbine system by using the CFD numerical simulation. These studies only consider effects of one-way wind-driving rain, but neglected the effect of rain on wind and didn't analyze wind effect and stability performance of the large wind turbine system under wind-rain coupling effect.
In this study, the surrounding wind field of the 5 MW wind turbine tower-blade system which was developed by NUAA under different yaw angles and wind-rain load was simulated firstly by CFD two-way coupling technology. Secondly, the influencing law of yaw angle on wind-driven rainfall, additional acting force of raindrops and rain-induced pressure coefficient was discussed. The velocity flow line, turbulence energy strength, raindrop running speed and trajectory action mechanism on the structural surface in the wind-rain coupling field was disclosed. Moreover, a new model of wind-rain equivalent pressure coefficient under different yaw angles was constructed and the corresponding distribution laws of wind-rain equivalent pressure coefficient were analyzed. Finally, large-scale wind turbine tower-blade coupling models under different yaw angles were constructed by combining the finite element method. Structural responses, buckling stability and ultimate bearing capacity of the large-scale wind turbine system under different yaw angles and two conditions (wind condition and wind-rain condition) were discussed.
Section snippets
Engineering project
Main structural design parameters and the model of 5 MW horizontal axis wind turbine with three blades which were developed by NAUU are shown in Table 1. The tower is a long thickness-variable structure. The top wall thickness is 40 mm and the bottom wall thickness is 90 mm. The cabin size is 18 m (Length) × 6 m (Width) × 6 m (Height). The dip angle of blade is 5° and the cut-out wind speed is 25 m/s. The included angle between any two blades is 120°. All blades distribute evenly along the
Rainfall intensity
Classification of rainfall intensity is shown in Table 3 (GB/T 28592-2012, 2012, [24]. Different classification of rainfall intensity may cause great difference of measurement to the same rainfall even. The structure checking based on hourly rainfall intensity can reflect effects of instantaneous rainfall intensity on stress performance of the structure under the mostly concerned extreme climatic conditions more intuitively. Considering the common rainstorm in inland China, rainfall intensity
Meshing of the computational domain in the wind-rain field
To ensure that the wind turbine is in the rainfall region and wake flow is developed completely, the computational domain was set 32D × 8D × 5D (down-wind X × across-wind Y × vertical direction Z and D is blade diameter in the wind turbine) and the wind turbine was put in the origin of the coordinate system, with x-axis consistent with the down wind direction. With considerations to both computational efficiency and accuracy and complexity of blade shape, the hybrid discrete meshing was adopted
Analysis of wind field
The surfaces at 37.2 m and 111.6 m height of the tower were used as the typical sections of the non-disturbance zone and significant disturbance zone. Contour map of pressure coefficient and circumferential distribution of pressure coefficient on these two typical sections of the tower under different yaw angles are shown in Fig. 6, Fig. 7. It finds that:
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Circumferential distribution of pressure coefficient in the non-disturbance zone is generally consistent and symmetric under different yaw
Finite element modeling and analysis of dynamic characteristics
A tower-blade integrated finite element model of the large-scale wind turbine system was constructed based on the large universal finite element analysis software ANSYS. The simulation used Shell63 unit for tower and blade, Beam189 unit for cabin, Solid65 unit for the ring base and Combin14 unit for interaction between the foundation and ring base. Natural frequency of vibration and mode of vibration of the wind turbine system were solved by Block Lanczos approach [34,35]. The finite element
Conclusions
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Blade and tower structures in the wind turbine system suffer the strongest wind-induced disturbance when the yaw angle is 0°. With the increase of yaw angle, effects of disturbance on streamline and wake flow on the tower decline gradually. The maximum negative pressure in the significant disturbance zone of the tower decreases firstly and then increases. The impact positions of raindrops mainly concentrate in the range of 40° at two windward sides of blade and tower structures. The maximum
Acknowledgments
This work was jointly funded by the National Basic Research Program of China (“973” Program) under Grant (2014CB046200), Natural Science Foundation of China (51878351, 51761165022, U1733129), open fund for Jiangsu Key Laboratory of Hi-Tech Research for Wind Turbine Design (ZAA20160013), Jiangsu Outstanding Youth Foundation (BK20160083), and China Postdoctoral Science Foundation (2015T80551).
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