Study of Centrifugal Stiffening on the Free Vibrations and Dynamic Response of Offshore Wind Turbine Blades

Abstract

Due to their large and increasing size and the corrosive nature of salt water and high wind speeds, offshore wind turbines are required to be more robust, more rugged and more reliable than their onshore counterparts. The dynamic characteristics of the blade and its response to applied forces may be influenced dramatically by rotor rotational speed, which may even threaten the stability of the wind turbine. An accurate and computationally efficient structural dynamics model is essential for offshore wind turbines. A comprehensive model that takes the centrifugal stiffening effect into consideration could make rapid and accurate decisions with live data sensed from the structure. Moreover, this can enhance both the performance and reliability of wind turbines. When a rotating blade deflects in its plane of rotation or perpendicular to it, the centrifugal force exerts an inertia force that increases the natural frequencies and changes the mode shapes, leading to changes in the dynamic response of the blade. However, in the previous literature, studies of centrifugal stiffening are rarely found. This study investigates the influence of centrifugal stiffening on the free vibrations and dynamic response of offshore wind turbine blades. The National Renewable Energy Laboratory (NREL) 5 MW blade benchmark was considered to study the effect of angular speed in the flap-wise and edge-wise directions. The results demonstrate that the angular speed directly affects the modal features, which directly impacts the dynamic response. The results also show that the angular velocity effect in the flap-wise direction is more significant than its effect in the edge-wise direction.

Dynamic Responses Analysis of A 5MW NREL Wind Turbine Blade under Flap-Wise and Edge-Wise Vibrations

“…Table 4, row 1 represents the results obtaind by Li Z et al, [15], where a geometrically exact beam model and BEM theory were used to obtain the blade deformation in both directions of the 5MW NREL blade. Similarly, row 2 represents the results of Sabale and Gopal [18] where BEM-EBM were used for the same blade, while row 3 represents the results obtained from [11] where the B-Mods FAST software developed by NREL was used. Row 4 represents the data obtained from reference [23] using a CFD flow solver and blade elastic deformation; row 5 represents the results obtained from Ponta et al, [13], where the generalised Timoshenko beam theory was adopted to model the 5MW NREL blade.…”

“…The flap-wise tip deflection agrees with [20], where the elastic deformation of the blade was calculated using the FE commercial software ABAQUS, although the edgewise deflection was significantly larger. The method adopted in [15], [18], and [20] for the structural analysis was the geometrically exact beam theory, and some differences can be noticed in the results. [13] adopted the Timoshenko beam theory for free vibrations, but it can be seen that the flap-wise deformation is small compared with other results.…”

“…Table 4. Comparison of the dynamic response under nominal working conditions of the present study and references [11], [13], [15], and [17][18][19][20] where the same model of 5MW NREL wind turbine blade was adopted. A comparison of simulated natural frequencies indicates that the free vibration analysis in the flap-wise direction is approximately the same for the Rayleigh and Rayleigh with adding the blade control angles such as the pitch pre-cone angles.…”

“…In the same study, the aerodynamic loading was calculated using the 3D aerodynamics of the rotating blade based on the blade momentum theory. The work in [18] investigated the influence of centrifugal stiffening on the free vibrations and dynamic response of offshore wind turbine blades. The study adopted the Rayleigh beam theory to analyse the blade structure.…”

“…Several techniques are used to calculate the aerodynamic loading based on the above combinations of structural analysis of wind turbine blades. BEM theory was used in [15], and the same BEM theory was employed to calculate the aerodynamic forces [17][18][19][20]. The computational fluid dynamics technique was adopted in [9], [21] and [23].…”