Coupled Bending-Twist Vibration of a Horizontal Axis Wind Turbine Blade Subjected to Turbulent Wind Flow

Abstract

In the preliminary design phase of an offshore wind turbine, a complete aerodynamic model is not required to assess its feasibility. As a result, engineers often switch to a simple model which is reliable and efficient, economically and computationally. Such a simplified model is based on the research interest. The main objective of this project is to study the aerodynamic interaction of a Horizontal Axis Wind Turbine (HAWT) blade. The aerodynamic interaction is basically the dependency of the relative wind velocity experienced by the blade on its structural response. Since the aerodynamic forces depend on the relative wind velocity, the interaction influences the force experienced by the blade. In addition, the cross-section of the blade is generally asymmetric, which results in a coupled bending-twist vibration of the blade, consequently influencing the aerodynamic interaction. Therefore, this project incorporates the coupled bending-twist vibration of the blade along with the introduction of gravity. A rotating blade is modeled accounting the bending-twist coupling by assuming a singly symmetric cross-section of the blade, which introduces an eccentricity between the center of mass and the elastic axis. The centrifugal force introduced due to the rotation of the blade, which induces an additional bending stiffness to the blade, is incorporated into the model. The relative wind velocity experienced by the blade is then defined by generating the wind profile from the Kaimal turbulence spectrum and incorporating the structural response. Due to the inclusion of torsional motion, the relative wind velocity varies along the chord line of the airfoil. Therefore, a point is selected along the chord line, generally the third-quarter chord point from the leading edge of the airfoil, for the definition of the relative wind velocity. Henceforth, the aerodynamic loads are defined and parametric studies are performed in order to assess the influence of gravity, aerodynamic interaction and bending-twist coupling on the structural response of the blade. It is found that the gravity induces a harmonic motion to the blade when rotating. In the case of a standstill blade, it only induces a static deformation due to the self-weight of the blade. The aerodynamic interaction is found to introduce an additional mass and damping to the system due to the dependency of the aerodynamic forces on the structural response acceleration and velocity, respectively. These added mass and stiffness result in lowering of the natural frequency of the system and significant damping of the structural response, predominantly in the flapwise direction when rotating. The bending-twist coupling is found to be dominant for the edgewise-torsional vibration.