Structural Dynamics of Small Wind Turbine Blade under Aerodynamic Loading

Abstract

Aeroacoustic noise emissio n is a concern for residential wind turbines. Small wind turbines operate at high RPMs and may spin even when pointed out of the wind. In order to better understand the aeroacoustic phenomena related to small wind turbines, a dynamic model of the turbine has been created with the hope of expanding the model to include aeroacoustic s. For this study, an aerodynamic model of a wind turbine blade was coupled with a linear finite element model of a small blade. The goal of this analysis is to create an initia l model that can be validated and expanded to predict aeroacoustic noise emissions. This study is directed toward a generic small three -bladed upwind wind turbine with blade lengths of approximately one meter and a linearly varying blade twist angle. Pre ssure distribution and blade loading were obtained numerically using STAR -CD Computation Fluid Dynamics (CFD) modeling software, for a given blade configuration. The calculation used the Reynolds -Averaged Navier -Stokes (RANS) equations with the k-� turbul ence model with the blade section on rotating reference frame. The CFD model was coupled to a finite element model (FEM) to predict blade deflection. The aerodynamic loads that were calculated using the CFD model were fed into a FEM written in Matlab. T he FEM was used to determine the equations of motion for the system. The FEM consisted of a coupled linear 3 -degree of freedom (DOF) per element beam finite element model of the blade. The stiffness matrix was approximated based on generic dimensions for the prescribed wind turbine blade length and included centrifical stiffening. Given the aerodynamic loads and initial conditions, the equations of motion were solved by direct integration over a small period of time. Results from the FEM were used to up date the CFD model and calculate the turbine loading as a transient condition. As an initial study, the model was used to compute a few iterations of the wind turbine blade dynamics.