Blade Tip flow and Noise Prediction by Large-Eddy Simulation in Horizontal Axis Wind Turbines

Abstract

This chapter investigates the physical mechanisms associated with the tip vortex noise caused by rotating wind turbines. A computational model for aerodynamic noise prediction that takes into account the true shape of the wind turbine blade geometry was developed. This is an important step towards wind turbine blade design with respect to aerodynamic noise reduction. The method developed in this research allows arbitrary blade tip shapes to be modeled and their acoustic emissions to be estimated. For validation purposes, turbulent fluctuations associated with the tip vortex and the acoustic fields generated by a finite blade are simulated using Large-Eddy Simulation (LES). Complex flow features associated with the tip vortex are captured and validated with experimental measurements. It is found that the tip vortex constitutes a major noise source. The flow around a NACA0012 blade section at a Reynolds number of 2.87 × 106 is calculated using compressible LES. The broadband aerodynamic noise emitted by a large rotating wind turbine blade of arbitrary shape is predicted, with particular emphasis on tip noise. A large-scale unsteady compressible LES combined with direct noise simulation using 300 million grid points is carried out on the Earth Simulator. Simulations for the W1NDMELIII wind turbine rotor blades are performed for two blade tip geometries. It is found that the ogee type tip shape reduces overall aerodynamic noise by 2 dB.