Branko Zajamsek

Experimental investigation of trailing edge noise from stationary and rotating airfoils

Trailing edge noise from stationary and rotating NACA 0012 airfoils is characterised and compared with a noise prediction based on the semi-empirical Brooks, Pope, and Marcolini (BPM) model. The NACA 0012 is symmetrical airfoil with no camber and 12% thickness to chord length ratio. Acoustic measurements were conducted in an anechoic wind tunnel using a stationary NACA 0012 airfoil at 0° pitch angle. Airfoil self-noise emissions from rotating NACA 0012 airfoils mounted at 0° and 10° pitch angles on a rotor-rig are studied in an anechoic room. The measurements were carried out using microphone arrays for noise localisation and magnitude estimation using beamforming post-processing. Results show good agreement between peak radiating trailing edge noise emissions of stationary and rotating NACA 0012 airfoils in terms of the Strouhal number. Furthermore, it is shown that noise predictions based on the BPM model considering only two dimensional flow effects, are in good agreement with measurements for rotating airfoils, at these particular conditions.

Numerical simulation of blade-passage noise

Numerical simulations are used to investigate the noise generated by the passage of a rotor blade past a fixed object (the blade-passage effects), which was studied by simulating a three-bladed rotor that is supported by a vertical cylindrical tower. To isolate the blade-passage effects, no incoming wind was introduced in the simulation. The symmetric blade was set to zero pitch angle relative to the plane of rotation and two blade-tower distances were investigated. The sliding mesh method was used to simulate the rotation of the blades and Curles acoustic analogy was used to predict the noise generated from the simulated flow data. Intense force fluctuations occur during the interaction on both the tower and the passing blade, and these are the primary sources of blade-passage noise. The contribution of the force fluctuations on the support tower to blade-passage noise, which previously had been ignored, was revealed to be more significant than that of the blades. The numerical model successfully predicts the noise spectra, which are validated by the very good agreement with experimental measurements. The simulations provide a framework to better understand blade-tower interaction noise in various applications.

Infrasound and Low-Frequency Noise from Wind Turbines

Infrasound, low-frequency noise (ILFN) and amplitude modulation of the noise are known to disturb some residents living near wind farms. However, the mechanisms responsible for ILFN and amplitude modulation are not well understood. In an attempt to shed some light on these mechanisms, acoustic measurements were taken close to a wind farm, at residences located two or more kilometres from the nearest turbine in a wind farm and in an anechoic chamber using a scale-model, electrically-driven, wind turbine. The measured spectra reveal distinct peaks at the frequencies corresponding to the blade-pass frequency and its harmonics, and the characteristics of these peaks are remarkably similar for field and laboratory measurements, indicating that the zero mean flow simulation is a good representation of an actual wind turbine. Near field acoustic holography measurements on the scale-model turbine confirm that tonal components at the frequencies corresponding to the blade-pass frequency and its harmonics are generated as a result of blade-tower interaction, suggesting that it is likely to be an important mechanism of infrasound generation for industrial wind turbines. Inaccuracies in the assumed location of sources of noise generated by a wind turbine affect the accuracy of community noise predictions. This is because the source height affects the distance from the turbine beyond which sound rays arrive at the receiver having been reflected from the ground more than once, thus reducing the attenuation with distance from the turbine.

Examination of the predictions versus measurements of an acoustic interaction model for spinning modes, and concurrent low frequency amplitude modulation acoustic signatures from a 3MW wind turbine

A recently presented hypothesis and model relating to the generation of spinning modes from wind turbines, as a direct result of acoustic interaction involving the tower, results in a far field infrasound sound pressure level prediction, which is higher than that predicted by point source method. The model also predicts a significant attenuation of the fundamental blade passing frequency component relative to the second and higher harmonics. The model concurrently predicts a low frequency (~20 Hz), amplitude modulated harmonic series as a side effect of the acoustic interaction on a 1.6 MW 80 m diameter wind turbine. This study examines the model predictions of a 3.0 MW 90 m diameter wind turbine, and compares the predictions to measurements of the low frequency harmonic series and blade passing frequency harmonics at several different distances from the wind turbine.