Stefan Oerlemans

Reduction of Wind Turbine Noise Using Optimized Airfoils and Trailing-Edge Serrations

Acoustic field measurements were carried out on a 94-m-diam three-bladed wind turbine with one standard blade, one blade with trailing-edge serrations, and one blade with an optimized airfoil shape. A large horizontal microphone array, positioned at a distance of about one rotor diameter from the turbine, was used to locate and quantify the noise sources in the rotor plane and on the individual blades. The acoustic source maps show that for an observer at the array position, the dominant source for the baseline blade is trailing-edge noise from the blade outboard region. Because of convective amplification and directivity, practically all of this noise is produced during the downward movement of the blade, which causes the typical swishing noise during the passage of the blades. Both modified blades show a significant trailing-edge noise reduction at low frequencies, which is more prominent for the serrated blade. However, the modified blades also show tip noise at high frequencies, which is mainly radiated during the upward part of the revolution and is most important at low wind speeds due to high tip loading. Nevertheless, average overall noise reductions of 0.5 and 3.2 dB are obtained for the optimized blade and the serrated blade, respectively.

Acoustic Array Measurements of a 1:10.6 Scaled Airbus A340 Model

Acoustic array measurements were carried out on a 1:10.6 scaled Airbus A340 model. Tests were done in both the open jet and a closed test section of the DNW-LLF wind tunnel. The purpose of these measurements was to investigate several noise reduction concepts for high-lift devices (slats and flaps) in landing configuration. The possibilities and limitations of arrays for the determination of quantitative results are discussed. Besides the identification of dominant noise source regions with conventional beamforming, local source spectra were determined using a power integration method. For the open jet results, the integration method was applied with and without the ’diagonal removal’ (DR) technique, in which the main diagonal of the cross-power matrix is discarded. It is shown that application of DR results in meaningful local spectra, whereas without DR the results are obscured by the influence of the main diagonal. On the other hand, by comparing integrated spectra with absolute sound levels on farfield microphones, it is shown that the application of DR, in combination with coherence loss, results in significantly reduced absolute levels. However, while the absolute sound levels can be too low, level differences between configurations can be accurately determined under certain conditions. From the closed test section array results, dominant source regions are identified as a function of frequency and angle-ofattack. These results are quantified by application of the integration method to several source areas on the wing. The local and overall effect of noise reduction devices is assessed. The effect of coherence loss on the absolute levels is investigated by varying the effective array size.

Reduction of Landing Gear Noise Using an Air Curtain

Acoustic and aerodynamic measurements were performed in NLRs Small Anechoic Wind Tunnel on the air curtain concept, which is intended for landing gear noise reduction. The idea of this concept is to apply an upstream blowing slot to deflect the flow around a landing gear (component), thus reducing the local flow speeds and therefore the aerodynamically generated noise. Prior to the wind tunnel tests, a design study was carried out to assess the possible benefit of an air curtain and to define the test set-up. For the subsequent proof-of-concepts tests, two-dimensional half-models were mounted on an endplate which was attached to the lower edge of the wind tunnel nozzle. Blowing was applied through a slot in the endplate, upstream of the model. Microphone array and PIV measurements were performed to characterize the acoustics and aerodynamics of the air curtain. Tests were done for different wind tunnel speeds, blowing speeds, slot geometries and model geometries. For the relatively quiet baseline half-models in the present tests, broadband noise reductions of 3-5 dB were obtained using an air curtain with normal blowing (i.e. perpendicular to the main flow). The noise reductions could be increased to 5-10 dB by oblique blowing (30° upstream) or by applying a small flow deflector directly before the blowing slot. For full models larger noise reductions are anticipated.

Spectral broadening by shear layers of open jet wind tunnels

The presence of shear layers in open jet wind tunnels complicates aeroacoustic measurements. Tones from wind tunnel models are subject to spectral broadening (or ‘haystacking’) when propagating through the turbulent shear layer flow. For example, in measurements on contra-rotating propellers this obscures the identification and quantification of rotor tones. This paper describes a theoretical and experimental study into spectral broadening by the shear layers of open jet wind tunnels. A simple physical model is derived which predicts the amount of broadening as a function of a single parameter, which is proportional to wind speed, source frequency and shear layer thickness. The theory is compared against experimental data from five different wind tunnels. Especially for the smaller wind tunnels the agreement between theory and experiment is generally good, which makes it possible to retrieve the original level of a tone from a broadened spectrum.

Reduction of Landing Gear Noise using Meshes

Acoustic and resistance measurements were performed in NLRs Small Anechoic Wind Tunnel on a large number of meshes intended for landing gear noise reduction. The meshes were tested on generic bluff bodies which simulated single and combined landing gear struts. An out-of-flow microphone array was used to localize and quantify the noise sources on the model. It is found that the meshes yield a drastic broadband noise reduction for a wide range of mesh materials and mesh shapes. The noise reduction occurs for all tested angles of attack and for all tested models. For the combined bodies it is found to be sufficient to treat only the upstream component with a mesh.