Boundary-layer Trailing-edge Noise Research Articles

Sensitivity Analysis of Wind Turbine Broadband Noise Estimation to Semi-Empirical Models Parameters

The continuous increase of energy demand and the rising concerns on climate change, are pushing the European Union decarbonization strategies and transition toward renewable based energy systems, with wind energy playing a leading role. It is therefore necessary to have a better understanding of how wind turbines (WTs) impact on their surroundings, including their noise emissions. Among the different methods to compute noise emissions of WTs, semi-empirical models are a valid choice to have a-priori estimations of noise spectra and sound pressure levels. These models are based on correlation laws for different physical mechanisms that contribute to noise generation. Popular models for dominant noise sources include the Amiet approach for inflow turbulence noise and the Lowson model for turbulent boundary layer-trailing edge noise. Determining the parameters involved in these models can be challenging, potentially leading to significant errors in noise prediction. In this study, we conducted a novel sensitivity analysis of the models by varying different parameters such as turbulent intensity and dissipation, boundary layer thickness, and temperature. The selected test case is the reference multi-MW horizontal axis wind turbine Neg-Micon 80. The results of the multilevel-multivariate analysis, involving 63,360 combinations of the input parameters, clearly demonstrate a significant dependence of these models on atmospheric turbulence parameters. Furthermore, these models exhibit an higher sensitivity to input parameters at lower frequencies of the noise spectrum, which are generally associated with higher values of sound pressure level.

Broadband trailing edge noise reduction through porous velvet-coated serrations

This study experimentally investigates the potential of using combinations of trailing edge serrations and thin porous membrane/velvet structures for turbulent boundary layer trailing edge noise reduction. The experiments were conducted in an anechoic wind tunnel, with a flat plate model as the baseline model. The chord-based Reynolds number ranged between 2×105 and 5×105, and the boundary layers were fully tripped near the leading edge. Two different installation methods were tested, where the serration structure was aligned/misaligned with the undisturbed wake flow. It was observed that the noise reduction capability of the conventional serrations deteriorates significantly when the serrations are misaligned with the flow, while the performances of the combined structures are only slightly affected by flow misalignment. A novel combined treatment is developed, in which the trailing edge serrations are surrounded by serrated porous velvet structures. This treatment is found to outperform the unmodified serrations and can achieve approximately 10 dB noise reduction in both flow-aligned and flow-misaligned conditions, within a wide frequency range corresponding to a boundary layer thickness-based Strouhal number Stδ between 0.3 to 1. A 30%–40% increase in the aerodynamic drag due to the velvet structures was observed. Further hotwire wake survey revealed the possible mechanisms for the additional noise reduction capability of the combined treatments.

Effects of yawed inflow on the aerodynamic and aeroacoustic performance of ducted wind turbines

Ducted Wind Turbines (DWTs) can be used for energy harvesting in urban areas where non-uniform inflows might be the cause of aerodynamic and acoustic performance degradation. For this reason, an aerodynamic and aeroacoustic analysis of DWTs in yawed inflow condition is performed for two duct geometries: a baseline commercial DWT model, DonQi®, and one with a duct having a higher cross-section camber with respect to the baseline, named DonQi D5. The latter has been obtained from a previous optimization study. A numerical investigation using Lattice-Boltzmann Very-Large-Eddy Simulations is presented. Data confirm that the aerodynamic performance improvement, i.e. increase of the power coefficient, is proportional to the increase of the duct thrust force coefficient. It is found that, placing the DWT at a yaw angle of 7.5°, the aerodynamic performances of the DonQi D5 DWT model are less affected by the yaw angle. On the other hand, this configuration shows an increase of broadband noise with respect to the baseline DonQi® one, both in non-yawed and yawed inflow conditions. This is associated to turbulent boundary layer trailing edge noise due to the turbulent flow structures developing along the surface of the duct.

Surface roughness effect on rotor broadband noise

The change of helicopter rotor broadband noise due to different surface roughness during ice accretion is investigated. Comprehensive rotor broadband noise measurements are carried out on rotor blades with different roughness sizes and rotation speeds in two facilities: the Adverse Environment Rotor Test Stand facility at The Pennsylvania State University, and the University of Maryland Acoustic Chamber. In both facilities, the measured high-frequency broadband noise increases significantly with increasing surface roughness height. Rotor broadband noise source identification is conducted and the broadband noise related to ice accretion is thought to be turbulent boundary layer-trailing edge noise. Theory suggests turbulent boundary layer-trailing edge noise scales with Mach number to the fifth power, which is also observed in the experimental data confirming that the dominant broadband noise mechanism during ice accretion is trailing edge noise. A correlation between the ice-induced surface roughness and the broadband noise level is developed. The correlation is strong, which can be used as an ice accretion early detection tool for helicopters, as well as to quantify the ice-induced roughness at the early stage of rotor ice accretion. The trailing edge noise theories developed by Ffowcs Williams and Hall, and Howe both identify two important parameters: boundary layer thickness and turbulence intensity. Numerical studies of two-dimensional airfoils with different ice-induced surface roughness heights are conducted to investigate the extent that surface roughness impacts the boundary layer thickness and turbulence intensity (and ultimately the turbulent boundary layer-trailing edge noise). The results show that boundary layer thickness and turbulence intensity at the trailing edge increase with the increased roughness height. Using Howe’s trailing edge noise model, the increased sound pressure level of the trailing edge noise due to the increased displacement thickness and normalized integrated turbulence intensity are 6.2 and 1.6 dB for large and small accreted ice roughness heights, respectively. The estimated increased sound pressure level values agree reasonably well with the experimental results, which are 5.8 and 2.6 dB for large and small roughness height, respectively.

Experimental Investigation of the Surface Pressure Field for Prediction of Trailing Edge Noise of Wind Turbine Aero Foils

This paper concerns the characterisation of turbulent boundary layer trailing edge noise by measuring the surface pressure field. Two aerofoils typically used at the outer blade section of modern MW wind turbines were tested in an anechoic wind tunnel for Reynolds numbers ranging from 1 million to 1.9 million and angles of attack ranging from −10° to 14°. The emitted trailing noise from the aerofoils was measured with a microphone array at a distance of 1.6 m away from the aerofoil. The two-dimensional surface pressure field, which is considered the source of the emitted trailing edge noise, was measured with pinhole microphones distributed in streamwise and spanwise direction on the surface of the aerofoil. Two acoustic formulations relating the fluctuating surface pressure field to far field trailing edge noise were investigated. The measurements of the fluctuating surface pressure field were used as input to the model. There was a factor of 2 as difference between the two models. The prediction of the far field trailing edge noise with one model was in excellent agreement with the microphone array measurements in a frequency range of 500-2000 Hz. This opens up the possibility of quantifying trailing edge noise with surface pressure measurements. In the high frequency range the models overpredicted the measured far field sound. For frequencies lower than 500 Hz more validation is necessary. The measurements of the surface pressure field revealed that the spanwise correlation length scale is only a function of the Reynolds number when properly normalised. This result can be used to reduce the number of surface pressure microphones necessary to characterise far field trailing edge noise.

Effect of angle of attack on rotor trailing-edge noise

Previous analyses of boundary layer trailing edge noise for large rotors have used zero blade angle of attack as input data. Attention is presently given to the important effects of blade angle of attack changes on rotor trailing edge noise in the case of a UH-1 helicopter. The primary effect is in the low to mid-frequency range, where noise level increases with angle of attack.

Trailing-Edge Noise from Hovering Rotors

A method has been developed to predict the high frequency broadband noise due to the interaction of convecting turbulent eddies with the trailing edges of a hovering rotor. The trailing edge noise from each blade was modeled as point dipole noise with spanwise loading corrections. This point dipole approximation was checked by applying the concept to a stationary airfoil in a moving medium with excellent results. In order to estimate the strength of the point dipole, the trailing edge noise theory of Amiet was used. The method was applied specifically to blade boundary layer turbulence and compared to incident atmospheric turbulence noise. The results indicate that the relative importance of these two mechanisms is related to the magnitudes of the intensity and of the length scales of the inflow and boundary layer turbulence. The results tend to fall below some available experimental data indicating that in those experiments other broadband noise sources were stronger than boundary layer-trailing edge noise. The approach which was developed is also applicable to other blade-turbulence interaction mechanisms such as local stall and tip noise.