This work discusses the physics of noise reduction achieved from serrated trailing–edges and its impact by the serration design. An experimental campaign is carried out with a benchmark 2D model based on a NACA 633–018 airfoil. Different trailing-edge serrations are tested under several flow speeds and angles of attack conditions to build a complete dataset of acoustic measurements. Systematic modifications of a reference sawtooth serration design are made to its scale and geometry. Scale modifications are based on sawtooth serrations and comprehend carefully considered variations of the serration height (2h), wavenumber (λ), and aspect ratio (2h/λ). Geometric shape modifications are represented by concave–shaped and combed–sawtooth serrations. This study represents a unique sensitivity–based parametric analysis on the scaling and geometric properties of trailing–edge serrations where the impacts of each modification are studied separately. The results obtained are used to provide guidelines for serration design choices and their impact on broadband noise reduction.@en

The present study focuses on the application of finlet rails as a passive technique of flow control to mitigate trailing-edge noise. Finlet rails are small cylinders whose axes are aligned along the streamwise direction, transversally positioned with respect to the trailing edge. In the first part of this study, the effects of finlet geometry on the aeroacoustic emission of a NACA 633−018 airfoil are investigated using an array of microphones. It is observed that reducing the transversal spacing of finlet rails leads to increasing the maximum noise reduction, found to be of 4 decibels at relatively low frequencies. An optimum for the height of the finlets was determined, equivalent to 1.6δ∗, where δ∗ is the displacement thickness of the boundary layer. With the aim of unveiling the underlying physical mechanism for finlet rails, PIV at high spatial resolution is applied around the surface treatment. It is found that the turbulence energy is lifted-up and moved away from the scattering edge, which attenuates the wall-pressure fluctuations. The observed attenuation of the wall-pressure fluctuations occurs at the energy-containing scales, which is an important difference with finlet fences. In the region underneath the finlet rails, the transversal size of the energetic structures diminishes when the surface treatment is applied. The combination of the lift-up of the turbulence structures, that reduces the wall-pressure fluctuations, with the smaller turbulence scales is responsible for the noise reduction observed for finlet rails.@en

The newly refurbished vertical tunnel (V-tunnel) at Delft University of Technology has been redesigned as a state-of-the-art facility for research in aeroacoustics (A-tunnel), as well as fundamental studies in laminar-turbulent transition and flow control. This manuscript focuses on the design and refurbishment aspects of the facility, including a description of the main modifications in the supporting structures and the drive system of the fan, with details of the flow conditioning and anechoic performance. A rigorous aeroacoustic and aerodynamic characterization of the facility is also presented, benchmarking the flow quality and acoustic performance of the new wind tunnel with respect to other aeroacoustic facilities across the world.@en

This paper explores the advantages of combining asynchronous microphone array measurements for acoustic source mapping in two and three–dimensional applications. Four different approaches are considered, three consisting of the combination of the source maps (arithmetic mean, geometric mean, and minimum value), and a fourth one obtained by combining the cross–spectral matrices of all the asynchronous measurements into a larger matrix and applying beamforming with it. Both synthetic and experimental test cases convey enhanced results concerning the single measurement baseline, especially by reducing the spurious sidelobes. Two aeroacoustic experiments are considered, the first features a distributed sound source over a flat plate model tested in a wind tunnel and the second features a hovering drone with multiple sound sources, representing typical challenges for 2D and 3D source localization, respectively. For the 2D source mapping configuration, the sidelobe level was reduced up to 5 dB with respect to the baseline, while maintaining a similar beamwidth of the main lobe. For the 3D test cases, the approaches enable volumetric source mapping capabilities, even when planar microphone arrays are considered, commonly available in typical test facilities. Overall, the minimum value approach presents the best performance for all cases in terms of reduced main lobe beamwidth and sidelobe levels.@en

A physical description of the flow mechanisms that govern the distribution of the wall-pressure fluctuations over the surface of a serrated trailing edge is proposed. Three main mechanisms that define the variation of turbulent pressure fluctuations across the serrated edge are discussed and semi-empirical models are formulated accordingly. It is shown that the intensity of the wall-pressure fluctuations increases at the tips under the effect of an increased convective velocity as a result of sidewise momentum diffusion. Furthermore, the change of impedance across the edge causes a local reduction of the pressure fluctuations in the vicinity of the trailing edge. Finally, aerodynamic loading over the serrations due to the non-symmetric flow created at different angles of attack establishes secondary flow patterns that induce higher wall-pressure fluctuations over the serration edges. The latter effect is present only for serrations under high aerodynamic loading, while the former ones are observed under any conditions. Semi-empirical models are formulated for predicting the variation of the wall-pressure fluctuations over the serration surface based on the three physical mechanisms described. These models are calibrated and compared against experiments conducted on a symmetric airfoil model at high Reynolds numbers.@en

The impact of aerodynamic loading on a serrated trailing edge is studied experimentally. Aerodynamic and acoustic measurements are conducted on a sawtooth-shaped trailing edge, retrofitted to a flat plate featuring a trailing-edge flap, and placed at incidence to the free-stream flow. The turbulent flow across the trailing edge is inspected by time-resolved three-dimensional velocity field measurements obtained from 4D-PIV, while the wall-pressure fluctuations are measured with surface-embedded microphones. Results discuss the relation between the velocity fluctuations over the serrations, the surface pressure fluctuations, and the far-field noise spectra. The aerodynamic analysis discusses the effect of counter-rotating vortex pairs, generated by the pressure imbalance across the edges of the serrations under loading. It is shown that the interaction of these vortices with the incoming turbulence affects the intensity of the wall-pressure spectrum at the outer rim of the serration surface. On the suction side, the intensity of the pressure fluctuations from the incoming boundary layer dominates over that induced by the vortex pairs. On the pressure side, instead, the velocity gradient prescribed by the vortex pairs produces a significant increase of the pressure fluctuations around the edges. The resulting spatial distribution of the wall-pressure fluctuations directly affects the far-field noise. Scattering predictions carried out with the wall-pressure fluctuations in the centre and root (on the suction side) exhibit good agreement with the measured noise in the low-frequency range, whereas using the surface pressure data at the tip of the serration (on the pressure side) yields a better prediction in the high-frequency range.@en

Wind-turbine noise can restrict the growing implementation of renewable energy sources and their application close to urban environments. The largest contributor to the noise of modern turbines is the scattering of the turbulent fluctuations at the blade trailing edge. This source of noise is directly correlated with the turbine’s extracted power. Therefore, operating in noise-restricted environments and at night times entails lower energy production. An extensively applied solution for reducing the noise of wind turbines is the use of trailing-edge serrations, i.e. imposing periodic variations in the geometry of the blade trailing edge. Serrations reduce the effectiveness of the scattering at the trailing edge as the turbulent fluctuations reach the trailing edge at different times along the blade span, consequently reducing the wind-turbine noise. Although extensive literature and knowledge exist on serrations, their measured performance does not compare with the predicted one. Even more problematic, the trends predicted for the geometric alterations of the serrations are not observed in reality. Notably, two things are worth mentioning: first, geometries shown as optimal by theory perform worse than other concepts, and second, the noise from serrations is affected by the angle between the insert and the flow. As a result, the design of trailing-edge serrations still requires dedicated experiments and numerical simulations, hampering the assessment of several geometries necessary for complete optimization of the serration design. This work seeks a physical interpretation of the noise generation mechanisms of trailing edges with serrated add-ons. This interpretation is focused on understanding the underlying physical principles of the flow surrounding a serrated trailing edge. This is carried out in this work with three studies, respectively on the observation, modelling, and control of the flow and acoustic properties of serrated trailing edges...@en

iew Video Presentation: https://doi-org.tudelft.idm.oclc.org/10.2514/6.2021-2179.vid The unsteady surface pressure fluctuations over trailing-edge serrations retrofitted to the benchmark NACA 633-018 profile are experimentally measured. Miniaturized unsteady pressure transducers installed on the serration surface are used to describe the temporal and spatial distribution of the wall pressure fluctuations over the trailing-edge add-on. The measured spectra, convection velocity, and correlation lengths of the surface pressure fluctuations are presented for the most upstream sensor and compared against the data at different locations along the serration surface. Different angles of attack are tested in a Reynolds number of 2,000,000 (based on the airfoil chord) in order to describe the properties of the wall pressure fluctuations at a range of representative conditions for wind turbine applications. The study details the characteristics of the wall-pressure fluctuations on the serration surface. In particular, the distribution of the wall-pressure fluctuations with and without aerodynamic loading caused by the variation of the airfoil angle of attack is shown, highlighting the impact of the vortex pairs formed around the edges of the serrations under loading on the wall-pressure fluctuations. The experimental results focus on the modifications of the wall-pressure fluctuations over the serration surface. Results show that, at small angles of attack, the amplitude of the wall-pressure fluctuations is higher at the serration root for low frequencies and at the serration tip for high frequencies. As the angle of attack increases, the aerodynamic loading over the serrations causes an increase in the wall pressure fluctuations around the edges of the serrations. This increase depends on the incoming fluctuations from the boundary layer and is related to the degradation of serration acoustic performance at angle. Furthermore, the experimental results corroborate to the benchmark activities for trailing-edge serration noise carried out in collaboration with DTU and DLR, providing information about the pressure fluctuations measured near the edges of the serration.@en

This work proposes a semi-empirical framework to predict the noise of wind turbines with serrated trailing edge blades. The framework is employed for studying the reduction of the noise of the SWT 2.3-93 benchmark wind turbine. The framework is verified against field acoustic measurements of the real wind-turbine model and of noise reduction measured for airfoil geometries with serrated trailing edges. Two different serration design strategies are proposed, respectively one with the same serration geometry along the blade and one with serrations scaled with the local boundary-layer properties along the radius. Results show the predicted noise reduction obtained with each of the add-ons and explore the benefits of tailoring the design of the serrations according to the varying flow conditions along the blade span.@en

Experimental results on trailing-edge (TE) noise from a NACA 633 –018 airfoil are presented for a chord-based Reynolds number Rec range between 2 × 105 and 3 × 106. Far-field TE noise from the baseline airfoil with a straight TE and TE serrations is measured with varying Rec, angle of attack, and serration shape and flap angle. Additionally, aerodynamic coefficients and boundary-layer parameters at the TE are also reported. To cover such a broad Rec range, two NACA 633 –018 airfoil models were tested in two different wind tunnels. The measurements include the emitted noise with natural and forced transition locations. For the straight TE, the forced transition location results in up to 5 dB increase of the far-field TE noise level, compared to the natural one. Scaling of the far-field noise spectra from the baseline TE shows that the Strouhal numbers St at which the peak noise level is measured reduce as Rec increases. TE noise spectra for the cases with the TE serrations are found to be dependent on the airfoil lift and Rec. The present data are to be included in the framework of the Benchmark Problems for Airframe Noise Computations category I and are publicly available in a repository with the following digital object identifier (DOI): https://doi.org/10.4121/20940646.@en

Abstract: Measurements of distributed surface pressure fluctuations over trailing-edge serrations at a Reynolds number Re θ = 4900 are performed with time-resolved 3D particle imaging velocimetry using helium filled soap bubbles as flow tracers. The sparse velocity vector field obtained with Lagrangian particle tracking is densely reconstructed using VIC+, a data assimilation technique based on the vortex-in-cell method. The instantaneous pressure distribution is inferred by invoking the momentum equation. Experiments are performed first over a flat plate, as assessment of the technique, where the properties of the convecting turbulent boundary layer are assessed and surface pressure fluctuations are validated against synchronous surface microphone measurements. The analysis of the flow over the trailing-edge serration focuses on the characterization of the spatial distribution and spectral coherence of surface pressure fluctuations, i.e. the flow features responsible for the acoustic scattering. Present results indicate that the measurement technique is suited to describe the spatio-temporal development of the pressure fluctuations over the serration surface at the proposed scale of the experiments. Graphic abstract: [Figure not available: see fulltext.].@en

An experimental aero-acoustic characterisation of the NACA 63 3-018 airfoil is presented in this study, featuring trailing-edge noise emissions with and without serrations. Measurements have been carried out for a chord-based Reynolds number range between 0.18 × 10 6 and 4.8 × 10 6 . Two airfoil models with different chord lengths have been tested in five different wind tunnels. The goal is to compare the measurements in different facilities, quantify the uncertainties, and establish a validation database that can serve as a benchmark for computational studies. The tests have been performed with clean and forced-transition boundary layers for a variety of angles of attack. The effect on the spectral slope and peak levels is evaluated. Scaling laws have been applied to compare different test conditions. The quality and nature of the collapse, as well as the applicability limits of the scaling, are examined. Different serration geometries have been tested at different flap angles. The noise reduction dependence on the aerodynamic loading is discussed. This work is based on an initiative of Task 39 "Quiet Wind Turbine Technology" of the Technology Collaboration Programme (TCP) of the International Energy Agency (IEA). @en