Trailing Edge Noise Reduction Research Articles

Reduction of Flow-Induced Trailing Edge Noise of Semi-Infinite Flat Plate by Structural Resonance

A unique method for the reduction in flow-induced trailing edge noise scattering for a semi-infinite thin plate is proposed by utilizing short flexible panels mounted on the surface of the plate. The proposed configuration differs from conventional cantilevered elastic trailing edges due to the limitations of their structural integrity and applicability. The key idea is to design short panels that vibrate in structural resonance under the fluid loading for absorbing the energy from the flow to sustain their vibration. A time-domain direct aeroacoustic simulation coupled with structural dynamics is carried out at a low Reynolds number of 50,000 to explore the aeroacoustic-structural interactions. The effectiveness of noise reduction of the designed configuration is analyzed by comprehensive aeroacoustic analysis where a significant noise reduction is observed for the plate mounted with three flexible panels without any adverse effects on the plate aerodynamics. The structural analysis shows a systematic vibration pattern for all the panels which clearly indicates the presence of complex fluid-structural interaction under resonance conditions. Design strategy of the proposed configuration along with its limitations are also discussed.

A Deep Learning Approach to Optimize Airfoil Shape for Reduced Trailing Edge Noise in Urban Air Mobility Rotors

The objective of this research is to optimize the airfoil shape of UAM VTOL rotors for trailing edge noise reduction. Our study focuses on employing deep learning techniques for airfoil shape optimization with an emphasis on noise reduction. We implement a neural network that combines a Variational Autoencoder and Generative Adversarial Networks to learn the representation of airfoil shapes. The generated airfoil shapes are then evaluated for aerodynamic performance using XFOIL and trailing edge noise using acoustic diffraction model. The optimization process employs a Particle Swarm Optimization algorithm to find optimal latent features for airfoil generation. The results show that the optimized airfoil shapes have improved aerodynamic performance and reduced noise levels compared to the airfoil VR-12 used in NASA's conceptual UAM. These findings demonstrate that our optimization process has the potential to reduce trailing edge noise while ensuring enhanced aerodynamic performance, which warrants further investigation.

Experimental Study on Wind Turbine Airfoil Trailing Edge Noise Reduction Using Wavy Leading Edges

Aerodynamic noise produced by the rotating blade is an important hindrance for the rapid development of modern wind turbines. Among the various noise sources, the airfoil trailing edge noise contributes a lot to the wind turbine noise. The control of wind turbine airfoil trailing edge self-noise by bio-inspired sinusoidal wavy leading edges is experimentally studied in a semi-anechoic chamber. The noise radiated by the baseline NACA 0012 airfoil and various wavy airfoils is measured using a planar microphone array consisting of fifty-two microphones. The noise source identifications are achieved by using the CLEAN-SC method. The effects of velocity and angle of attack on noise radiation of the baseline airfoil are analyzed in detail. The noise control law of the wavy amplitude and wavelength on airfoil trailing edge noise is explored. Based on the acoustic beamforming results, the noise control effects of the wavy leading edges are intuitively demonstrated. In general, the wavy leading edge with a larger amplitude and smaller wavelength has a better effect on the airfoil trailing edge noise reduction. The maximum sound pressure level reduction can be up to 33.9 dB. The results of this study are expected to provide important information for wind turbine aerodynamic noise control.

Numerical study of trailing-edge noise reduction mechanism of wind turbine with a novel trailing-edge serration

The serration assembly installed at the trailing-edge of wind turbines can reduce the turbulent boundary-layer trailing-edge noise. This work studies the aeroacoustic response and noise reduction capability of four different trailing-edge serrations, involving a novel Multi-Flapped-Serration with Iron-Shaped Edges (MFS-Iron) presented here. These four serrations are retrofitted to S809 and NACA643418 airfoils. The Large-eddy simulation (LES) method and Ffowcs Williams and Hawkings integration (FW-H) methods are respectively employed to predict the unsteady flow field and far-field noise based on optimized structured and unstructured hybrid grids. From the numerical simulation results, we explore the trailing-edge noise reduction mechanism. The results show that the proposed MFS-Iron can effectively realize noise reduction under appropriate design parameters and optimize the aerodynamic layout. When using the MFS-Iron serration, the cross-flow intensity between the serration gap is significantly reduced, and the original two groups of symmetrical horseshoe vortex are mixed into a single vortex, which has a positive impact on the suppression of high-frequency noise. By further studying the Lamb vector divergence vortex dynamics process near the surface of serrated trailing-edge, it is found that the Lamb vector divergence is directly related to the intensity of the noise source.

Trailing-edge noise reduction through finlet-induced turbulence

Biologically inspired finlet treatments have been shown to effectively reduce the trailing-edge noise of a flat plate and hence are a viable noise-suppression technology for engineering applications. The present work performs a thorough experimental investigation on the near-field dynamics of finlet surface treatments applied to a flat plate. To examine the underlying noise-reduction mechanism, the manipulated flow field is analysed using data from detailed static, unsteady wall-pressure as well as velocity measurements and their correlations. Specifically, the densely populated dynamic transducers allow for the tracking of the turbulent boundary-layer development from upstream to the wake of the finlet-treated area (see supplementary movies), which elucidates the formation of ‘finlet-induced turbulence’ through flow–finlet interaction. Associated turbulence structures are found to further develop within the treated area and structures shed from the top of the finlets are observed to mix and merge with the turbulence being channelled through the space between the finlets in the finlet wake. While the mixing process increases the spanwise turbulence length scale, it significantly attenuates the unsteady wall-pressure fluctuation at the trailing edge and thus leads to broadband reduction of the trailing-edge noise. Moreover, it corroborates the findings of earlier studies suggesting that there exists an optimal distance between finlets and trailing-edge where the mixing effects are most beneficial.

A computational study of trailing edge noise suppression with embedded structural compliance.

A unique concept for suppression of trailing edge noise scattering from a splitter plate in a low Reynolds number flow is proposed. The key idea of the concept is the adoption of a structural compliance system embedded with a finite number of elastic panels. Specific compliance system designs are devised for promotion of panel structural resonance that effectively absorbs broadband flow/acoustic fluctuation energy responsible for noise scattering. The concept is examined using high-fidelity direct aeroacoustic simulation together with spatiotemporal aeroacoustic-structural interaction analysis. The concept is confirmed feasible and outperforms many similar trailing edge noise reduction approaches reported in the literature.

Airfoil trailing-edge noise and drag reduction at a moderate Reynolds number using wavy geometries

This study utilizes a hybrid aeroacoustic model to investigate how airfoils with spanwise wavy geometries can be used to reduce trailing-edge noise alongside improving the aerodynamic performance. A smooth airfoil is compared to four variants, which have spanwise surface waves of different wavelengths, at a Reynolds number of Re = 64 000 and an angle-of-attack of 1°. The first three variants have a geometry modified by a single wavelength, whereas the fourth has a surface composed of two wavelengths, which creates a more irregular surface variation. The results show that modest noise reductions of around 4 dB are achieved for the first three variants, but a much larger reduction of 17.7 dB is achieved for the fourth variant. The mechanisms behind the noise reduction are explored, and it is shown that the geometry reduces the spanwise correlation of the pressure fluctuations and also modifies the boundary layer dynamics, which contributes to the large reduction. It is further shown that a wavy geometry can reduce the drag force by reducing the shear stress over parts of the airfoil surface and by limiting the flow separation on the suction side. The fourth variant is also assessed across a wider range of angles (0°≤α≤4°) and is shown to produce less noise than the smooth wing across the entire range as well as a drag reduction for α≥1°.

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.

Trailing-edge noise reduction potential of a locally applied shallow dimpled surface

There is a strong need for airframe noise reduction methods that preserve the aerodynamic performance of lift-generating airframe components. Inspired by the drag reduction studies of van Nesselrooij et al. (2016) and Spalart et al. (2019), the current study explored the trailing-edge noise reduction potential of shallow dimples applied near the trailing edge of a NACA0012 airfoil (approximately the latter 18% of the chord). The effect of shallow dimples on the aerodynamic coefficients is studied by Reynolds Averaged Navier–Stokes (RANS). To investigate the noise reduction credentials of dimples, high-fidelity Overset-Large Eddy Simulations are carried out for a chord based Reynolds number Rec of 4.2 ×105 at zero angle of attack. Near-field analysis revealed that this local application of dimples eliminated the tonal vortex shedding noise, which is otherwise present in the reference configuration without dimples. Furthermore, the application of shallow dimples resulted in the breakdown of the span-wise coherence in the low-to-mid frequency range and a reduction in the convective velocity of the stream-wise convecting eddies. These effects have a favourable impact on the trailing edge noise reduction with a modest 1.8% increase in the overall drag (less than 1 drag count). The far-field sound characteristics revealed that the dimple treatment resulted in the reduction of overall sound pressure levels in the downstream radiation direction (≈ 5.5 dB) and an increase in the upstream radiation direction (≈ 3.5 dB). In a nutshell, this manuscript argues for the inclusion of shallow dimples to the list of potential candidates for passive noise reduction methods, such as serrations, porous materials, and finlets.

Parametric Airfoil Design and Sensitivity Analysis for Turbulent Boundary-Layer Trailing-Edge Noise Reduction

This paper presents the investigation of airfoil turbulent boundary-layer trailing-edge noise reduction using a parametric airfoil design. The airfoil design tool performs a parametric design starting from a baseline airfoil and then morphing it with a number of design parameters, which include camber, camber crest position, thickness, thickness crest position, leading-edge radius, trailing-edge camber, and boat-tail angle. A predictive method that combines an empirical wall-pressure spectrum model and an acoustic diffraction model is used to predict airfoil turbulent boundary-layer trailing-edge noise. In this study, a NACA 0012 airfoil and a NACA 64-618 airfoil are used to analyze the effects of the design parameters on airfoil aerodynamic performance and trailing-edge noise. The sensitivity of different parameters is performed with Morris’s method in order to further assess their relative influences. Overall, it is found that thickness, trailing-edge camber, and boat-tail angle have a significant influence on noise levels.

Airfoil Trailing-Edge Noise Reduction by Application of Finlets

This experimental study investigates the effect of biologically inspired finlets on the reduction of trailing-edge noise of a NACA 0012 airfoil. At a chord-based Reynolds number of 400,000, the far-field noise measurements show that the finlets are capable of reducing the trailing-edge noise effectively from 1000 to 4000 Hz with up to 6 dB reduction, for effective angles of attack from 0 to 8 deg. By correlating the far-field noise to the boundary-layer measurements, an optimal finlet height to boundary-layer thickness ratio to achieve the highest noise reduction is discussed. Static pressure and dynamic pressure fluctuations are examined, particularly within the finlet-treated area, to capture the flow dynamics through the treatments. The unsteady surface pressure fluctuation spectra show strong reduction at frequencies higher than 1000 Hz in the finlet wake, complying with the far-field measurements. Both the auto- and cross-correlation results of the unsteady surface pressure reveal the formation of large-scale turbulence at the entrance and toward the exit of the finlets. Moreover, they suggest that the formation and subsequent mixing of these large-scale structures with the boundary layer can be beneficial to reducing the unsteady surface pressure fluctuations. The findings will be useful to potential optimization of the finlets.

Trailing-edge noise reduction of a wing by a surface modification

There is an increased emphasis on reducing airframe noise in the last decades. Airframe noise is sound generated by the interaction of a turbulent flow with the aircraft geometry, and significantly contributes to the overall noise production during the landing phase. One examples of airframe noise is the noise generated at a wing's trailing edge, i.e., trailing-edge noise. In this contribution, we numerically explore the local application of riblets for the purpose of trailing-edge noise reduction. Two configurations are studied: i) a clean NACA0012 wing section as a reference, and ii) the same configuration with riblets installed at the wing's aft part. The numerical investigation follows a hybrid computational aeroacoustics approach, where the time-average flow is studied by means of RANS. Noise sources are generated by means of a stochastic approach called Fast Random Particle Mesh method. The results show a deceleration of the flow behind the riblets. Furthermore, the turbulent kinetic energy indicates increased unsteadiness behind the riblets which is shifted away from the wall due to the presence of the riblets. Lastly, the sound sources are investigated by means of the 3D Lamb-vector, which indicates a slight reduction in magnitude near the trailing edge.

Flexural Wave Modulation and Mitigation in Airfoils Using Acoustic Black Holes

This study introduces a framework for the design and implementation of acoustic black holes (ABHs) in airfoils. A generalized multi-parameter damped-ABH generation function is mapped onto NACA series airfoils. Representative geometries and a uniformly distributed baseline, all with the same mass of structure and damping are fabricated using multi-material PolyJet 3D printing. Laser Doppler vibrometer measurements along the airfoil chord in response to a broadband 0.1 - 12 kHz excitation show a decrease in trailing edge vibrations by as much as 10 dB, a broadband 5 dB reduction across the entire chord as well as substantial spatial and temporal modulation of flexural waves by ABH-embedded foils. Finite element analysis (FEA) models are developed and validated based on the measured data. Furthermore, a parametric FEA study is performed on a set of comparable designs to elucidate the scope of modulation achievable. These findings are applicable to trailing-edge noise reduction, flow control, structural enhancement and energy harvesting for airfoils.

Improved delayed detached eddy simulation of a porous wavy trailing edge

This paper proposes a novel trailing-edge treatment to reduce trailing-edge noise. Prior studies showed that both porous materials and wavy trailing edges have decent trailing-edge noise reduction. Therefore, a combination of porosity and waviness, or a porous wavy trailing edge, could possibly result in greater noise reduction. Improved delayed detached eddy simulations are performed to compare the aeroacoustic performance of a porous wavy trailing edge to those of clean airfoil, porous trailing edge, and wavy trailing edge. It is revealed that a porous wavy trailing edge reduces the overall sound pressure level by 8.1 dB without compromising the aerodynamic efficiency. The detailed flow physics of noise reduction with a porous wavy trailing edge is also investigated. Overall, the current research provides a low-noise solution for wind turbines and the future's ultra-quiet aircraft.

Numerical Analysis of Airfoil Trailing-Edge Noise for Straight and Serrated Edges at Incidence

The goal of this paper is to perform a detailed analysis of the hydrodynamic near field and acoustic far field of a NACA 64-618 airfoil with and without serrations. The impact of serrations is investigated across different airfoil angles of attack and serration flap angles. The natural boundary-layer transition cases 6 and 7 of the AIAA Benchmark Problems for Airframe Noise Computations V Workshop Category 1 are considered as benchmark for the straight edge cases. The numerical simulations are performed using the fully explicit, transient, and compressible lattice Boltzmann equation implemented in the computational fluid dynamics/computational aero-acoustics (CFD/CAA) solver SIMULIA PowerFLOW®. The acoustic far field is obtained by using the Ffowcs-Williams and Hawkings integral solution applied to the airfoil surface. A mesh resolution study is performed on the straight trailing-edge cases to demonstrate the grid independence of the numerical solution. The numerical results compare favorably against the experimental data. The impact of the serration flap angle on the effectiveness of sawtooth serrations in reducing noise is investigated by considering three different serration flap angles. It is found that the serration flap angle primarily affects the trailing-edge noise reduction through a modification of 1) the effective angle at which the turbulent structures are convected over the serrated edge; 2) the convection velocity and spanwise coherence length along the serration; and 3) the intensity of the hydrodynamic wall-pressure fluctuations that are scattered along the serrated edge. The first and last phenomena are expected to play the most important role on the far-field noise reduction.

The Application of a Linear Microphone Array in the Quantitative Evaluation of the Blade Trailing-Edge Noise Reduction

This paper concerns the application of a linear microphone array in the quantitative evaluation of blade trailing-edge (TE) noise reduction. The noise radiation from the blades with straight and serrated TEs is measured in an indoor open-jet wind tunnel. The array data are processed using the inverse method based on the Clean algorithm based on spatial source coherence (Clean-SC). In order to obtain correct application and achieve the best effect for the microphone array test, the computing software for array data reduction is firstly developed and assessed by Sarradj’s benchmark case. The assessment results show that the present array data processing method has a good accuracy with an error less than 0.5 dB in a wide frequency range. Then, a linear array with 32 microphones is designed to identify the noise source of a NACA65(12)-10 blade. The performance of the Clean-SC algorithm is compared with the Clean algorithm based on point spread functions (Clean-PSF) method for experimentally identifying the noise sources of the blade. The results show that there is about a 2 dB error when using the Clean-PSF algorithm due to the interference of different aerodynamic noise sources. Experimental studies are conducted to study the blade TE noise reduction using serrated TEs. The TE noise for the blade with and without sawtooth configurations is measured with the flow speeds from 20 m/s to 70 m/s, and the corresponding Reynolds numbers based on the chord are from 200,000 to 700,000. Parametric studies of the sawtooth amplitude and wavelength are conducted to understand the noise reduction law. It is observed that the TE noise reduction is sensitive to both the amplitude and wavelength. The flow speed also affects the noise reduction in the serrated TEs. To obtain the best noise suppression effect, the sawtooth configuration should be carefully designed according to the actual working conditions and airflow parameters.

Numerical investigation of porous materials for trailing edge noise reduction

In the framework of the German Collaborative Research Center CRC 880: Fundamentals of High Lift for Future Civil Aircraft porous materials as a means towards the reduction of airfoil trailing edge noise are investigated. At DLR, both experimental and numerical approaches are pursued to understand the physics behind the noise reduction. The present paper focuses on the numerical investigations, for which the experimental data serves as an evaluation basis. From the analysis of homogeneous materials, first steps are made towards the design of aeroacoustically tailored materials. It is assumed that materials with locally varying permeability may be suitable to achieve maximum noise reduction, as they provide a smooth transition from the solid airfoil to the free flow in the wake. The simulation results support this understanding, however it is revealed that high local gradients in the material properties themselves may act as acoustic sources.

Experimental investigation of porous materials for trailing-edge noise reduction

The introduction of quiet short take-off and landing for civil aircraft operations in close proximity to the population poses important technological challenges. One critical aspect is the realization of extreme lift augmentation at low acoustic emissions. The aircraft concept selected to achieve this goal is a high-lift system equipped with an active flow-control non-slotted flap and a droop nose. For this specific configuration, trailing edge noise becomes a dominant noise source. Porous materials as a passive means for trailing-edge noise reduction are selected and characterized. Results of extensive experimental investigations in the acoustic wind tunnel Braunschweig are presented and discussed to point out the potential and limitations of the selected porous devices. Practical issues related to material manufacturing and integration into the wind tunnel model are addressed. The noise reduction potential of passive porous trailing-edge devices is found to strongly depend on both these aspects. Issues related to the characterization of the porous materials properties are described. Although porous materials are found to be successful at reducing trailing-edge noise emissions, the results indicate that there is still a need for more generic investigations to further clarify the parametric dependencies between noise reduction and material properties.

Uniform flow injection into a turbulent boundary layer for trailing edge noise reduction

The hydrodynamic effects of inclined uniform continuous blowing on a turbulent boundary layer are investigated experimentally. A laminar flow is introduced into the boundary layer through a fence on a flat plate at a distance of 3.38δ0 upstream of the trailing edge. The effects of this open-loop technique of flow control are examined at different angles of injection and at different blowing rates. Surface pressure fluctuations acquired from flush-mounted microphones are used to estimate the trailing edge noise. Injection angles of 70° and 90° in combination with strong blowing rates enable a noise reduction of up to 15 dB at mid and high frequencies, f > 300 Hz. Similar aeroacoustic performances are obtained at a blowing angle of 50° but at lower blowing rates. At low frequencies, a penalty is expected, with the trailing edge noise increasing for all the injection angles and blowing rates under analysis. Mean velocity profiles from hot-wire anemometry reveal that high injection angles and strong blowing rates induce a flow separation, which is expected to deteriorate the aerodynamic performances. When applying a uniform blowing at 50°, however, no flow separation occurs. From an aeroacoustic and aerodynamic point of view, uniform blowing applied at 50° and at intermediate blowing rates is found to be the most promising setting.

Numerical study of 3-D finlets using Reynolds-averaged Navier–Stokes computational fluid dynamics for trailing edge noise reduction

This paper uses Reynolds-averaged Navier–Stokes computational fluid dynamics to study trailing edge noise reduction with 3-D finlets. Reynolds-averaged Navier–Stokes computational fluid dynamics provides boundary layer parameters near a trailing edge for an empirical wall pressure spectrum model, and then an acoustic model predicts far-field noise based on pressure fluctuations obtained from the wall pressure spectrum model. First, this numerical approach is validated against experiments. Second, a comprehensive trend analysis is conducted to give insight into the design of 3-D finlets under different flow conditions. A data-driven turbulence spanwise length scale model is developed to tackle finlets with small spacing. Combined with acoustic results, detailed computational flow field results are analyzed to understand the physical mechanism of noise reduction. While the major part of the proposed mechanism is the same as prior work, several new observations are shown which better understand the physical mechanism of noise reduction with 3-D finlets. The goals of the current paper are to provide an efficient Reynolds-averaged Navier–Stokes-based approach to predict trailing edge noise of 3-D finlets, to give complete trend analysis results with various finlets under different flow conditions, and to advance an understanding of the underlying physics.