Trailing edge noise

The aerodynamic noise production mechanisms of modern horizontal axis wind turbines are reviewed. An engineering analysis of the time and frequency scales from three noise sources, leading edge turbulence interaction noise, trailing edge noise and blade-tower interaction noise is presented. The analysis shows that noise sources are present from low frequencies (1-4 Hz) to over 500 Hz for a representative wind turbine. The results of the analysis are used to explain amplitude modulation observed during noise measurements at a European wind farm. Daytime noise measurements close to a South Australian wind farm are also presented that show amplitude modulation. The paper concludes with a description of conceptual ideas for the control of wind turbine noise.

This paper presents an experimental investigation of the flow-induced noise created by a wall-mounted finite airfoil at low-to-moderate Reynolds number and zero angle of attack. Far-field noise measurements have been taken at a single observer location and with two perpendicular microphone arrays in an anechoic wind tunnel at Reynolds numbers of Rec=9.2×104–1.6×105, based on chord, and for a variety of airfoil aspect ratios (length to chord ratio of L/C=0.2–2, corresponding to length to thickness ratio of L/T=1.7–16.7). Additionally, surface oil-film visualisation images and unsteady velocity measurements taken in the near trailing edge wake are related to far-field noise measurements to determine the flow mechanisms responsible for noise generation. The results show that the wall-mounted finite airfoil radiates noise similar to a two-dimensional airfoil when L/T>8.3. Despite the incoming boundary layer height at the junction being 1.30≤δ/T≤1.46, junction and tip flow suppresses tonal noise production for airfoil׳s up to L/T=8.3 at Rec=9.2×104–1.2×105. Trailing edge noise is found to be the dominant airfoil noise generation mechanism at frequencies above 1 kHz with the position of the noise source along the trailing edge determined by the proportion of the airfoil span influenced by flow at the airfoil–wall junction.

This paper presents the results of an experimental investigation exploring the noise reduction potential of sawtooth trailing edge serrations on a flat plate at low-to-moderate Reynolds number (1.6e5 < Rec < 4.2e5). Acoustic and aerodynamic measurements have been taken using a flat plate with both sharp and serrated trailing edges in the anechoic wind tunnel at the University of Adelaide. Trailing edge serrations are found to achieve up to 13 dB of attenuation in the narrowband noise levels without modifying the directivity of the radiated noise. The noise reduction achieved with trailing edge serrations is found to be dependent on their geometrical wavelength and Strouhal number, St = fd/U, where f is frequency, d is boundary layer thickness and U is free-stream velocity. Far-field acoustic data are compared with theoretical noise reduction predictions showing that significant differences exist between measurements and theory. Velocity data measured in the very near trailing edge wake with hot-wire anemometry are related to the far-field noise measurements to give insight into the trailing edge serration noise reduction mechanism. The results suggest that for this particular configuration, the noise reduction capability of trailing edge serrations is related to their influence on the hydrodynamic field at the source location.

This paper presents the results of an experimental investigation of the flow-induced sound produced by a smooth, wall-mounted finite length airfoil with flat ended tip and a
tripped turbulent boundary layer. Acoustic measurements have been taken in the Stability Wind Tunnel at Virginia Tech with a microphone array at a range of Reynolds numbers, angles of attack and for a variety of airfoil aspect ratios. Spectral data show the dominant noise sources are airfoil trailing edge noise and tip vortex formation noise. Acoustic data are also used to evaluate semi-empirical prediction of wall-mounted finite airfoil trailing edge and tip noise with the so-called Brooks, Pope and Marcolini (BPM) model. The prediction method employs the BPM trailing edge noise model modified to incorporate span-wise variations in flow properties in combination with the BPM flat tip noise model. Three-dimensional trailing edge noise predictions agree well with measured spectra at a Strouhal number of StC < 18, based on airfoil chord. The BPM tip noise model under-predicts the peak level and frequency of tip noise contributions at StC > 18. A new empirical model of flat tip airfoil noise is presented that provides accurate estimation of the wall-mounted finite airfoil dominant tip noise contribution to within 1.7 dB.

"This paper presents experimental data concerning the flow and noise generated by a sharp edged flat plate at low-to-moderate Reynolds number (Reynolds number based on chord of 2.0 x 10^5 to 5.0 x 10^5). The data are used to evaluate a variety of semi-empirical trailing edge noise prediction methods. All were found to under-predict noise at lower frequencies. Examination of the velocity spectra in the near wake reveals that there are energetic velocity fluctuations at low frequency about the trailing edge. A semi-empirical model of the surface pressure spectrum is derived for predicting the trailing edge noise at low-to-moderate Reynolds number.

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Turbulent trailing edge noise is one of the most important sources of noise from wind turbines. Most of the understanding concerning the physics of turbulent trailing edge noise comes from experimental data obtained using NACA 0012 airfoil profiles in anechoic wind tunnel facilities. Further, nearly all semi-empirical models used for design purposes are based or validated against this data set. This paper compares turbulent trailing edge data for NACA 0012 airfoils from a variety of wind tunnel facilities as well as some modern computational studies. Even though the comparison is limited to zero angle of attack, the results are able to show the limitations of the current data set and the accuracy of two semi-empirical noise models. There is reasonable agreement across different data sets at moderate Reynolds numbers, but there is limited agreement at higher Reynolds numbers. With the design Reynolds number for modern and future wind turbine blades now exceeding the Reynolds number of the test data, there is a need to perform new experiments and update our existing prediction models.

This paper presents the results of an experimental investigation of the sound produced by flow interaction with a wall-mounted finite length airfoil at low-to-moderate Reynolds number. Acoustic measurements have been taken in an anechoic wind tunnel at a range of Reynolds numbers, angles of attack and for a variety of airfoil aspect ratios (airfoil length to chord ratio) with a single microphone and two perpendicular planar microphone
arrays. For comparison, measurements have also been taken with a semi-infinite or two- dimensional airfoil and a half-span airfoil with tip flow but no boundary layer impingement. The experimental data is used to examine changes in wall-mounted finite airfoil noise
production as a function of Reynolds number, angle of attack and airfoil aspect ratio. Additionally, the data gives insight into the airfoil noise generation mechanisms and the influence of flow at the airfoil tip and wall junction on noise production.

We demonstrate a new demultiplexer based on XPM in a fiber that utilizes the derivative feature of XPM induced spectral broadening. The basic idea is that the leading edge of the control pulse generates a red shift of the spectrum of the XPM modulated input signal, and the trailing edge generates a blue shift. We have previously reported wavelength conversion using this technique where the incoming data phase modulate continuous wave (CW) light with subsequent conversion to amplitude modulation. Here, only one of the OTDM data channels in the high bit-rate data is spectrally broadened and that channel can then be extracted with a narrow band optical band-pass filter at either side of the original spectrum. Thus, only one edge of the control pulse governs the width of the demultiplexing switch window, if dispersive walk-off is neglected. Therefore a control pulse broader than the actual bit slot can be used for demultiplexing. Another important feature of this demultiplexer compared ...

This paper explores the noise reduction potential of sawtooth trailing edge serrations on a flat plate at low-tomoderate Reynolds number. The noise radiated by a flat plate with both sharp and serrated trailing edges has been measured in an anechoic wind tunnel at the University of Adelaide. The noise measurements have been taken at a range of flow speeds (Reynolds numbers of Rec < 4.5 × 10^5, based on chord) for two different sawtooth geometries. Trailing edge serrations are found to achieve an average attenuation of up to 7 dB over a certain Strouhal number range, where Strouhal number is St = fδ/U∞, f is frequency, δ is boundary layer thickness and U∞ is the free stream velocity. The results of this study are compared with theoretical noise reduction predictions showing that significant differences exist between measurements and theory.

This paper presents results of an empirical study on the flow and noise generated by the trailing edge of a flat plate encountering low turbulence fluid flow. Simultaneous measurements of flow and far-field noise data have been taken in an anechoic wind tunnel at the University of Adelaide. Velocity data about the trailing edge and in the wake have been measured using hot-wire anemometry. These data are related to far-field noise measurements obtained with two microphones: one mounted above and one mounted below the trailing edge. Spectral maps of the fluctuating velocity about the trailing edge are also presented. Experimental data of this kind provides further insight into the trailing edge noise mechanism and can be used to validate theoretical noise predictions calculated using computational aeroacoustic techniques.

This paper presents the results of an experimental investigation exploring the noise-reduction potential of sawtooth trailing-edge serrations on a flat plate at low-to-moderate Reynolds number (1.6e5<Rec<4.2e5). Acoustic measurements have been taken using a flat plate with both sharp and serrated trailing edges in an anechoic wind tunnel. Trailing-edge serrations are found to achieve reductions of up to 13 dB in the narrowband noise levels, and this is mainly due to attenuation of vortex shedding at the trailing edge. Velocity data have also been measured in the very-near trailing-edge wake using hot-wire anemometry, and these data are related to the far-field noise measurements to give insight into the trailing-edge serration noise-reduction mechanism. The results show that, for this particular configuration, the noise-reduction mechanism of trailing-edge serrations is dominated by their influence on the hydrodynamic field at the source location. Therefore, the assumption that the turbulent field is unaffected by the serrations is not valid and explains why theory is not able to explain experimental observations.

"This paper presents an experimental study on the noise reduction potential of sawtooth trailing edge serrations at low-to-moderate Reynolds number (1.6e5 < Rec < 4.2e5, based on chord). The noise radiated by flat plates with both straight and serrated trailing edges has been measured in an anechoic wind tunnel at the University of Adelaide. The trailing edge serrations were found to reduce narrowband noise levels by up to 13 dB. The straight and serrated trailing edge far-field acoustic spectra are scaled according to existing theory and compared to predictions of trailing edge noise obtained using current semi-empirical methods. In addition, velocity spectra measured in the trailing edge near wake region using hot-wire anemometry are related to the far-field noise measurements to give further insight into the serration noise reduction mechanism.

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This paper describes the airfoil trailing edge noise generation mechanism and how flow over an airfoil can create tonal or broadband noise. Examples of vortex shedding as well as tonal and broadband noise spectra are presented. A brief review of how trailing edge noise can be be predicted computationally is given and some results shown using a new industrially friendly computationalmethodology that couples with conventional steady flow simulation software. The paper concludes with a discussion of passive trailing edge noise control devices and their effectiveness.

This paper presents results of a Computational Fluid Dynamic (CFD) study of a sharp-edged symmetric flat strut at Reynolds number 500,000 based on chord at zero degrees angle of attack, and the subsequent estimation of far-field noise generated at the trailing-edge. Flow field results obtained using Improved Delayed Detached Eddy Simulation (IDDES) modelling and Reynolds-Averaged Navier Stokes (RANS) modelling techniques are compared with empirical wind-tunnel data. The flow is observed to be physically complex in nature, exhibiting numerical solutions that are sensitive to the mesh grid and freestream turbulence intensity. Although originally developed for use specifically with RANS-generated flow data, the RANS-based Statistical Noise Model (RSNM) technique, which estimates far-field noise from mean turbulence data via an acoustic Green's function and a statistical turbulence correlation model, is used here to estimate far-field noise spectra from both RANS and IDDES flow data. Far-field noise is also estimated from the IDDES model using the permeable surface form of the Ffowcs Williams and Hawkings (FWH) solver. The FWH estimate gives the closest match to experimental data, while the RSNM-generated noise estimate from the IDDES data appears to be more successful at capturing the large turbulent structures within the flow than the RANS data.

Junction flows occur when a boundary layer encounters a wall-mounted obstacle and are a source of unwanted noise for a wide range of engineering applications that include air, water and land vehicles, wind turbines and turbomachinery. This paper examines the sound produced by two types of junction flows: a wall-mounted finite length circular cylinder and a wall-mounted finite length NACA 0012 airfoil. Noise measurements have been taken in an anechoic wind tunnel at the University of Adelaide at a range of flow speeds and for a variety of aspect ratios (length to cylinder diameter or airfoil thickness ratio) to determine the influence of these parameters on noise generation. The experimental data presented in this paper give improved insight into the underlying sound generation physics and can be used to validate numerical predictions of junction flow noise.

The effect of leading-edge geometry on the wake/boundary-layer interaction was studied in a low-speed single-stage HP compressor. Both a 3:1 elliptic and a circular leading edge were tested on a controlled diffusion aerofoil stator blade. Experiments were undertaken on the stator suction surface; these included hotwire boundary-layer traverses, surface hotfilm measurements, and high resolution leading-edge pressure measurements. Steady computational fluid dynamics (CFD) predictions were also performed to aid the interpretation of the results. The two leading-edge shapes gave rise to significantly different flows. For a blade with an elliptic leading edge (Blade A), the leading-edge boundary layer remained attached and laminar in the absence of wakes. The wake presence led to the formation of a thickened laminar boundary layer in which turbulent disturbances were observed to form. Measurements of the trailing-edge boundary layer indicated that the wake/leading-edge interaction for Bl...

The flow and noise created by trailing edge serrations has been studied experimentally, at low Reynolds number (Rec = 1e5 - 1.3e5, based on chord). Wide trailing edge serrations with a wavelength (s) to amplitude (2h) ratio of s/h = 0.6 were found to reduce the overall sound pressure level by up to 11 dB. In contrast, narrower trailing edge serrations with s/h = 0.2 produce high amplitude tonal noise and an increase in the overall sound pressure level of up to 4 dB. Unsteady flow data in near the trailing edge wake show that the narrow serrations are responsible for the formation of intense vortices across the span and this results in the production of high amplitude tonal noise. Wide serrations reduce turbulent velocity fluctuations at low frequencies which explains the lower radiated noise.

"This paper presents results of a comprehensive empirical study on the flow and noise generated by the trailing edge of a flat strut at low-to-moderate Reynolds number. Simultaneous measurements of the flow and far-field noise have been taken for three flat plate models with new trailing edge geometries in an anechoic wind tunnel at the University of Adelaide. The far-field acoustic data are scaled according to existing theory and compared with predictions of trailing edge noise obtained using current semi-empirical methods. Mean and unsteady velocity data have been measured in the very near trailing edge wake with hot-wire anemometry and these data are related to the far-field noise measurements. Additionally, spectral maps of the
fluctuating velocity about the trailing edge are presented. Experimental data of this kind provides greater insight into the trailing edge noise mechanism and can be used to further develop and validate trailing edge noise prediction models."

Numerical predictions of three-dimensional flow and heat transfer are presented for a rotating two-pass rectangular channel with 45-deg rib turbulators and channel aspect ratio of 2:1. The rib height-to-hydraulic diameter ratio e/Dh is 0.094 and the rib-pitch-to-height ratio P/e is 10. Two channel orientations are studied: β=90deg and 135 deg, corresponding to the mid-portion and the trailing edge regions of a turbine blade, respectively. The focus of this study is twofold; namely, to investigate the effect of the channel aspect ratio and the channel orientation on the nature of the flow and heat transfer enhancement. A multi-block Reynolds-averaged Navier-Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure. In the present method, the convective transport equations for momentum, energy, and turbulence quantities are solved in curvilinear, body-fitted coordinates using the finite-analytic method. The numerical results compare reasonably ...

A high-fidelity computational analysis carefully validated against concurrently obtained experimental results is employed to examine self-noise radiation of airfoils at transitional flow regimes, with a focus on elucidating the connection between the unsteady behavior of the laminar separation bubble (LSB) and the acoustic feedback-loop (AFL) resonant interactions observed in the airfoil boundary layers. The employed parametric study examines AFL sensitivity to the changes in the upstream flow conditions and the airfoil loading. Implicit Large-Eddy Simulations are performed for a NACA-0012 airfoil in selected transitional-flow regimes for which experimental measurements recorded characteristic multiple-tone acoustic spectra with a dual ladder-type frequency structure. The switch between the tone-producing and no-tone-producing regimes is traced to the LSB size and position as a function of the flow Reynolds number and the airfoil angle of attack, and further substantiated by the lin...

Derivative-free optimization techniques are applied in conjunction with large-eddy simulation (LES) to reduce the noise generated by turbulent flow over a hydrofoil trailing edge. A cost function proportional to the radiated acoustic power is derived based on the Ffowcs Williams and Hall solution to Lighthill&#39;s equation. Optimization is performed using the surrogate-management framework with filter-based constraints for lift and drag. To make the optimization more efficient, a novel method has been developed to incorporate Reynolds-averaged Navier–Stokes (RANS) calculations for constraint evaluation. Separation of the constraint and cost-function computations using this method results in fewer expensive LES computations. This work demonstrates the ability to fully couple optimization to large-eddy simulation for time-accurate turbulent flow. The results demonstrate an 89% reduction in noise power, which comes about primarily by the elimination of low-frequency vortex shedding. T...

Turbulent boundary layers separating from a blunt trailing edge give rise to organized vortical structures in the downstream wake. The perturbation of this inherent flow-instability at f0 by controlled oscillations of the edge at fe produces corresponding, organized components of unsteady surface pressure along the edge. For edge excitation near the ‘natural’ vortex shedding frequency f0, the phase between the local pressure fluctuations and the edge displacement shows large changes for small changes in excitation frequency. Moreover, in this range of excitation, there is quenching (or attenuation) of the surface pressure component at f0 and resonant peaking of the component at fe. These phenomena are related to the change in sign of the energy transfer between the fluid and the body. Integration of the instantaneous pressure distributions along the surfaces of the edge leads to the instantaneous lift at fe and f0 acting upon the oscillating trailing edge. The location of the lift v...

A series of direct numerical simulations of pulsatile flows in pipes with a constriction are presented here. Results capture the flow features reported in earlier experiments in the literature and confirm a qualitatively similar multi-step process to transition to turbulence observed in planar configurations. In particular, an instability of the shear layer leads to the formation of an array of vortices rings. Transition to turbulence takes place as these vortex rings undergo three-dimensional instabilities. We will present a systematic study of the effects of: 1. geometry of the constriction; 2. percent occlusion; 3. inflow conditions, to the above transition process. In addition, the effects of blood rheology on the results will be explored via numerical experiments with a variety of non-Newtonian models.

This paper presents a recent experimental program that measures the flow-induced noise generated by a wall-mounted finite airfoil in the Stability Wind Tunnel at Virginia Tech, USA. Far-field noise measurements have been taken at a single observer location and with a microphone array at a Reynolds number of Rec = 1.6×106 (based on airfoil chord) and for a variety of airfoil angles of attack from a = 0◦ to 8◦. The experimental data presented in this paper give insight into the affect of three-dimensionality on noise generation and show how flow at the junction and tip influence noise production at the source locations.

This letter presents an experimental study on the tonal noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number. Flow and far-field noise data reveal that, in this particular case, the tonal noise appears to be governed by vortex shedding processes. Also related to the existence of the tonal noise is a region of separated flow slightly upstream of the trailing edge. Hydrodynamic fluctuations at selected vortex shedding frequencies are strongly amplified by the inflectional mean velocity profile in the separated shear layer. The amplified hydrodynamic fluctuations are diffracted by the trailing edge, producing strong tonal noise.