Serration manufacturing effects on propeller trailing edge noise mechanisms

Experimental results on trailing-edge (TE) noise from a NACA airfoil are presented for a chord-based Reynolds number range between and . Far-field TE noise from the baseline airfoil with a straight TE and TE serrations is measured with varying , 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 range, two NACA 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 at which the peak noise level is measured reduce as increases. TE noise spectra for the cases with the TE serrations are found to be dependent on the airfoil lift and . 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.

This paper discusses unique measurements of wall-pressure fluctuations (WPFs), spanwise correlation length, and far-field noise of a NACA 0008 airfoil subjected to uniform, non-turbulent and turbulent inflows. From these measurements, the near-field characteristics of trailing-edge (TE) and leading-edge (LE) noise of a thin airfoil are analyzed. The competing nature of the LE and TE noise mechanisms of an airfoil subjected to a turbulent inflow is also investigated. Experiments were performed in the Aeroacoustic Wind Tunnel of the University of Twente for a chord-based Reynolds number ranging from 3.2×105 to 9×105 for a uniform inflow and a rod-generated turbulent inflow with a turbulence intensity of approximately 18%. The effective angles of attack ranged from -5∘ to 5∘. Measurements of the boundary layer at the TE, the WPFs along the chord and span, and the far-field radiated noise were performed. For the uniform inflow case, laminar-boundary-layer noise, turbulent-boundary-layer (TBL) noise, and blunt TE noise sources are identified. The far-field noise spectrum presents similar components in the frequency domain as the WPF spectrum and the spanwise correlation length at the TE. The angle of attack mainly affects the WPF spectrum and spanwise correlation length for chord-based Strouhal numbers St<10. The angle of attack slightly affects the TE noise with a maximum variation of 3 dB for 10<St<60. For the turbulent inflow case, it is observed that the turbulence significantly affects the WPFs and spanwise correlation length along the chord, increasing considerably compared to the case of uniform inflow. The highest WPF spectral level and the larger spanwise correlation length occur at positions close to the LE. The results show that the turbulent inflow yields a higher WPF spectral level and larger spanwise correlation length at the TE, resulting in higher levels of TE noise. However, LE noise is still the dominant noise source for St<25.2. For higher frequencies, the TE noise level is expected to become higher than the LE noise level.

Trailing edge (TE) noise measurements for a NACA 63–215 airfoil model are presented, providing benchmark experimental data for a cambered airfoil. The effects of flow Mach number and angle of attack of the airfoil model with different TE bluntnesses are shown. Far-field noise spectra and directivity are obtained using a directional microphone array. Standard and diagonal removal beamforming techniques are evaluated employing tailored weighting functions for quantitatively accounting for the distributed line character of TE noise. Diagonal removal processing is used for the primary database as it successfully removes noise contaminants. Some TE noise predictions are reported to help interpret the data with respect to effects of flow speed, angle of attack, and TE bluntness on spectral shape and peak levels. Important findings include the validation of a TE noise directivity function for different airfoil angles of attack and the demonstration of the importance of the directivity function's convective amplification terms.

This study concerns the mechanisms of the turbulence broadband noise reduction for an airfoil with the trailing edge serrations. An open jet wind tunnel test of a SD2030 airfoil (with 4% camber, 8.0% thickness) with and without the trailing edge serrations was comparatively investigated. The Dantec Dynamics 3D hot wire anemometry was used to measure the development and intensity of turbulence boundary layer and wake of the airfoil. The noise source identifying technique with the microphone array was used to focus on the trailing edge noise radiation. The far-field acoustics DSP and fluctuating turbulence spatiotemporal information around airfoil were compared for the airfoil with standard straight trailing-edge and the airfoil with the serrated trailing-edges. It is shown that trailing edge noise could be effectively reduced with the using of serrated trailing edge, and the magnitude and frequency range of airfoil trailing edge noise reduction are changed with the increase of main flow velocity. In the frequency range of 10 kHz, the largest noise reduction of sound pressure level is about 5 dB with the using of serrated trailing edge in the test flow speed range. The predicted results of Howe’s theoretical model are in a good agreement with the experiment results when St = ωδ/Uc ≈ 1.2. However, Howe’s theoretical model over-predicted noise reduction levels in high frequency while lowly-predicted the noise suppression in low frequency range. Near-field turbulence measurements show that serrated trailing edge could reduce turbulence strength on some directions and on some positions, and could also increase turbulence strength on other directions and on other positions. This result indicates that noise reduction mechanism with the serrated trailing edge is very complicated. Some special point around serrated trailing edge should be noted in the investigation of the noise reduction of airfoil with serrated trailing edge.

In the current study, we investigate a route to reduction of the turbulent boundary layer–trailing edge interaction noise. The trailing edge noise is generated by surface pressure fluctuations beneath a turbulent boundary and scattered at the trailing edge of wind turbine blades. Trailing edge noise is considered to be the dominant noise source of modern wind turbines. Therefore, efforts are constantly made to attenuate the noise. Today, noise emission can be reduced by proper airfoil design or passive devices, such as trailing edge serrations. A further improved candidate technology for trailing edge noise attenuation is active flow control in the form of wall‐normal suction. With active flow control, the boundary layer features responsible for trailing edge noise generation can be manipulated, and correspondingly the trailing edge noise can be reduced. Detailed experimental investigations were performed at the Universities of Tel‐Aviv and Stuttgart. The tests showed that steady wall‐normal suction has a positive effect on the trailing edge noise by reducing the boundary layer thickness, and with it the integral length scales of the eddies within the boundary layer. Copyright © 2014 John Wiley &amp; Sons, Ltd.

This paper is a continuation of a series of study on the mechanism of the broadband noise reduction for turbomachinery blade using trailing edge serrations. The noise reduction potential of turbine blade with trailing edge serrations is experimentally assessed as well as the various parameters on the noise reduction effect. Special focus is put on whether the trailing edge serrations affect turbine cascade tailing edge noise in the same way as they do on the isolated airfoil. Five different trailing edge serrations were designed for a turbine linear cascade to investigate the effects of serration geometry parameter on the noise reduction. A linear microphone array was used to quantify the difference of sound source levels of turbine cascade with and without trailing edge modifications. The experiment was carried out at various velocities and the Reynolds number (based on cascade inlet velocity and chord) ranges from 1.3×105 to 3.3×105. The experiment results show that trailing edge serrations can reduce turbine trailing edge noise in a wide frequency range that we are interested (from 1600Hz to 10000Hz) and a maximum noise reduction of about 5dB is obtained in the mid frequency range (2000Hz to 4000Hz). The results show that the serration length has an important effect on the noise reduction effect and the longer serration in the experiment lead to more noise reduction. However, serration wavelength has only a little effect on the noise reduction although the wider trailing edge serrations tested in the experiment can achieve slightly more noise reduction. This is quite different from that for airfoils. At all the velocities tested, the cascade trailing edge noise is effectively reduced and the maximum noise reduction occurs at St=2fh/U≈1.

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.

This paper is one of the series study on the mechanisms of the turbulence broadband noise reduction for turbomachinery blade with the trailing edge serrations. The trailing edge noise reduction potential of compressor blade with trailing edge serrations is experimentally investigated in a linear cascade wind tunnel with 5 compressor blades. The measurement technique of the linear microphone array was used to quantify differences of sound source levels of compressor blades withand withouttrailing edge modifications. The results show that compressor cascade trailing edge noise is effectively reduced with the using of serrated trailing edge. Serrations with sharper geometry have more noise reduction potential. The Experimental results also show that compressor turbulent trailing edge noise is considerably reduced in the frequency range of 1600-5000Hz. The largest noise reduction of sound pressure level is about 2 dB with the using of serrated trailing edge in the test. The comparison of present results with the previous turbine cascade noise reduction experimental results indicated that as inflow velocity increases, the noise reduction effect of serrated trailing edge for turbine cascade becomes better, and the serration with blunt geometry have more noise reduction potential for turbine cascade, these are opposite to the results of compressor cascade.

Trailing edge (TE) noise measurements for a NACA 63-215 airfoil model are presented, providing benchmark experimental data for a cambered airfoil. The effects of flow Mach number and angle of attack of the airfoil model with different TE bluntnesses are shown. Far-field noise spectra and directivity are obtained using a directional microphone array. Standard and diagonal removal beamforming techniques are evaluated employing tailored weighting functions for quantitatively accounting for the distributed line character of TE noise. Diagonal removal processing is used for the primary database as it successfully removes noise contaminates. Some TE noise predictions are reported to help interpret the data, with respect to flow speed, angle of attack, and TE bluntness on spectral shape and peak levels. Important findings include the validation of a TE noise directivity function for different airfoil angles of attack and the demonstration of the importance of the directivity function s convective amplification terms.

This thesis deals with the detection of aeroacoustic sound sources on aircraft and wind turbines using phased microphone arrays. First, the reliability of the array technique is assessed using airframe noise measurements in open and closed wind tunnels. It is demonstrated that quantitative acoustic measurements are possible in both wind tunnels. Then, the array technique is applied to characterize the noise sources on two modern large wind turbines. It is shown that practically all noise emitted to the ground is produced by the outer part of the blades during their downward movement. This asymmetric source pattern, which causes the typical swishing noise during the passage of the blades, can be explained by trailing edge noise directivity and convective amplification. Next, a semi-empirical prediction method is developed for the noise from large wind turbines. The prediction code is successfully validated against the experimental results, not only with regard to sound levels, spectra, and directivity, but also with regard to the noise source distribution in the rotor plane and the temporal variation in sound level (swish). The validated prediction method is then applied to calculate wind turbine noise footprints, which show that large swish amplitudes can occur even at large distance. The influence of airfoil shape on blade noise is investigated through acoustic wind tunnel tests on a series of wind turbine airfoils. Measurements are carried out at various wind speeds and angles of attack, with and without upstream turbulence and boundary layer tripping. The speed dependence, directivity, and tonal behaviour are determined for both trailing edge noise and inflow turbulence noise. Finally, two noise reduction concepts are tested on a large wind turbine: acoustically optimized airfoils and trailing edge serrations. Both blade modifications yield a significant trailing edge noise reduction at low frequencies, but also cause increased tip noise at high frequencies. Nevertheless, it is demonstrated that using trailing edge serrations wind turbine noise can be halved without adverse effects on the aerodynamic performance.

Fan Broadband Self Noise Prediction Model

This paper document the evaluation of a zonal RANS-LES approach for the prediction of broadband and tonal noise generated by the flow past an airfoil trailing edge at a high Reynolds number. A multi-domain decomposition is considered, where the acoustic sources are resolved with a LES sub-domain embedded in the RANS domain. At the RANS-LES interface, a stochastic vortex method is used to ge nerate synthetic turbulent perturbations. The simulations are performed with the general-purpose unstructured control-volume code FLUENT. The far-field noise is calculated using the aeroacoustic analogy of FfowcsWilliams and Hawkings. The results of the simulation are compared with available acoustic and mean velocity measurements. The investigation demonstrates the ability of this approach to predict the aerodynamic and aeroacoustic properties of the flow. Two simulations are performed in order to address the s ensitivity of the results to the perturbation model. The comparison clearly indicates the critical influence of the model I. Introduction HIS study deals with numerical prediction of airfoi l trailing edge noise. Trailing-edge aeroacoustics is of importance in both aeronautical and naval applicati ons. The dipole sound produced by the edge scatteri ng of pressure fluctuations at a trailing edge is most of ten an undesirable effect. These pressure fluctuati ons are created by turbulent eddies as they convect over the trailing edge. This causes edge scattering of noise to the f ar field. This scattering mechanism can produce strong broadband and/or tonal noise which is radiated to the far fiel d. In this work, an Hybrid zonal RANS/LES unsteady CFD simulation is used to get a prediction of the acou stic sources, which are then use as an entry data of an acoustic propagation model. The case under study c orresponds to the recent experiment conducted by Kunze 1 . The trailing edge shape considered is identical w ith on of the trailing edge shapes previously investigated by Blake 2 .

Serrated trailing edge structure based on the principle of bionics of the airfoil was an effective means to reduce the airfoil trailing edge noise, but the physical mechanism of noise reduction is still not fully studied. Characteristics of turbulent wake flow and noise results of airfoils with and without serrated trailing edge would be comparatively analyzed through hot wire anemometer and line array. It can be seen wake flow turbulence and the turbulence intensity in three directions of serrated trailing edge airfoil are significantly reduced compared to baseline airfoil. The sound field showed trailing edge noise had a huge decrease on serrated trailing edge airfoil, the leading edge noise is nearly the same. Serrated trailing edge structure widens the wake region and accelerates the large vortex to breakdown, resulting in additional horseshoe vortex. Turbulence decay rate along the flow direction becomes larger.

This experimental work investigates the trailing-edge (TE) noise from a NACA0012 airfoil in an open-jet wind tunnel, for chord-Reynolds numbers between 1e5 and 4.6e5 and angles of attack between 0 and 6 degrees. The range of parameters for which TE noise is either tonal or broadband in the present experiments is in good agreement with existing literature results. One of the main objectives of this work is to test the assumption of recent modelling approaches based on the linearised Navier-Stokes operator. These studies focus on spanwise coherent structures in the turbulent boundary layer to investigate the mechanisms responsible for trailing edge noise, as they always satisfy the trailing edge scattering condition. However, numerical simulations routinely use a narrow numerical domain and periodic lateral boundary conditions, which could favour spanwise coherent dynamics. Therefore, particular emphasis is placed on the experimental characterisation of the spanwise wavenumber content of the pressure fluctuations on the airfoil surface and in the acoustic field. A good agreement with theoretical and numerical observations is found, as the spanwise wavenumber contents of the acoustic field are in good agreement with the edge scattering condition. Furthermore, the coherence between the surface pressure fluctuations and the acoustic fields is significantly improved when considering spanwise-coherent structures by spanwise averaging of the temporal signals, even in the case of 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.