© 2023, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.Correction Notice Figure 9 caption should read: Acoustic spectra contour plot for the L/D= 3, r/D = 0.6 and ɑ = 0 deg jet/flat plate configuration, φ ≈ 0 deg (black curves) and 180 deg (blue curves), i.e. plate unshielded and shielded sides, respectively. Frames, from top to bottom: θ ≈ 90, 70, 50 and 30 deg. Frames, from left to right: Mach numbers of 0.4, 0.7 and 0.9. Acknowledgements should read: The authors would like to thank Ms. Barbara G. Hasparyk for her technical support with the assembling of the antenna, microphones calibration, writing the post-processing codes and many insightful conversations. We are also in debt with the Prométée plateforme technical staff and with Prof. JeanChristophe Robinet (DynFluid Laboratory). Part of this research was funded by Airbus and we thank Dr. Jérôme Hubert and Dr. Grégoire Pont for their inputs.

© 2023, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.This paper reports acoustic and velocity-field measurements on subsonic elliptical jets of 1:2 aspect ratio and Dh = 50 mm nozzle hydraulic diameter. The experiments were performed in the 0.4 to 0.9 Mach number (Mj) range, and all tests were conducted in an anechoic facility. Acoustic power spectral density (PSD) results were obtained with an antenna containing 18 azimuthally distributed microphones at a radius R/Dh = 14.3, and for polar angles in the 20 deg ≤_θ≤ 95 deg range. We show that the acoustic field of the 1:2 aspect ratio elliptical nozzle is very similar to the (axisymmetric) round nozzle acoustic field measurements, even for θ = 90 deg, i.e. the position closest to the nozzle exit. The spectra of both nozzles, round and elliptical, have approximately the same signature, i.e. broadband, and noise intensity levels. Stereo particle image velocimetry (stereo PIV) measurements enabled the assessment of the three velocity components for a series of cross-flow planes up to the streamwise distance x/Dh = 15 (750 mm). Such measurements were conducted only for Mj = 0.4 and 0.7. The results show that at least up to x/Dh = 3, the jet flow field keeps a proper elliptical profile, including a well-defined shear-layer. Far downstream, e.g. x/Dh = 8, it is observed a merging of the shear-layer with the jet core flow and, regarding mean flow and root-mean-square (rms) metrics, the developed flow field becomes approximately round.

© 2023, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.This paper reports aeroacoustic experiments on round jets containing tab elements adhered to the nozzle internal surface with the purpose of generating steady streaks in the nozzle boundary-layer. Such streaks are theoretically expected to reduce growth rates associated with the Kelvin-Helmholtz mechanism and, in turn, to reduce jet noise. Nozzle configurations with and without a boundary-layer transition trigger element (carborundum trip), were studied. Stereo particle image velocimetry (stereo PIV) was employed to measure the three components of the velocity for a series of planes parallel to the nozzle exit at Mj = 0.7 in the 0.03 xD 10 streamwise range, where x is the streamwise distance and D is the jet diameter. Such measurements clearly show alternating regions of high and low speed flow due to the streaks that were induced by the tabs on the nozzle boundary-layer and are sustained in the jet shear-layer up to at least xD = 3. The acoustic experiments were performed in an anechoic facility, using an azimuthal array containing 18 equally-spaced microphones to characterize the acoustic field. The antenna was employed to conduct measurements at 15 streamwise stations in the 20 deg θ 90 deg polar range. All acoustic experiments were conducted in the 0.4 Mj 0.9 Mach number range. The presence of the tab elements leads to noise reductions of up to 6 dB/St, observed for Strouhal numbers in the 0.1 St 0.5 range, Mj = 0.4, axisymmetric azimuthal mode and untripped boundary-layer case. When the trip mechanism is present, the noise reduction is up to 3 dB/St. An overall sound pressure level (OASPL) reduction of up to 3 dB was measured for axisymmetric mode of the tabbed case for Mj = 0.4. As the tabs were designed based on boundary-layer measurements at Mj = 0.4, the noise reduction decreases with increasing Mach number. Nevertheless, significant noise reductions of up to 1.5 dB are still observed up to Mj = 0.9 and axisymmetric mode. Moreover, the noise reduction is up to 6 dB for the two first helical modes. The noise reduction was measured at both lower and higher polar angles and for almost the entire frequency range, up to at least St 2. Streak-inducing devices such as the present tabs are thus a promising approach to reduce jet noise

© 2023, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.An array of cylindrical roughness elements was used to reduce the tonal noise introduced by the separation bubble over a NACA 0012 airfoil at low angles of attack. Experiments were performed for four configurations: a baseline smooth airfoils, two with roughness elements at only one of the airfoil surfaces (pressure side or suction side), and the other with roughness elements at both airfoil surfaces. Arowof spanwise periodically spaced cylinderswas placed close to the mid-chord position in order to induce streaks that render the bubble three-dimensional, decreasing separation and stabilizing the Kelvin-Helmholtz instability of the separated shear layer, which is related to tonal noise. Our results show a decrease, and in some cases the total suppression, of the tonal noise at Reynolds numbers ranging from 0.6×105 to 2.5×105, and angles of attack ranging from 0 to 4 degrees.

© 2023, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.This paper reports experimental aeroacoustic results on subsonic installed jet obtained in an anechoic facility under isothermal condition for 0.4 ≤ Mj ≤ 0.9 Mach number range in order to extend the knowledge of jet/plate acoustics. A flat plate is employed to mimic a flap (or wing) and the acoustic measurements were performed with an azimuthal microphones array sweeping the 20 deg ≤ θ ≤ 95 deg polar range, and the microphones positioned at a radius of R/D = 14.3. The studied parametric space includes 0.5 ≤ r/D ≤ 1.5 radial distance between the flat plate trailing-edge and the jet axis, L/D = 2 and 3 streamwise distances between the flat plate trailing-edge and nozzle lip and 0 deg ≤ α ≤ 45 deg flat plate incidence angles. Overall, three different spectral signatures were identified for the installed jet noise: broadband, weak (linear) tones and strong (non-linear) tones. The regime of strong amplitude level harmonic (non-linear) peaks was detected in the noise spectra for the 0.5 ≤ r/D ≤ 0.6 radial position for Mj ≥ 0.7. Weak amplitude level tones were identified in the 0.5 ≤ r/D ≤ 0.9 for Mj ≥ 0.6 and only broadband noise was observed for r/D > 0.9. For radial positions r/D < 0.9 and Mj < 0.6, as well as for r/D ≥ 1 and all studied Mach numbers, the installation noise is essentially of broadband nature. A transition regime, in which it is not possible to verify if the weak peaks are present or not, was also identified for some plate configurations (r/D > 0.6) and Mach numbers (Mj ≤ 0.6). The stronger peaks were measured for the r/D = 0.5, L/D = 3 and α = 45 deg configuration, with spectral levels up to 145 dB/St at the polar angle θ = 90 deg.

© The Author(s), 2021. Published by Cambridge University Press on behalf of Royal Aeronautical Society.This paper introduces the Low Acoustic Noise and Turbulence (LANT) wind tunnel of the Sao Carlos School of Engineering, University of Sao Paulo (USP-EESC), Brazil. The closed-loop wind tunnel features several devices to improve flow uniformity, reduce swirl, and lower the background acoustic noise and turbulence, enabling stability and aeroacoustic experiments. The design criteria was based on the best practices reported, in particular for low turbulence wind tunnels. Yet, these criteria are conflicting and we discuss the decisions that had to be made and present flow quality results that were achieved. The 16-bladed axial fan with 13-blade stators is driven by a variable-speed electric motor. At the corners, 100 mm dense acoustic foam is installed on the vertical walls, floor and ceiling, and the turning vanes are filled with acoustic-absorbing material. The long settling chamber contains a 3.175 mm mesh hexagonal honeycomb and five fine mesh nylon screens, ending in a 7:1 area ratio short contraction. The 3-m long closed-working section has a cross-section area. At 15 m/s the working section wall boundary layer is less than 100 mm thick, providing an area of at least where the streamwise flow uniformity was within 1%. In the 10-30 m/s flow speed range, the turbulence intensity ranged from 0.05% to 0.071% and the background acoustic noise level, obtained with an inflow microphone, ranged from 90 and 110 dB. A benchmark experiment on a flat plate boundary layer produced an almost perfect two-dimensional Blasius profile up to. A beamforming benchmark experiment on aeroacoustics accurately identified the sound emitted by a cylinder immersed in the flow.

© 2022 12th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2022. All rights reserved.A resolvent-based methodology is employed to obtain non-causal spatio-temporal estimates of turbulent pipe flow from low-rank probe measurements of wall shear-stress fluctuations. DNS and LES pipe flow numerical simulations at friction Reynolds number of 550 are used as databases. We consider one of the DNS databases as the true spatio-temporal flow field, from which the low-rank measurements are extracted. Such database is also employed to verify the accuracy of the linear estimators. The estimator needs a model for the nonlinear (or forcing) terms of the Navier-Stokes equations system, which are obtained from a DNS database and from a series of computationally cheaper LES databases with grids coarser than the DNS. Comparisons between the reference DNS and the estimates indicate that sufficiently accurate results can be achieved with cheaper LES containing up to 10% of the number of grid points of the DNS, with estimates closely matching the reference DNS results up to the buffer-layer and reasonable agreement up to the beginning of the log layer.

© 2022, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.The leading-edge slat has been identified as a relevant noise source since the 1970’s. However, many issues regarding the noise generation mechanisms and the appropriate tools to study them are still under debate. Particularly, details of the narrow-band peak generation mechanism and the use of rigid-wall wind tunnels are open issues. The purpose of this paper is to revisit earlier results in the light of recent advances achieved in this topic, in an attempt to better understand these aspects of the slat noise. It is shown that a feedback model that assumes the trailing edge as the main emission spot and a one-quarter-period phase difference between the hydrodynamic and the acoustic fluctuations in this position is corroborated by measurements from different research groups. Based on this feedback model, the uneven relevance of the slat gap, overlap and deflection angle to the peak amplitude can be explained. Mach scaling laws for the narrow-band peak amplitudes substantially higher than the expected fifth power is pointed out in several previously published data, which suggests the feedback nature of the underlying mechanism. Finally, the implications of this mechanism to the set-up of wind-tunnel tests are discussed.

© Authors 2021We employ a resolvent-based methodology to estimate velocity and pressure fluctuations within turbulent channel flows at friction Reynolds numbers of approximately 180, 550 and 1000 using measurements of shear stress and pressure at the walls, taken from direct numerical simulation (DNS) databases. Martini et al. (J. Fluid Mech., vol. 900, 2021, p. A2) showed that the resolvent-based estimator is optimal when the true space-Time forcing statistics are utilised, thus providing an upper bound for the accuracy of any linear estimator. We use this framework to determine the flow structures that can be linearly estimated from wall measurements, and we characterise these structures and the estimation errors in both physical and wavenumber space. We also compare these results to those obtained using approximate forcing models-an eddy-viscosity model and white-noise forcing-and demonstrate the significant benefit of using true forcing statistics. All models lead to accurate results up to the buffer layer, but only using the true forcing statistics allows accurate estimation of large-scale logarithmic-layer structures, with significant correlation between the estimates and DNS results throughout the channel. The eddy-viscosity model displays an intermediate behaviour, which may be related to its ability to partially capture the forcing colour. Our results show that structures that leave a footprint on the channel walls can be accurately estimated using the linear resolvent-based methodology, and the presence of large-scale wall-Attached structures enables accurate estimations through the logarithmic layer.

© The slat represents an important airframe noise source as it extends over almost the entire aircraft wingspan. Most studies of slat noise consider idealized geometries. However, for practical applications, several elements are installed on its cove, such as bulb seals to avoid direct contact with the main wing surface. Previous investigations of an unswept and untapered MD30P30N airfoil reported that the flow dynamics and the corresponding acoustic noise are very sensitive to the presence and location of the bulb seal. For certain locations a second recirculation bubble is created inside the slat cove and the acoustic narrowband peaks are intensified. The present paper shows that the two-bubble topology promotes the recirculation of turbulence within the slat cove. Spectral proper orthogonal decomposition analysis based on the radiated pressure intensity is used to identify the flow structures responsible for sound generation. Even though the recirculating turbulence is mostly incoherent, it interacts with the coherent Kelvin-Helmholtz vortices in the initial part of the mixing layer. Then, vortex merging and straining lead to the formation of complex vortex clusters. Our results show that the origin and evolution of these clusters are consistent with Rossiter's mechanism responsible for the narrowband peaks. The enhanced recirculation accelerates the cluster evolution leading to wider clusters and lower-frequency Rossiter modes.

© 2019 Elsevier LtdPhased-array microphone techniques are widely used tools for the measurement and analysis of aeroacoustic noise sources. Nevertheless, beamforming results of sources distributed along a line, such as airfoil trailing-edge and slat, have not been entirely understood. This paper investigates the performance of beamforming methods in representing sources distributed along a line and estimating their spectral level. Conventional beamforming and the deconvolution techniques DAMAS and CLEAN-SC were employed for both synthetic and physical sources. The physical source is the slat of an MD30P30N high-lift model tested in a closed-section wind-tunnel with array microphones mounted flush to the tunnel wall. The synthetic source consisted of a large number of uncorrelated aligned monopole point sources. DAMAS was found to be the most accurate approach for both synthetic and physical test cases. The physical source results were not as accurate as the synthetic source ones, nevertheless, they were substantially improved by array shading and by an acoustic treatment (foam coating) on the working section walls. The most effective array shading methodology was based on the mean coherence level of each microphone, here referred to as CW (Coherence Weighting shading).

© 2019 Elsevier LtdSlat noise experiments are sometimes performed to provide data for the validation of numerical procedures. Under such circumstances, closed-section wind-tunnels, in contrast with open-section ones, offer well-defined boundary conditions for simulations. Nevertheless, many difficulties arise in aeroacoustic experiments with two-dimensional lifting body tests in such wind-tunnels. Reverberation and acoustic images can also interfere with the measurements. This paper addresses the effects of the aforementioned issues on the MD30P30N airfoil slat aeroacoustic measurements. Conventional beamforming and DAMAS deconvolution techniques were employed for the acoustic data post-processing. Pressure taps were used in the assessment of spanwise and chordwise pressures on the high-lift airfoil model surfaces. The interaction between the airfoil and the wind-tunnel wall boundary-layer was mitigated by the application of suction on the tunnel wall and rendered a more two-dimensional flow. Acoustic polyurethane -based foam was applied on the test-section walls for reducing reverberation effects on the slat noise source maps. The results showed the error in the noise spectrum associated with the three-dimensional effect at the model ends may correspond to a decrement of 4 deg or higher of the entire wing angle of attack. The acoustic treatment over the wind-tunnel closed-section walls mitigated the mirror sources, nevertheless, the effects of such source images on both noise source maps and noise spectra in the region of interest were considered small.

© 2019, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.The current aircraft noise has a large contribution provenient from the airframe in which the most relevant sources are the landing gear, the flap and the slat. The leading edge slat is a distributed noise source since it spans almost the whole wing extension. The presence of excrescences inside the slat cove can change significantly the recirculating flow and the characteristics of noise generated. Previous experiments showed substantial increasing in the narrow band peaks for the MD30P30N high-lift model in which the slat geometry included a bulb seal attached at some positions of its cove. We present simulations performed with the same sealed slat geometry. Mean flow shows the presence of an additional recirculation bubble for some positions of the seal. For these cases, Proper Orthogonal Decomposition was used to identify the most significant coherent structures which could be related to the slat cavity modes. These structures are confined inside the most external and narrow recirculation which enhances their interaction with the mixing layer creating a more complex scenario than that observed for slat geometries without any excrescences.

© 2018 by University of São Paulo.This paper reports a study of the sound emitted by a slat of a two-dimensional three-element MD30P30N airfoil, on which a two-dimensional excrescence was installed to represent a sealing device. Experiments were conducted in a closed-test-section wind-tunnel, and a conventional beamforming technique combined with DAMAS deconvolution method was employed for the evaluation of the radiated noise. Nine seal positions along the slat cove wall and six different seal cross sections were tested for 3-9 deg angles of attack and 34 m/s freestream speed. Mean static pressure measurements on the airfoil surface indicated that the seals did not impact the airfoil lift but significantly affected the slat noise signature, particularly the low-frequency narrowband peaks. At some positions, the narrowband peaks were increased up to 18 dB/Hz, whereas for other positions, they were virtually suppressed. At the noisiest position, even a seal whose height was 1.33% of the slat chord strongly affected the slat noise signature. Lattice Boltzmann numerical simulations in combination with the Ffowc Williams-Hawkings analogy were conducted for experimental cases selected and revealed the strong effect of the seal on the mean flow and the turbulence recirculation in the slat cove.

© 2018, The Brazilian Society of Mechanical Sciences and Engineering.The design of a microphone array affects the performance of beamforming algorithms in the localization and evaluation of acoustic noise sources. This paper addresses a circular phased array design for near-field aeroacoustic measurements in a closed-test-section wind tunnel. Microphones were distributed in rings and occupied an equal aperture area—each ring could have a different number of microphones. The array performance was evaluated through the dynamic range and array resolution given by the beamwidth. The array designed for the exploration of the novel approach had 112 microphones and 950-mm aperture. In comparison with classical designs, also optimized for the same number of microphones and aperture, the approach provided the best array resolution and a high dynamic range level almost uniform over the frequency range of interest (800 to 20,000 Hz). Microphone shading was also assessed for improving the array performance, and the employment of only the outermost microphones (the innermost ones were shaded) reduced approximately 40% the array beamwidth.

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.This paper introduces the new low-turbulence, low-background-acoustic-noise, closed-circuit wind-tunnel of the University of Sao Paulo, Sao Carlos School of Engineering (USP-EESC), containing an overview of design criteria and facility specifications. The designed wind-tunnel includes features to improve the flow quality through the reduction of turbulent and acoustic disturbances. Such features suit the wind-tunnel for pure aerodynamic characterization as well as for aeroacoustic experiments. A long settling chamber, a hexagonal honeycomb and five nylon screens were included to minimize flow disturbances in the test-section. Turning vanes are filled with insulation, aiming at reduction and absorption of background acoustic noise, whereas dense acoustic foam was installed on the corners walls, floor and ceiling to provide secondary means of background acoustic noise suppression. The facility has a closed test-section of 1×1 m2cross-sectional area and 3 m length. A purposely designed short contraction with a ratio of 7:1 was used. A variable-speed motor of 110 kW powers a 16-bladed axial fan containing 13 stators and the expected top freestream speed in the test-section is 60 m/s. Preliminary results include the test-section flow characterization, regarding turbulence level and background acoustic noise measurements. An approximately 0.108% turbulence level was obtained for 20 m/s freestream speed, with the measurements conducted with constant temperature hot-wire anemometer circuit and the signal bandpass filtered between 5 and 10,000 Hz. The acoustic characterization was performed with a 112-microphone phased array and data were post-processed with in-house beamforming codes.

Copyright © 2017 by University of São Paulo. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.This study investigates the slat noise of a two-dimensional scaled, unswept, and untapered MD30P30N high-lift model. The experimental data refer to aeroacoustic and aerodynamic measurements in a closed-section wind tunnel forawide rangeofanglesofattack(from-6 deg uptothe stall; approximatelyat18deg) and Mach numbers between 0.07 and 0.1. Three slat configurations (the original MD30P30N, another with a higher slat deflection, and one with smaller slat gap and overlap) are studied experimentally. The signal processing applied to the acoustic data involves conventional beamforming enhanced by two deconvolution algorithms, namely, DAMAS and CLEAN-SC. An original variation of the beamforming cluster approach that is based on the coherence level among microphone pairs is introduced, and it improves the results obtained by DAMAS. Below -2 deg and above 12 deg angles of attack, the slat noise is very small and mostly below the wind-tunnel background noise for all configurations. Between -2 and 12 deg angles of attack, the slat noise spectra are substantially affected by the slat configuration, although it always contains a dominant low-frequency content, a midfrequency broadband noise, and a single high-frequency broad peak. Within this range, the lower angles of attack display the strongest low-frequency narrowband peaks. In fact, at lower angles of attack, the low-frequency narrowband peaks scale with a Mach power above 10.

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.This study focuses on investigating the slat noise of a two-dimensional scaled, unswept and untapered, MD30P30N high-lift model. The database refers aeroacoustic and aerodynamic measurements in a closed-section wind tunnel for a wide range of angles of attack, namely from −6° and 18°, and Mach number ranging from 0.07 to 0.1. Aeroacoustic measurements were carried using a wall-mounted microphone array. The signal processing applied to the acoustic data involved conventional beamforming enhanced by two deconvolution algorithms, DAMAS and CLEAN-SC. Suction-based wall boundary-layer control was applied to provide a more two dimensional flow over the model span. Chord-wise and span-wise static pressure measurements were performed. Slat noise spectra revel low-frequency tones, broadband noise and a single broad tone in the higher frequency range. The results show all slat noise components are strongly affected by wing angle of attack. The low-frequency tones vanish at lower negative wing angles of attack but reach their higher level at lower positive angles of attack. The tones level variation within a narrow range of angles of attack may be over 10 dB/Hz. Overall, the dominant part in the slat noise spectra reduces as the wing angle of attack increases, rendering higher frequency noise components potential contributors to the overall noise.

© 2015, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved.This paper refers to an aeroacoustic experimental study of a slat containing a twodimensional protrusion installed at the interior of its cove, representing a sealing device. The experiments were based on a 2D model of the three-element MD30P30N geometry with 500 mm stowed chord. Four different positions of the seal were tested. The effect of the seal was assessed in what concerns the pressure coefficient on the airfoil surface, the frequency content of radiated noise and noise source field using conventional beamforming technique combined with DAMAS deconvolution method. This study was based on experimental data obtained at a wind tunnel of closed test section, using an array containing 62 microphones. Different angles of attack and free-stream speeds were evaluated. The main results evidenced that the presence of a seal at slat cove has a huge impact at its acoustic frequency spectrum components. Depending on the sealing device position, it could attenuate or intensify the low frequency tonal peaks, which are characteristic of slat models at low Reynolds number regime.