Far-field Noise Research Articles

Experimental investigation on aerodynamic noise of a small-scale multi-blade centrifugal fan

In this study, the aerodynamic noise of a small-scale centrifugal fan was experimentally investigated using acoustic testing techniques and time-resolved stereoscopic particle image velocimetry (SPIV). During the acoustic experiments, both far-field noise and near-field pressure fluctuations of the test fan were measured. The overall far-field noise towards the fan inlet side was found to be higher than that of the back side. The pressure fluctuations on the fan upper casing exceeded those on the side wall due to the uncontracted volute tongue, indicating pronounced flow-to-wall interactions. Moreover, based on a simultaneous measurement, the coherence between the near-field pressure fluctuations and far-field noise highlighted the significant contributions of impeller rotation to noise radiation. SPIV measurements uncovered the time-averaged and transient flow fields at the fan's inlet and outlet. The time-averaged results demonstrated the concentrated inlet flow and outlet flow separation, leading to high flow unsteadiness. Transient flow fields displayed an asymmetric jet-wake region characterized by both quasi-steady flow and rotational flow behaviours. The instantaneous flow results were analyzed using the dynamic mode decomposition (DMD) method, which clearly recognized the jet-wake patterns with frequencies corresponding to the rotational frequency. The observed consistency in frequency characteristics among noise, pressure fluctuations, and unsteady flow affirms that flow dynamics are crucial to the primary noise mechanisms.

The impact of non-frozen turbulence on the modelling of the noise from serrated trailing edges

Serrations are commonly employed to mitigate the turbulent boundary layer trailing-edge noise. However, significant discrepancies persist between model predictions and experimental observations. In this paper, we show that this results from the frozen turbulence assumption. A fully developed turbulent boundary layer over a flat plate is first simulated using the large-eddy simulation method, with the turbulence at the inlet generated using the digital filter method. The space–time correlations and spectral characteristics of wall pressure fluctuations are examined. The simulation results demonstrate that the coherence function decays in the streamwise direction, deviating from the constant value of unity assumed in the frozen turbulence assumption. By considering an exponential decay function, we relax the frozen turbulence assumption and develop a prediction model that accounts for the intrinsic non-frozen nature of turbulent boundary layers. To facilitate a direct comparison with frozen models, a correction coefficient is introduced to account for the influence of non-frozen turbulence. The comparison between the new and original models demonstrates that the new model predicts lower noise reductions, aligning more closely with the experimental observations. The physical mechanism underlying the overprediction of the noise model assuming frozen turbulence is discussed. The overprediction is due to the decoherence of the phase variation along the serrated trailing edge. Consequently, the ratio of the serration amplitude to the streamwise frequency-dependent correlation length is identified as a crucial parameter in determining the correct prediction of far-field noise.

Pressure And Noise Estimation Of Tandem Cylinder Flow Based On PIV

The present work proposes an optimized method for pressure reconstruction and far-field noise prediction for tandem cylinder flow based on time-resolved planar particle image velocimetry (PIV). Proper orthogonal decomposition (POD) low-order reconstruction is applied to mitigate the incoherent noise introduced during PIV measurement and processing and enhances the identification of dynamic characteristics of relative structures within the flow field. As a result, PIV-based pressure reconstruction through solving the Poisson equation encompasses fewer errors arising from contaminated boundary conditions and Poisson source terms, thereby mitigating the propagation of errors into pressure. The time-marching algorithm accelerates the convergence and stabilizes the iterative progress while solving the Poisson equation, leading a considerable computation savings for cases with multiple snapshots like aeroacoustics relative investigation. The reconstructed wall pressure is compared with the reference pressure fluctuation signal simultaneously measured, yielding good agreement. Finally, the far-field noise was predicted through Curle’s analogy based on reconstructed wall pressure, considering the spanwise correction derived through an additional spanwise planar PIV. The predicted far-field noise agrees well with the reference microphone measurement. The overall assessment demonstrates that the employment of POD low-order reconstruction and time-marching algorithm significantly improve the speed and accuracy for estimating pressure field and far-field noise.

Jet Installation Noise Modelling for Round and Chevron Jets

Wall-Modelled Large Eddy Simulations (LES) are conducted using a high-resolution CABARET method, accelerated on Graphics Processing Units (GPUs), for a canonical configuration that includes a flat plate within the linear hydrodynamic region of a single-stream jet. This configuration was previously investigated through experiments at the University of Bristol. The simulations investigate jets at acoustic Mach numbers of 0.5 and 0.9, focusing on two types of nozzle geometries: round and chevron nozzles. These nozzles are scaled-down versions (3:1 scale) of NASA’s SMC000 and SMC006 nozzles. The parameters from the LES, including flow and noise solutions, are validated by comparison with experimental data. Notably, the mean flow velocity and turbulence distribution are compared with NASA’s PIV measurements. Additionally, the near-field and far-field pressure spectra are evaluated in comparison with data from the Bristol experiments. For far-field noise predictions, a range of techniques are employed, ranging from the Ffowcs Williams–Hawkings (FW–H) method in both permeable and impermeable control surface formulations, to the trailing edge scattering model by Lyu and Dowling, which is based on the Amiet trailing edge noise theory. The permeable control surface FW–H solution, incorporating all jet mixing and installation noise sources, is within 2 dB of the experimental data across most frequencies and observer angles for all considered jet cases. Moreover, the impermeable control surface FW–H solution, accounting for some quadrupole noise contributions, proves adequate for accurate noise spectra predictions across all frequencies at larger observer angles. The implemented edge-scattering model successfully captures the mechanism of low-frequency sound amplification, dominant at low frequencies and high observer angles. Furthermore, this mechanism is shown to be effectively consistent for both M=0.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M=0.5$$\\end{document} and M=0.9\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M=0.9$$\\end{document}, and for jets from both round and chevron nozzles.

Effect of trailing-edge blowing on the acoustic and aerodynamic characteristics of a flatback airfoil

This study investigated the impact of trailing-edge uniform air blowing on the acoustic and aerodynamic characteristics of a flatback airfoil. Near- and far-field pressure fluctuations, surface static pressure distribution, as well as boundary layer and wake flow measurements were conducted to comprehensively understand the effects of the method on both the noise generation mechanism and the aerodynamic characteristics of the airfoil. It was revealed that tonal noise originates from surface pressure fluctuations induced by upstream flow disturbances due to vortex shedding. The application of blowing was found to shift large-scale vortices generated during vortex shedding further downstream, resulting in the suppression of surface pressure fluctuations on both the pressure and suction sides of the airfoil, consequently reducing far-field noise. Additionally, blowing enhanced spanwise coherence at the vortex shedding frequency. In terms of aerodynamic behavior, blowing was shown to increase base pressure, leading to drag reduction without affecting lift. Interestingly, the significant drag reduction was found to occur at the same blowing parameter associated with maximum tonal noise reduction.

Flow and Aeroacoustic Characteristics of Underexpanded Supersonic Jets Exhausting from a Conical Converging Nozzle

Ensuring the safety of space flights and solving the problems of reducing acoustic loads during the launch of space vehicles requires not only the development of new technical systems for launch complexes, but also methods for the numerical simulation of fluid and aeroacoustic fields generated by supersonic jets. The growing regulations for space vehicle noise also explain the interest in developing models and techniques that anticipate flow and the aeroacoustic characteristics of supersonic jets. Together with integral techniques for computing far-field noise, development of relevant mathematical models and implementation of numerical tools, the concepts of computational fluid dynamics (CFD) and computational aeroacoustics (CAA) are covered. The noise generated by a supersonic underexpanded jet is used to illustrate the capabilities of current numerical modelling and simulation tools. The jet structure, flow properties, and aeroacoustic quantities are affected by the nozzle pressure ratio. The outcomes of numerical simulation are contrasted with existing experimental and computational data. The available numerical modelling and simulation tools facilitate the development of novel computational methods and methodologies for challenges in CFD and CAA, in addition to solving research and engineering problems.

Data Reduction Technologies in Prediction of Propeller Noise

High-fidelity computations are often used in predicting the tonal and broadband noise of propellers and rotors associated with Advanced Air Mobility Vehicles (AAMVs). But LES is both CPU and storage intensive. We present here an investigation of the feasibility of reduction methods such as Proper Orthogonal Decomposition as well as Dynamic Mode Decomposition for reduction of data obtained via LES to be used further to obtain additional parameters. Specifically, we investigate how accurate reduced models of the high-fidelity computations can be used to predict the far-field noise. It is found that POD is capable of accurately reconstructing the parameters of interest with 15–40% of the total mode energies, whereas the DMD can only reconstruct primitive parameters such as velocity and pressure loosely. A rank truncation convergence criterion > 99.8% is needed for better performance of the DMD algorithm. In the far-field spectra, DMD can only predict the tonal contents in the lower and mid frequencies, while the POD can reproduce all frequencies of interest.

On the aeroacoustic simulation and far-field noise modeling of a cylinder turbulent wake at the Reynolds number of 3900

The turbulent wake flows of circular cylinders have been under extensive study due to their strong fluctuating forces and noise levels, yet the relationship between them has rarely been examined. In this study, a high-fidelity numerical simulation is performed for a cylinder wake flow at the Reynolds number of 3900, aimed at far-field noise modeling based on the investigation of noise source characteristics. Despite irregular multi-scale turbulent wake eddies, the primary vortex shedding that resembles the Karman vortex shedding is observed, and the modes extracted by the dynamic mode decomposition technique at the primary shedding frequency and its higher harmonics appear as nearly two-dimensional spanwise structures. A non-permeable Ffowcs Williams and Hawkings (FW-H) approach is used to formulate the far-field noise spectra as the surface integral of unsteady flow samples on the cylinder surface and the volume integral of unsteady stress in the turbulent wake. The former can be modeled as the dipole sound of unsteady lift and drag, and the latter can be modeled as the tonal noise of discrete two-dimensional (i.e., spanwise-averaged) vortex modes. The results show that the vortex sound is extremely weak. Thus, the far-field noise can be modeled almost entirely by the aerodynamic forces, and the quantitative relationship between them is established by the convective FW-H solutions.

Vortex-induced noise suppression of a cylinder with blowing through porous media

To mitigate vortex shedding for flow and noise control of a circular cylinder, an experimental approach combining air blowing and porous coating was implemented simultaneously as a hybrid method. Localized air blowing was symmetrically applied through structured porous media at four angles, corresponding to different regions of the flow field: boundary layers, shear layers on the cylinder, separated shear layers, and the cylinder's base. The study involved synchronizing near-field pressure fluctuation and far-field noise measurements with flow field measurements obtained via particle image velocimetry. Near-field pressure measurements were taken around the cylinder's circumference using a remote sensing method. This comprehensive investigation revealed that vortex shedding primarily induced pressure fluctuations at the cylinder's shoulders, resulting in the propagation of acoustic waves to the far field. The hybrid method, alongside the separate application of porous coating and local blowing, showcased substantial efficacy in mitigating near-field pressure, consequently leading to a reduction in far-field noise. These techniques achieved this by strategically shifting the vortex formation region further downstream and expanding the wake region compared to the baseline. Notably, the hybrid method, particularly when local blowing was applied at the base of the porous coated cylinder, exhibited a significantly enhanced impact in this regard, resembling the behavior observed with the individual application of porous coating.

Characteristics of the flow around finite wall-mounted square cylinders and the mechanism of tonal noise

Motivated by the need to reduce the noise from train pantographs, numerical simulations are carried out for the flow over finite wall-mounted square cylinders with different aspect ratios at a Reynolds number of 1.5×104. Five aspect ratios (height-to-width ratios) are taken into account, namely, 1.4, 4.3, 7.1, 10, and 12.9. The effect of the aspect ratio on the aerodynamic coefficients, the near-wall flow topologies, and the pressure distributions are studied in detail to give insight into the noise generation mechanisms. The pressure rate of change dp/dt on the cylinder surfaces is adopted to evaluate the dipole noise source. It turns out that distributions of dp/dt are closely related to flow evolutions near the free ends and the wall-mounting junctions of cylinders with different aspect ratios. High levels of dp/dt are found close to lateral trailing edges of the cylinder, while the strength grows quickly as the aspect ratio is increased. The far-field noise emitted from these cylinders is predicted using the Ffowcs Williams–Hawkings acoustic analogy and validated with wind tunnel measurements available in the literature. For receivers located in the cross-flow direction, a single acoustic tone near a Strouhal number of 0.1 is observed for cylinders with aspect ratios not greater than seven, while an additional tone at a higher Strouhal number occurs as the aspect ratio is further increased. The underlying mechanism of the tonal noise emitted to the far field is also investigated by combining the noise source localization and dynamic mode decompositions.

Aerodynamic noise reduction of high-speed pantograph by introducing planar jet on leeward surface of panhead

Combining the improved delayed detached eddy simulation (IDDES) model and Ffowcs Williams-Hawkings (FW–H) equation, the potential of panhead leeward-side jet in pantograph aerodynamic noise control is numerically investigated. A typical double-strip pantograph is used as the baseline model. Two identical planar jets are arranged on the leeward side of the two strips to achieve active flow control. The results show that the planar jet can reduce the aerodynamic drag of the upstream strip and suppress its lift fluctuation, while the jet does not produce equivalent level of positive effects on the downstream strip. Benefiting from the suppression of lift fluctuation of the upstream strip, the far-field noise of the model equipped with jet achieves a 7 dB reduction in the main direction of panhead lift fluctuation, and the noise on the pantograph side is also reduced by 1–2 dB. The flow-field analysis reveals that the planar jet pushes the coherent flow structures in the panhead's wake downstream, which makes the high-intensity flow-field fluctuation away from the upstream strip. The absence of the positive effect of the jet on wake flow modification of the downstream strip can be attributed to the distinct inflow conditions of the two strips.

Interior noise prediction of inter-coach space of high-speed maglev trains based on wavenumber decomposition on aerodynamic excitation

Current research on noise and vibration control of high-speed maglev trains pays more attention to far-field noise, while the level of interior noise has a direct impact on the ride comfort and should be placed equal weight on. In this paper, inter-coach space, one of the main pressure fluctuation sources of a specific type of high-speed maglev train with a design speed of 600 km·h−1, is taken as the research object. The turbulent and acoustic components of wall pressure fluctuations (WPF) are separated based on a wavenumber-frequency analysis approach, and then each component is applied as different forms of source input to the vibroacoustic model, namely, finite element method-boundary element method (FEM-BEM) and statistical energy analysis (SEA) for low- and high-frequency ranges respectively, to investigate the contribution of both components to interior acoustic cavity in all frequency range. It can be seen quantitatively from the results that the amplitude of turbulent component is generally much higher than that of the acoustic one, but it can be vice versa when it comes to the interior response. The conclusion drawn in this paper are able to provide guidance for future researches on more targeted interior noise control of high-speed maglev trains.

An Improved Boundary Element Method for Predicting Half-Space Scattered Noise Combined with Permeable Boundaries

The boundary element method is widely used in practical engineering problems, especially in the field of acoustics. For flow-induced noise, the main target of acoustic calculations is to solve the wave equation with the flow field information. However, the sound field distribution of noncompact structures in half-space is especially complex because of the strong scattering effect, while the object surface boundary integration often brings a large workload and generates numerical singularities. In this paper, an improved boundary element method for predicting the aeroacoustic noise of noncompact structures is proposed, which can consider the characteristic distribution of sound field induced by complex structures in half-space. The smooth permeable boundary surrounding the object is used as the integration boundary, while the scattering effect of the ground boundary is investigated by combining the mirror Green’s function method, and the numerical prediction of aeroacoustic noise is carried out for the dipole source and NACA0012 airfoil in half-space. Numerical results show that the far-field noise obtained by using the permeable surface is consistent with that obtained by integrating the direct object boundary under the influence of ground boundary scattering. The mirror image Green’s function method is able to finely capture the ground scattering effect, which has a significant effect on the sound field as the frequency increases.

Acoustic wind tunnel experimental study of aerodynamic noise distribution in high-rise buildings

Wind-induced aerodynamic disturbance has become a major noise source affecting residents in super high-rise buildings. To investigate the distribution characteristics of aerodynamic noise on high-rise building surfaces in strong wind conditions and the impact of wind speed and direction, this paper pioneers the application of the acoustic wind tunnel method based on microphone array localization to the experimental study of the surface acoustic pressure field of a high-rise building model, the far-field noise spectrum of which is obtained using the microphone array sensor. The beamforming calculation method was used to obtain the spectrum and localization cloud. By comparing the spectra and cloud diagrams under different process conditions and based on existing building wind pressure reports, it is concluded that: the noise source is always located at the height of the connecting corridor where there is large wind pressure pulsation in the two building models; the aerodynamic noise is more pronounced around building gaps where the airflow is more disturbed and also in the oncoming wind trailing direction where the wind pressure pulsation is more pronounced. An increase in wind speed increased noise source amplitude from 30 dB to 60 dB, and the location of the noise source shifted backwards to the tail of the incoming wind. This paper shows that the acoustic wind tunnel method based on microphone array localization technology can obtain the aerodynamic noise intensity on the surfaces of super high-rise buildings and reasonably predict the location of the sound source. This article puts forward suggestions to reduce noise in high-rise buildings based on experimental results.

Suppression effect of the leading-edge groove on deep cavity noise at low speeds: Groove length

Detached Eddy Simulations combined with Ffowcs Williams–Hawkings acoustic analogy of subsonic flows over a deep cavity preceded by deep grooves with different lengths were conducted to investigate the effects of groove length on the flow characteristics and the noise suppression in the vicinity of the deep cavity. The length-to-depth ratio of the deep cavity is 2/3, and the length-to-depth ratios of the grooves are 1/2. The distance between the grooves and the cavity remains constant at twice the groove length. The groove length ranges from 5 mm to 30 mm. The Reynolds number based on the cavity length is 8 × 10−5. The analysis results indicate that all grooves studied in this paper have some effect on the distribution of sound energy of near-field noise and near-/far-field noise amplitude, but have no effect on the characteristic frequencies of cavity noise and far-field noise directivity. When the groove length is 15 mm and 20 mm, the deep grooves are most effective in suppressing deep cavity noise. The time-averaged velocities in the incoming boundary layer, shear layer, and wake region decrease, and the height of the boundary layer downstream of the cavity increases, which reduces the intensity of the interaction between the flow structure and solid wall downstream of the cavity, resulting in a reduction in flow-acoustic feedback noise. Meanwhile, the suppression effect of the groove flow on the cavity flow effectively alters the flow characteristics of the entire cavity mouth, the rear edge wall, and part of the bottom surface region, reducing the energy at these locations. This is the primary reason for the reduction in broadband noise and acoustic resonance noise.

Acoustics and Forces from a Subscale Electric Vertical-Takeoff-and-Landing Rotor in Edgewise Flight

The far-field noise and forces of a subscale two-blade rotor were measured in an anechoic wind tunnel for both hover and edgewise flight conditions. The mean thrust and torque coefficients increased with the edgewise advance ratio in relation to the hover coefficients. The tonal and broadband noise contributions were separated and analyzed in both the time and frequency domains. The sound pressure level (SPL) at the blade pass frequency (BPF) had the greatest contribution to the overall SPL, and the dominant broadband noise source changed from high-frequency trailing edge noise to midfrequency blade wake interaction noise with an increasing edgewise advance ratio. The tip Mach number scaling of the noise sources was examined and found to be highly dependent on the oncoming freestream velocity. The BPF SPL directivity showed a dependency on the advance ratio, with a peak magnitude below and upstream of the rotor’s retreating side. The broadband noise had a dipole directivity with a minimum in the rotor plane. The midfrequency broadband noise had signatures indicative of blade wake interaction noise, and the high-frequency noise had signatures indicative of amplitude modulated trailing edge noise. The results demonstrate the importance of unsteady loading noise and broadband noise for subscale urban air mobility rotors.

Low-cost wind turbine aeroacoustic predictions using actuator lines

Aerodynamic noise is a limitation for further exploitation of wind energy resources. As this type of noise is caused by the interaction of turbulent flow with the airframe, a detailed resolution of the flow is necessary to obtain an accurate prediction of the far-field noise. Computational fluid dynamic (CFD) solvers simulate the flow field but only at a high computational cost, which is much increased when the acoustic field is resolved. Therefore, wind turbine noise predictions using numerical approaches remain a challenge. This paper presents a methodology that couples (relatively fast) wind turbine CFD simulations using actuator lines with a fast turn-around noise prediction method. The flow field is simulated using actuator lines and large eddy simulations. The noise prediction method is based on the Amiet–Schlinker’s theory for rotatory noise sources, considering leading- and trailing-edge noise as unique noise sources. A 2D panel code (XFOIL) calculates the sectional forces and boundary layer quantities. The resulting methodology for the noise prediction method is of high fidelity since the wind turbine geometry is accounted for in both flow and acoustics predictions. Results are compared with field measurements of a full-scale wind turbine for two operational conditions, validating the results of this research.

On the application of acoustic liners for the reduction of jet-flap installation noise

In this work, an experimental and numerical analysis of the feasibility of using acoustic liners to reduce the acoustic far-field radiation of a subsonic jet near a flat plate is presented. The Boundary Element Method (BEM) with a wavepacket model of the jet noise and considering an acoustic impedance boundary condition was used to calculate the far-field noise for different conditions. A parametric optimization was performed in order to find an optimum impedance in terms of the Overall Sound Pressure Level reduction. Semi-empirical models of acoustic liners were then used to design a liner with an impedance as close as possible to the optimum impedance. The liners were built using additive manufacturing and tested in terms of their acoustic impedance and the resulting noise reduction when applied to a region of the flat plate. The BEM simulations, considering the impedance, showed good agreement with experimental data. Overall, by means of the BEM model, optimization and liner semi-empirical models, it was possible to design an acoustic liner which significantly reduced the installation noise at the far-field, with the overall noise reduction between 2 dB to 4 dB depending on observation angle.

Cylinder flow and noise control by active base blowing

An extensive experimental investigation was undertaken to control the flow and noise characteristics influenced by vortex shedding from a circular cylinder by implementing air blowing at the base of the cylinder. The study synchronised near-field pressure and far-field noise measurements with the wake velocity field to understand the noise reduction mechanism of base blowing. Surface pressure fluctuations were measured using pressure taps distributed around the cylinder's circumference through a remote-sensing method, while velocity measurements were obtained using planar particle image velocimetry at the midspan to examine the flow dynamics. The study unveiled the crucial role of near-field pressure, particularly induced at the shoulders of the cylinder, in generating far-field noise. The rapid vertical flow movement, arising from the interaction between shear layers, was identified as a mechanism responsible for inducing surface pressure fluctuations. This phenomenon occurred as high-momentum fluid moved from the free stream into the interior of the vortex-formation region. By applying base blowing, a remarkable reduction in both near-field pressure and far-field noise was achieved at the fundamental vortex-shedding frequency, with reductions of approximately 20 and 25 dB, respectively, compared with the baseline. Additionally, base blowing caused the shear layers to roll up farther downstream than in the baseline by decreasing the entrainment of fluid-bearing opposite vorticity by the shear layer upstream of the growing vortex. Consequently, there was a substantial decrease in turbulent kinetic energy and Reynolds stress near the cylinder, resulting in slower vertical flow movement and weaker near-field pressure.

Relevance of quadrupolar sound diffraction on flow-induced noise from porous-coated cylinders

This paper elucidates the link between the near-wake development of a circular cylinder coated with a porous material in a low Mach-number flow and the related aerodynamic sound attenuation. It accomplishes this by formulating the cylinder flow-induced noise as a diffraction problem. The necessity for such an approach is driven by experimental evidence obtained through acoustic beamforming and particle-image-velocimetry measurements, which reveal that the dominant noise sources for a coated cylinder are not localised on the body surface but rather in the wake, specifically at the outbreak position of the shedding instability. The acoustic field at the vortex-shedding frequency can be hence modelled by considering a compact lateral quadrupole at this location and employing an exact Green’s function tailored to a cylindrical geometry. Because of the diffraction of the sound waves radiated by the quadrupolar source on the cylinder surface, the resulting far-field directivity pattern resembles that of a dipole. The study demonstrates that the porous coating has the two-fold effect of decreasing the strength of the point quadrupole in the wake and moving its origin further downstream, reducing, in turn, the efficiency of the sound scattering. Consequently, the diffracted part of the acoustic field, which dominates the far-field noise for a bare cylinder in accordance with classical theory, provides a contribution that is comparable to the direct part. The results eventually indicate that quadrupolar sources must be considered to accurately predict the noise radiated from a porous-coated cylinder, even at low Mach numbers.