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The surface pressure field generated by a supersonic impinging jet on a vertical flat plate was measured using a pressure-sensitive paint (PSP) and a double-pulsed laser. The Mach number of the jet was M j = 1.23 and the position of the flat plate was h/D = 4.5. A significant peak at St = 0.41 (15.2 kHz) was observed in the spectra measured by a microphone, and unsteady pressure transducers. The feedback loop involved in the acoustic loading occurred at this frequency. Coherent structures of the flow were extracted by applying the azimuthal Fourier decomposition and the dynamic mode decomposition (DMD) to PSP images on the impingement plate. Axisymmetric modes expanding outward from the impingement point of the jet were observed for the azimuthal mode m = 0. Two helical modes related to the feedback loop were identified for |m| = 1. It was confirmed that the RMS values of the amplitudes of these DMD modes were larger than the other modes. The frequencies of these DMD modes were St = 0.39 (14.3 kHz) and St = 0.37 (13.7 kHz), respectively. Coherent structures associated with phenomena faster than 10 kHz were successfully extracted from PSP measurement data. [ABSTRACT FROM AUTHOR]

This research aims to investigate the impact of a split airfoil on noise emissions from a horizontal-axis wind turbine. The objective is to comprehensively understand the airflow patterns around the airfoil to reduce noise emissions. The study rigorously examines a range of angles of attack, from 0° to 25°, for both the original airfoil and the airfoil with a split, using advanced computational aerodynamics coupled with analog acoustic analysis. The methodology involves two-dimensional flow simulations with Delayed Detached Eddy Simulation based on the Spalart-Allmaras model, enabling precise near-field flow calculations around the airfoil. Additionally, far-field noise predictions, employing the Ffowcs Williams and Hawkings analogy based on simulated sources, reveal the efficacy of the split airfoil design. Results indicate that the split airfoil design effectively reduces noise emissions across various angles of attack. These reductions translate into a significant decrease in the Overall Sound Pressure Level, ranging from 14% to 19%, and remarkable Sound Pressure Level reductions between 12% and 60% across diverse frequencies, showcasing substantial noise improvements in various frequency ranges.

This research aims to investigate the impact of a split airfoil on noise emissions from a horizontal-axis wind turbine. The objective is to comprehensively understand the airflow patterns around the airfoil to reduce noise emissions. The study rigorously examines a range of angles of attack, from 0° to 25°, for both the original airfoil and the airfoil with a split, using advanced computational aerodynamics coupled with analog acoustic analysis. The methodology involves two-dimensional flow simulations with Delayed Detached Eddy Simulation based on the Spalart-Allmaras model, enabling precise near-field flow calculations around the airfoil. Additionally, far-field noise predictions, employing the Ffowcs Williams and Hawkings analogy based on simulated sources, reveal the efficacy of the split airfoil design. Results indicate that the split airfoil design effectively reduces noise emissions across various angles of attack. These reductions translate into a significant decrease in the Overall Sound Pressure Level, ranging from 14% to 19%, and remarkable Sound Pressure Level reductions between 12% and 60% across diverse frequencies, showcasing substantial noise improvements in various frequency ranges. [ABSTRACT FROM AUTHOR]

A new approach to accelerating the evaluation of monopole and dipole source integrals via the fast multipole method (FMM) in the time domain for general three-dimensional (3-D) aeroacoustic problems is presented in this paper. In this approach, the aeroacoustic field is predicted via a hybrid method that uses computational fluid dynamics (CFD) for near-field flow field calculations and the Ffowcs Williams–Hawkings (FW-H) acoustic analogy for far-field sound field predictions. The evaluation of the surface integrals of the monopole and dipole source terms appearing in the FW-H formulation is accelerated by a 3-D FMM to reduce computational cost. The proposed method is referred to as Fast FW-H in this work. The performance and efficiency of the proposed methodology are demonstrated using several examples. First, aeroacoustic predictions for the cases of a stationary acoustic monopole, moving acoustic monopole and stationary acoustic dipole in a uniform flow are studied, generally showing good agreement with the analytical solutions. Second, the sound field radiating from a flow passing a finite-length circular cylinder and the propeller of an unmanned aerial vehicle (UAV) during forward flight are studied, and the computed results obtained via the FW-H and Fast FW-H methods in the time domain with a stationary, permeable surface are compared. The overall computational efficiency of the sound field solutions obtained via the Fast FW-H method is found to be approximately two times faster than the computational efficiency of the original FW-H method, indicating that this proposed approach can be an accurate and efficient computational tool for modelling far-field aeroacoustic problems.

A new approach to accelerating the evaluation of monopole and dipole source integrals via the fast multipole method (FMM) in the time domain for general three-dimensional (3-D) aeroacoustic problems is presented in this paper. In this approach, the aeroacoustic field is predicted via a hybrid method that uses computational fluid dynamics (CFD) for near-field flow field calculations and the Ffowcs Williams–Hawkings (FW-H) acoustic analogy for far-field sound field predictions. The evaluation of the surface integrals of the monopole and dipole source terms appearing in the FW-H formulation is accelerated by a 3-D FMM to reduce computational cost. The proposed method is referred to as Fast FW-H in this work. The performance and efficiency of the proposed methodology are demonstrated using several examples. First, aeroacoustic predictions for the cases of a stationary acoustic monopole, moving acoustic monopole and stationary acoustic dipole in a uniform flow are studied, generally showing good agreement with the analytical solutions. Second, the sound field radiating from a flow passing a finite-length circular cylinder and the propeller of an unmanned aerial vehicle (UAV) during forward flight are studied, and the computed results obtained via the FW-H and Fast FW-H methods in the time domain with a stationary, permeable surface are compared. The overall computational efficiency of the sound field solutions obtained via the Fast FW-H method is found to be approximately two times faster than the computational efficiency of the original FW-H method, indicating that this proposed approach can be an accurate and efficient computational tool for modelling far-field aeroacoustic problems. [ABSTRACT FROM AUTHOR]

In order to study the aeroacoustic characteristics of the new distributed tail rotor, the method used for the calculation of distributed ducted tail rotor aerodynamic and acoustic is established based on the sliding mesh method and penetrable surface. The Reynolds-averaged Navier-Stokes is used as the govern equation. The second order Roe scheme is used for spatial dispersion. The implicit LU-SGS scheme is applied to the pseudo-time marching. The S-A turbulence model is selected. The FW-H equation is used to calculate the noise. By the established method, the aerodynamic and acoustic characteristics of traditional tail rotor and distributed ducted tail rotor are calculated. The results indicated that compared with the traditional tail rotor, for the typical radiation direction, namely the tip-path-plane, the noise amplitude of distributed ducted tail rotor is reduced by 5—6 dB under the same aerodynamics, and with the decrease of rotor speed, the noise amplitude is decreased further. [ABSTRACT FROM AUTHOR]

The exact dynamic mode decomposition (DMD) was applied to the nonsequential image dataset obtained by the double-pulsed schlieren measurement of a supersonic impinging jet, and the effect of the dataset length on the obtained spatial modes and estimated frequencies of the aeroacoustic fields was investigated. The Mach number of the jet was 2.0, the Reynolds number based on the diameter of the nozzle exit was 1.0 × 10 6 and the distance between the nozzle exit and the flat plate was four times the nozzle diameter long. The DMD modes extract the characteristic pattern and its frequency that relate to the aeroacoustic fields. The estimated frequencies of DMD modes were compared with the acoustic spectra measured using microphones. The estimated frequency of the DMD mode that has the largest amplitude approximately coincides with that of the highest peak in the acoustic spectra regardless of the dataset length. However, the variation in the estimated frequencies of the high-order DMD modes increases when the dataset length is short. Although the estimated frequencies of the second and third DMD modes did not match the peak frequencies of the acoustic spectra, the estimation accuracy of the frequency of the modes can be improved by recalculating the frequency based on the wavelength of the corresponding spatial mode. The order of the amplitude of DMD modes did not agree with the order of the peak magnitude in the acoustic spectra, except for the first mode. This is because the schlieren method visualizes the density gradient resulting in emphasizing the high-frequency fluctuations. This mismatch was mitigated by correcting the acoustic spectrum considering the first derivative of the acoustic spectrum. Therefore, the verification of the estimation accuracy considering the data characteristics is important when the exact DMD analysis is applied to the noisy experimental data. [ABSTRACT FROM AUTHOR]

The present study numerically investigated a cylinder under oscillating motions at a low Reynolds number. The effects of two oscillation frequencies and amplitudes on the lift drag coefficient, near-field surface pressure fluctuation, and far-field noise were studied. The models were examined at a Mach number of 0.05, corresponding to a Reynolds number of 1.0 × 105. In this paper, the incompressible Navier–Stokes equations (INSE) and linearized perturbed compressible equations (LPCE) were coupled to form a hybrid noise prediction method, which was used to solve the flow field and acoustic radiation field. Based on the simulation results of the acoustic radiation field, the frequency characteristics of the acoustic waves were analyzed by the dynamic modal decomposition (DMD) method. It was observed that when the oscillation amplitude was the same, the variation amplitude and mean value of the lift-drag coefficient increased with the increase in the oscillation frequency. Under the same small oscillation frequency, the oscillation amplitude had little effect on the lift-drag coefficient. However, for the same large oscillation frequency, the variation amplitude of the lift-drag coefficient increased as the oscillation amplitude increased. In addition, both the amplitude and frequency had a significant effect on the directionality of the noise and the intensity of the sound waves. The main energy of the sound field was mainly concentrated on the first and second narrowband frequencies by using the DMD method to analyze the sound pressure level spectrum. [ABSTRACT FROM AUTHOR]

The serrated ventilation dampers for regulation of airflow volume were tested. The computer fluid flow analysis was conducted using the Comsol program. The nature of the flow through the serrated ventilation damper was analyzed to identify the different turbulence regions formed on the airflow field. Additionally, the aerodynamical and acoustical parameters of these blades of ventilation dampers with different serrated trailing and/or leading-edge were studied. The aeroacoustic studies were done in the reverberation room. The sound power level and loss coefficient were determined for the studied models. [ABSTRACT FROM AUTHOR]

Silencers are typical devices used for the reduction of noise in ventilation systems, which can be found in almost all industrial, service or residential installations. Determination of acoustic parameters for specific HVAC devices, like silencers, is within the scope of specialised laboratories. With the silencers, two main parameters should be taken care of: the first one is sound attenuation and the second one is pressure losses. In the presented paper, the focus is on measurement methods described by standard ISO 7235:2009. This standard specifies the methods for determining the sound power level of the flow noise generated by silencers, the total pressure of silencers and the insertion loss of silencers with and without airflow by using the substitute object. In this work, we focused on the correlation between the shape of the substitution duct and its acoustic parameters and its relation to this with the final result of insertion loss of a silencer. [ABSTRACT FROM AUTHOR]

A detailed investigation of the statistical properties of the near-field pressure fluctuations induced by an under-expanded jet, by varying the nozzle exit shapes has been presented. Experiments using different convergent Chevron nozzles were carried out in the anechoic chamber at the University of Bristol to assess the importance of the Chevron shape on the near pressure field emitted by a single stream under-expanded jet. Measurements were carried out through an axial microphone array traversed radially to various positions for jet in an under-expanded condition at Mach number M = 1.3. The intermittent behavior is investigated considering the standard statistical indicators and a wavelet-based conditional approach, including the phase angle. The intermittent degree of various features related to different scales, such as Screech tones and broadband shock associate noise were estimated. A series of recently developed wavelet-based tools were assessed as a viable approach to investigate the noise emitted by under-expanded jets. [ABSTRACT FROM AUTHOR]

The present paper investigates the aerodynamic and aeroacoustic characteristics of the H-rotor Darrieus vertical axis wind turbine (VAWT) combined with very promising energy conversion and steering technology; a fixed guide-vanes. The main scope of the current work is to enhance the aerodynamic performance and assess the noise production accomplished with such enhancement. The studies are carried out in two phases; the first phase is a parametric 2D CFD simulation employing the unsteady Reynolds-averaged Navier-Stokes (URANS) approach to optimize the design parameters of the guide-vanes. The second phase is a 3D CFD simulation of the full turbine using a higher-order numerical scheme and a hybrid RANS/LES (DDES) method. The guide-vanes show a superior power augmentation, about 42% increase in the power coefficient at λ = 2.75, with a slightly noisy operation and completely change the signal directivity. A remarkable difference in power coefficient is observed between 2D and 3D models at the high-speed ratios stems from the 3D effect. As a result, a 3D simulation of the capped Darrieus turbine is carried out, and then a noise assessment of such configuration is assessed. The results show a 20% increase in power coefficient by using the cap, without significant change in the noise signal. [ABSTRACT FROM AUTHOR]

The present study numerically investigated a cylinder under oscillating motions at a low Reynolds number. The effects of two oscillation frequencies and amplitudes on the lift drag coefficient, near-field surface pressure fluctuation, and far-field noise were studied. The models were examined at a Mach number of 0.05, corresponding to a Reynolds number of 1.0 × 105. In this paper, the incompressible Navier–Stokes equations (INSE) and linearized perturbed compressible equations (LPCE) were coupled to form a hybrid noise prediction method, which was used to solve the flow field and acoustic radiation field. Based on the simulation results of the acoustic radiation field, the frequency characteristics of the acoustic waves were analyzed by the dynamic modal decomposition (DMD) method. It was observed that when the oscillation amplitude was the same, the variation amplitude and mean value of the lift-drag coefficient increased with the increase in the oscillation frequency. Under the same small oscillation frequency, the oscillation amplitude had little effect on the lift-drag coefficient. However, for the same large oscillation frequency, the variation amplitude of the lift-drag coefficient increased as the oscillation amplitude increased. In addition, both the amplitude and frequency had a significant effect on the directionality of the noise and the intensity of the sound waves. The main energy of the sound field was mainly concentrated on the first and second narrowband frequencies by using the DMD method to analyze the sound pressure level spectrum.

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A detailed investigation of the statistical properties of the near-field pressure fluctuations induced by an under-expanded jet, by varying the nozzle exit shapes has been presented. Experiments using different convergent Chevron nozzles were carried out in the anechoic chamber at the University of Bristol to assess the importance of the Chevron shape on the near pressure field emitted by a single stream under-expanded jet. Measurements were carried out through an axial microphone array traversed radially to various positions for jet in an under-expanded condition at Mach number M = 1.3. The intermittent behavior is investigated considering the standard statistical indicators and a wavelet-based conditional approach, including the phase angle. The intermittent degree of various features related to different scales, such as Screech tones and broadband shock associate noise were estimated. A series of recently developed wavelet-based tools were assessed as a viable approach to investigate the noise emitted by under-expanded jets.

We analyze the differences between the Ffowcs–Williams and Hawking’s approach and a new sound propagation approach based on the finite element method used for solving Lighthill’s aeroacoustic wave equation for compressible flows. In addition, we discuss the applicability of both methods. The sound propagation approach based on Lighthill’s equation introduces a flow-interface boundary condition, similar to permeable boundaries in the Ffowcs–Williams and Hawking’s analogy, which allows the omission of complex geometries in propagation domains. This enables to reduce numeric effort and storage requirements. Thereby, the hybrid aeroacoustic workflow is considered, for which aeroacoustic source terms are computed to couple a flow and a separated acoustic propagation simulation. We present an extensive investigation of Lighthill’s source terms in the sense of the proposed weak formulation of Lighthill’s equation. For validation, measurements from a cold gas test rig are used. In addition, the possibilities of applying both sound propagation methods for investigating the influence of resonators and sound absorbers are discussed.

This paper is aimed at researching the interaction between aeroacoustic noise radiated from a rectangular cavity (gear bay) and from landing gear. It is a complicated flow-induced noise problem, involving the nonlinear, unsteady evolution of the turbulent structure inside the airflow bypassing the landing gear and the cavity. The generation and radiation mechanism of aeroacoustic noise are also concerned. In fact, it is a problem about the nonlinear interaction between the vortices shedding from the boundary layer of bluff bodies and the cavity-limited shear layer. To simplify this issue, a two-wheel landing gear named LAGOON is chosen as the landing gear model. The unsteady flow field and aerodynamic noise from it is simulated by applying the commercial software ANSYS Fluent. Good agreement is achieved between the numerical simulation and wind tunnel measurements in terms of the aerodynamic and aeroacoustic results. According to the size of LAGOON, a simple rectangular cavity is designed as the landing gear bay. Both the cavity combined with LAGOON and the cavity alone are simulated and compared. The results show that under the blocking effect of a strut, most small pieces of vortices at the trailing edge of the cavity bottom would dissipate rather than move forward along with the backflow, leading to the correlation of cavity resonance being more contrasting and increasing its amplitude. The blockage effect induced by rear wall could also enhance the turbulence kinetic energy at the wake of the strut, thus increasing the low-frequency noise radiated from the strut and cavity. [ABSTRACT FROM AUTHOR]

The aerodynamic and aeroacoustic behaviors of the Darrieus vertical axis wind turbine are very complicated. Hence, it is challenging to simulate the real flow behavior using the computational fluid dynamics approach. In the present study, the behavior of two different numerical techniques, Unsteady Reynolds Averaged Navier Stokes and Delayed Detached-Eddy Simulation (DDES), are compared to predict the aerodynamic and aeroacoustic performance. The DDES approach with the higher order scheme is used to investigate the noise mechanisms of Darrieus VAWT at several operating conditions. It is concluded that the dominant noise mechanism, when the turbine operates at low and high speed ratios, is a dipole noise, without any contribution from the monopole sound in the time averaged signal. Moreover, all the blade segments along the chord have a significant contribution to noise production. The leading edge segment is responsible for the noise reflects the load fluctuations by the variation of the effective angle of attack, flow blade relative velocity, and Blade vortex interaction. On the other hand, the rest of the blade segments reflect the load fluctuations by the massively separated flow at low speed ratio, while at high speed ratio it is as the same as the leading edge. [ABSTRACT FROM AUTHOR]

This work presents an experimental study to investigate the influence of step volute tongue on aerodynamic performance and aeroacoustic behavior of a forward-curved fan. The noise characteristics are analyzed and controlled based on measured acoustic pressures for various stepped tongues; meanwhile, fan performance is detected and optimized. The design parameters of the stepped tongues are presented to provide significant physical insight into increasing the static pressure as well as the efficiency of static pressure and reducing the fan noise generation. The comparison of the test results indicates that the improved static pressure and its efficiency of the HLHL model have increased by 15.67 Pa and 3.57%, respectively, by comparing with those of the baseline model. At 740 m3/h, a great correlation between different arrays of stepped tongue and the noise generation was observed. The tonal noise level of the HLHL model is a better optimization scheme because it decreases as much as 1.2 dB for the noise generation of forward-curved fan. In particular, it is found that some stepped tongues of the volute tongue achieved the goal of reducing noise generation and improving the performance of fan by experimental measurement at the same time. [ABSTRACT FROM AUTHOR]

Abstract Broadband shock-associated noise (BBSAN) is a high frequency noise generated in imperfectly expanded jets from the interaction of turbulent structures and shock-cells. This paper presents an azimuthal mode analysis of BBSAN in the jet, near-field and far-field from an unstructured large-eddy simulation of a non-screeching supersonic under-expanded jet. The studied supersonic jet is issued from a round convergent nozzle with a perfectly expanded Mach number of 1.15 and Reynolds number based on the nozzle diameter of 1.25 × 106. Aerodynamic and far-field acoustic numerical results are validated against experiments. Pressure fluctuations represented in the frequency-wavenumber domain follow the neutral waves from a vortex-sheet model of an ideally expanded jet. The far-field pressure field is decomposed in azimuthal modes based on Fourier coefficients, showing three different regions: broadband behaviour of the mixing noise for downstream angles below 50° with the first modes containing most of the energy, BBSAN with similar amplitudes for the first 4 azimuthal modes at central angles between 50° and 130°, and distinct modes at upstream angles above 130°. A decomposition in negative/positive phase-velocity waves of the jet and near-field pressure field allows the identification of these characteristic modes. The complexity of the flow is illustrated with the analysis of the root-mean-square and spectral contents of the first 4 azimuthal modes. The modes exhibit peaks at different shock-cells and radial positions as well as depicting particular radiation patterns that depend on both the mode and frequency. [ABSTRACT FROM AUTHOR]

Abstract A wall-modeled statistically converged Large Eddy Simulation (LES) of the turbulent flow in the NASA Source Diagnostic Test turbofan has been successfully performed for the first time. A good agreement with aerodynamic measurements is observed for both Reynolds Averaged Navier-Stokes and LES results, although the LES provides better results in the tip regions where large coherent structures appear and no flow separation on the stator vanes is observed. In the LES the boundary layer naturally transition to turbulence on the blade suction side but remains quasi laminar over most of its pressure side. The rotor-wake turbulence yielding the stage broadband noise is then seen to be quasi isotropic. Transition on the downstream stator vanes is not triggered by the wake impingement but rather occurs at mid-chord. Finally, acoustics are investigated using both Ffowcs Williams & Hawkings' and Goldstein's analogies from the recorded LES noise source on the stator vanes. The latter analogy provides levels closer to the measurements especially at high frequencies, although the results are most likely still influenced by too coherent rotor tip secondary flow at low frequencies. [ABSTRACT FROM AUTHOR]

The sound emission of airplanes has some applications such as localization, classification, and detecting fault. Therefore, investigation of issues, which affects the airplanes sounds, is important. In recent years, pollution of dust in all cities of the Iran shows an increasing trend. In the literature, all variables affecting the sound emission such as temperature, pressure, and relative humidity have been investigated, but there are not any researches about the influence of dust on the atmospheric attenuation coefficient. The experimental tests have been carried out with 3 sensitive microphones, 950m away from the takeoff area of Imam Khomeini international airport for 6 different airplanes, including Airbus 320, 319, 321, Boeing 747, 777, and Embraer 190 at different atmospheric conditions. The air temperature was in the range of 20-40˚C and the relative humidity was in the range of 2-34%. At first, the experimental setup was validated by available data, considering different temperatures and relative humidities. In this research, a new variable, β, has been introduced to detect the dust effect, which is defined as: the difference between the calculated sound pressure level at no dust and the measured sound pressure level while the dust density is 1μgr/m3. Airbus 320 has the minimum dust atmospheric attenuation coefficient value (0.01202db*m3/μgr) and its maximum is related to the Embraer 190 (0.0154db*m3/μgr). Finally, the obtained results show that increasing in dust concentration (PM2.5 and PM10) leads to increase in atmospheric attenuation coefficient between airplane and microphones area, and the measured sound pressure level decreases. [ABSTRACT FROM AUTHOR]

Over the last 10 years, the use of micro air vehicles has rapidly covered a broad range of civilian and military applications. While most missions require optimizing the endurance, a growing number of applications also require acoustic covertness. For rotorcraft micro air vehicles, combining endurance and covertness heavily relies on the capability to design new propulsion systems. The present paper aims at describing a complete methodology for designing quiet and efficient micro air vehicle rotors, ranging from preliminary aerodynamic prediction to aeroacoustic optimization to experimental validation. The present approach is suitable for engineering purposes and can be applied to any multirotor micro air vehicle. A fast-response and reliable aerodynamic design method based on the blade-element momentum theory has been used and coupled with an extended acoustic model based on the Ffowcs Williams and Hawkings equation as well as analytical formulations for broadband noise. The aerodynamic and acoustic solvers have been coupled within an optimization tool. Key design parameters include the number of blades, twist and chord distribution along the blade, as well as the choice of an optimal airfoil. An experimental test bench suitable for non-anechoic environment has been developed in order to assess the benefit of the new rotor designs. Optimal rotors can maintain high aerodynamic efficiency and low acoustic signature with noise reductions in the order of 10 dB(A). [ABSTRACT FROM AUTHOR]

The present work investigates the flow and the sound field generated by high subsonic jets in crossflow (JICF). The problem arises when a jet is exhausted perpendicularly into a moving medium. Although being characterized as a very complex flow, the JICF has a well-known fluid dynamics, but a sound field yet to be more explored. Therefore, a hybrid methodology of low computational cost aeroacoustic prediction tool is applied in this work for the complete investigation of this problem. A single jet operating at Mach number 0.75 in a crossflow regime with effective velocity ratios of 4 and 8 is studied herein. The fluid dynamics is solved by the Reynolds Average Navier–Stokes (RANS) equations, and the noise calculations are performed using a statistical method known as the Lighthill Ray-Tracing (LRT) method. The numerical results for the acoustic and flow fields were in reasonable agreement with the experimental data available showing good applicability of this kind of methodology for solving JICF. [ABSTRACT FROM AUTHOR]

The effects of sound pressure level at two observation positions of a fixed and varied blade pitch angle at Low-Mach unsteady incompressible Reynolds-Average Navier-Stokes flow approach, on an H-rotor Vertical Axis Wind Turbine. The objective of the study is to compare the noise dissipation and output power/energy of the airfoil blades design of the vertical axis wind turbine in residential zones. The Ffowcs Williams-Hawkings (FHWH) techniques were applied to validate the output noise and vortex shedding of the different angles of attacks (AoA). The study postulated that the time history of the calculated sound pressure level at two observers positions: the aeroacoustic, blade vortex interaction noise, flow separations, dynamic stall experience from varied angled of attacks are found to produces less noise and vortex shedding compared to the fixed angle of attack.

This paper presents an analysis of sound pressure levels through theoretical modeling and experimental validation in a 1 kW small wind turbine. The models used in the theoretical analysis are BPM (Brooks, Pope, and Marcolini) and BM (Brooks and Marcolini), where wind turbine blades are divided in sections, and each section has its own contribution with respect to the total emitted sound pressure level. The noise propagation study and its experimental validation were accomplished within the requirements of the standard IEC 61400-11 Ed.3 and the standard NOM-081-SEMARNAT-1994. The comparative study of theoretical and experimental results showed that the BPM and BM methods have a maximum error of 5.5% corresponding to the rated wind speed of 10 m/s. However, at low wind speeds, the theoretical models fit well to experimental data, for example, in the range from 5 to 8 m/s. The experimental data showed that the rotor’s aerodynamic noise is more evident at low wind speed, because under these conditions, environmental noise is much less than wind turbine noise. Finally, to prevent possible negative effects on people’s health, there is a recommended minimum and suitable distance between small wind turbine installations and buildings.

Vertical axis wind turbines (VAWT) are a source of renewable energy and are used for both industrial and domestic purposes. The study of noise characteristics of a VAWT is an important performance parameter for the turbine. This study focuses on the development of a linear microphone array and measuring acoustic signals on a cambered five-bladed 45 W VAWT in an anechoic chamber at different tip speed ratios. The sound pressure level spectrum of VAWT shows that tonal noises such as blade passing frequencies dominate at lower frequencies whereas broadband noise corresponds to all audible ranges of frequencies. This study shows that the major portion of noise from the source is dominated by aerodynamic noises generated due to vortex generation and trailing edge serrations. The research also predicts that dynamic stall is evident in the lower Tip speed ratio (TSR) region making smaller TSR values unsuitable for a quiet VAWT. This paper compares the results of linear aeroacoustic array with a 128-MEMS acoustic camera with higher resolution. The study depicts a 3 dB margin between two systems at lower TSR values. The research approves the usage of the 8 mic linear array for small radius rotary machinery considering the results comparison with a NORSONIC camera and its resolution. These observations serve as a basis for noise reduction and blade optimization techniques.

The volume of general aviation transport is increasing and consequently this gives rise to the need for reducing noise levels while improving or at least maintaining the same level of aerodynamic performance. This can be achieved through the use of modern propellers and engines. In this paper we present a methodology to study and predict the installed propeller noise at the same time with the aerodynamic characterization, through the use of unsteady CFD and Acoustic-Analogies analysis. The analysis shows the influence of different acoustic sources generated by the installed propeller on the far field noise by monitoring the monopoles and dipoles with and without taking into account the quadrupoles. In this way we can observe the influence of the distance from the propeller to the engine intake on the overall sound. Time averaged aerodynamic performances are presented along with instantaneous results.

Abstract In response to the continuous increase in aircraft noise pollution, computational aeroacoustic analyses are mandatory during the aero-engine design loop. In order to investigate the acoustic generation and propagation phenomena within a multi-stage turbomachinery, an experimental campaign on direct and indirect noise coming from a high pressure axial turbine stage has been carried out by Politecnico di Milano in the context of the European research project RECORD. The purpose of this work is to numerically predict both the direct noise produced by stator/rotor interactions and the indirect noise generated by the non-acoustic fluctuations coming from an annular combustor that impinge on the HPT stage by using URANS analyses. The computational results are in good agreement with experimental measures, confirming the possibility to include the numerical method during the engine design loop to assess noise emissions and suggest low noise design solutions. [ABSTRACT FROM AUTHOR]

A numerical method for aeroacoustic source characterization of in-duct components at frequencies beyond the cut-on frequencies of several acoustic modes is presented. Assuming linearity and time invariance, any ducted component can be fully characterized using a network (multi-port) model including source strength and scattering. A two-step multi-source approach is applied to numerical data in order to educe the multi-port characteristics. First, a scale resolving compressible flow simulation, here the Improved Delayed Detached Eddy Simulation (IDDES), is run to compute the channel flow that also contains the acoustic sources. Second, a linear acoustic computation, here the Linearized Navier Stokes Equations (LNSE), around a mean flow is solved for different acoustic loads to determine the component's scattering. The work uncovers the high potential of two-step numerical multi-port eduction methods. Particularly, it is shown that the acoustic source power spectra can be accurately extracted from IDDES data and the total acoustic power prediction is very good. Furthermore, a good result in the scattering data obtained from a second computationally inexpensive LNSE computation is achieved. The approach is interesting when describing mid-size duct systems, for example ventilation systems in aircraft and buildings, with a moderate number of higher order modes propagating in the considered frequency range. Therefore, the increasing availability of compressible flow data opens a wide field of applications. [ABSTRACT FROM AUTHOR]

In this paper, results of WindPACT 1.5 MW baseline horizontal axes wind turbine aeroacoustic and aerodynamic optimization are presented. For this purpose, the blade twist, the chord distribution, the airfoils for all sections, and the rotational speed are optimized with Multi-Objective Particle Swarm Optimization (MOPSO) algorithm. The geometric class/shape function transformation technique and Bézier curve are used for geometry parameterization. Improved blade element momentum theory and Brooks, Pope and Marcolini semi-empirical methods are implemented for wind turbine power and aeroacoustic noise calculation in the optimization procedure. MOPSO parametric study is conducted to increase both robustness and speed of the optimization cycle. Optimization objective functions are power output and overall average sound pressure level. After optimization, improved delayed detached eddy simulation is employed to verify the new wind turbine output power and noise sources. The noise propagation to the far field is calculated with the Ffowcs Williams and Hawkings acoustic analogy. Results show about 1 dB noise reduction as well as 6% power increment for the optimized WindPACT wind turbine. [ABSTRACT FROM AUTHOR]

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 × 10 5 to 9 × 10 5 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 S t < 10. The angle of attack slightly affects the TE noise with a maximum variation of 3 dB for 10 < S t < 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 S t < 25.2. For higher frequencies, the TE noise level is expected to become higher than the LE noise level. • Laminar-boundary-layer noise, turbulent-boundary-layer noise, and blunt trailing-edge noise are observed for uniform inflow • Radiated noise spectrum for uniform inflow has similar components as the wall-pressure spectrum at the trailing edge (TE) • Turbulent inflow strongly affects the wall-pressure fluctuations and spanwise correlation length along the airfoil chord • Leading-edge noise is dominant but TE noise increases due to the free-stream turbulence penetration in the boundary layer • The scaling of the noise sources with the free-stream velocity is analyzed [ABSTRACT FROM AUTHOR]

Comprehensive methods to cascade active multi-ports, e.g., for acoustic network prediction, have until now only been available for plane waves. This paper presents procedures to combine multi-ports with an arbitrary number of considered duct modes. A multi-port method is used to extract complex mode amplitudes from experimental data of single and tandem in-duct orifice plates for Helmholtz numbers up to around 4 and, hence, beyond the cut-on of several higher order modes. The theory of connecting single multi-ports to linear cascades is derived for the passive properties (the scattering of the system) and the active properties (the source cross-spectrum matrix of the system). One scope of this paper is to investigate the influence of the hydrodynamic near field on the accuracy of both the passive and the active predictions in multi-port cascades. The scattering and the source cross-spectrum matrix of tandem orifice configurations is measured for three cases, namely, with a distance between the plates of 10 duct diameter, for which the downstream orifice is outside the jet of the upstream orifice, 4 duct diameter, and 2 duct diameter (both inside the jet). The results are compared with predictions from single orifice measurements. It is shown that the scattering is only sensitive to disturbed inflow in certain frequency ranges where coupling between the flow and sound field exists, whereas the source cross-spectrum matrix is very sensitive to disturbed inflow for all frequencies. An important part of the analysis is based on an eigenvalue analysis of the scattering matrix and the source cross-spectrum matrix to evaluate the potential of sound amplification and dominant source mechanisms. [ABSTRACT FROM AUTHOR]