Aeroacoustic Properties Research Articles

Influence of characteristic lengths on the aeroacoustic properties of in-duct tube bank configurations

The flow over circular cylinders or tubes arranged in different configurations is widely employed in various engineering applications, ranging from combustion systems to HVAC systems. The Reynolds number serves as the governing parameter defining the flow around the tubes, determining the separation of the flow from the tube surface, and establishing a separation point. This separation point, in turn, dictates the formation of a jet with parameters such as jet height, jet velocity, and jet length. The jet formation, influenced by varying Reynolds numbers, introduces the possibility of various characteristic lengths, including the diameter of the tubes, height of the duct, gap between adjacent tubes, jet height, and jet length. This study aims to explore the profound influence of these characteristic lengths on the aeroacoustic properties and the flow acoustic coupling at cylinders in cross flow. The jet parameters are derived from the analysis of experimental and analytical modeling data. Through a systematic investigation, we examine the interdependencies between these characteristic lengths and the resulting aeroacoustic properties.

Damage behavior of cold-rolled sintered fiber felts

Porous Materials, such as sintered fiber felts (SFFs), can contribute to the reduction of aircraft noise by acting as a low-noise trailing edge (TE). A rolling process with a time-varying rolling gap was sucessfully used to tailor the aeroacoustic properties of SFFs. To ensure the suitability of rolled porous materials for application, the mechanical properties after rolling must be investigated. The objective of this study was to clarify the effect of the rolling process on the damage evolution during tensile loading. A SFF consisting of a functional layer and a support grid made of alloy 1.4404 was used for rolling. Interrupted tensile tests in combination with computed tomography (CT) were used to investigate the damage evolution of as-received material, uniformly rolled material and material rolled with a gradient in thickness reduction. Metallographic examination and fracture surface analysis using scanning electron microscopy (SEM) complemented the CT analysis. Uniformly rolled material with a low degree of deformation (−0.74 ≤ φ < 0) failed identically to the as-received material. The functional layer failed first, starting from the center. The support grid subsequently failed due to the incremental failure of individual wires. The material rolled with a gradient in thickness reduction failed accordingly. A significant change in damage evolution occured for material rolled at high degrees of deformation φ ≤ −1.53, where the support grid fails first and the functional layer second. The rolling process using a time-varying rolling gap does not result in a detrimental mechanical behavior under tensile load. The results improve the general understanding of the effects of rolling processes on the properties of porous materials and allow the damage behavior to be taken into account with regard to its application as a TE.

Comparative numerical investigation of flow-induced noise characteristics of high-speed trains using high-resolution compressible Large Eddy Simulation

The South Korean Ministry of Land, Infrastructure and Transport has developed a “Comprehensive Plan for 400km/h High-Speed Rail” aimed at enhancing the operational speed of the high-speed railways, currently running up to 300km/h. A short-term goal is to elevate the operating speed of high-speed trains to 370 km/h, up from 320 km/h. Because aerodynamic noise proportionally escalates with the 6th powers of the speed, aerodynamic noise becomes more significant at higher speeds. Consequently, there's a pressing need for design solutions that reduce aerodynamic noise in high-speed trains. This study involves an aeroacoustic analysis using real-scale models of the current model and the preliminary design targeting 370 kph operation. Each model's 8-car formation has been simplified to a 5- car setup. A challenge in predicting aerodynamic noise is the generation of detailed sound sources in the near field and precise noise propagation in the acoustic field. For that, a three-dimensional compressible Large Eddy Simulation technique is employed, utilizing high-resolution grids. This allows for concurrent computation of the external flow and acoustic fields for a real-scale, high-speed train in an open environment. The analysis comprehensively examines the aerodynamic and aeroacoustic properties of each train car, including the major contributors to aerodynamic noise in high-speed trains. The radiated noise is predicted using the Ffowcs Williams and Hawkings equation and is further examined in relation to vortex sound sources.

Aeroacoustic two-port characterisation technique for ventilation valves

This paper describes a measurement method to characterise the low-frequency aeroacoustic properties of ventilation extraction and supply valves based on an active two-port formulation for network modelling purposes. The method uses a more compact controlled environment compared to commonly used semi-(anechoic) or reverberations rooms, and relies on a radiation model that assumes the valve radiates similarly to plane waves from an open duct. Using a loudspeaker excitation inside the set-up, a passive two-port model representing the reflection and transmission properties of the valve is obtained. This passive two-port model is then extended to an active two-port model including the source properties by characterising a reflection-free source vector. The resulting properties do not include the influence of the radiation impedance of the environment and can be used to estimate the reflection coefficient and the sound power radiated from the valve when the valve is backed by a different environment. Predictions of the one-port reflection coefficient and the predicted radiated airborne sound power, obtained from the experimentally characterised two-port properties of a ventilation valve, are compared to measurements using established methods in a semi-anechoic environment, showing good agreement in the targeted frequency range where plane wave propagation can be assumed.

Improvement of airflow uniformity and noise reduction with optimized V-shape configuration of perforated plate in the air distributor

The air distributor is the end component of the ventilation system, and its performance directly determines the comfortable and pleasant feeling of the residence. A perforated plate can be added to the air distributor to convert dynamic pressure into static pressure first and then distribute the air flow, which provides better self-adjustment compared with the embedded guide vane structure. However, the perforated plate can lead to a contradiction between head loss, noise and distribution uniformity. To reconcile this problem, a novel V-shape configuration of the perforated plate was put forward in this study. The internal flow and aeroacoustic properties of different types of perforated plates were systematically studied, and a full-scale test platform was built to verify the flow characteristics. The effects of hole size and installation position of the perforated plate on the uniform distribution of air flow were analyzed by the parametric analysis method. Then, the sound pressure level of the optimized perforated plate air distributor was further analyzed. The results show that, with the optimized perforated plate structure, the uniform flow performance was improved by about 30% and the overall sound pressure level was reduced by up to 12 dB.

Study On the Effect of Operating Conditions on Acoustic Three-Port Measurements of Perforates in presence of Grazing Flow

Different impedance eduction methods have been previously used to study the acoustic properties of perforated plates in the presence of grazing flow, giving varying results. The work presented here is to contribute to the ongoing experimental research on the acoustic behaviour of perforates under different acoustic excitations and flow conditions. Here, a three-port technique is used to study the aero-acoustic properties of perforates, which allows to observe the acoustic field on both sides of the perforate. Experimentally determined characteristics of the perforate are dependent on the operating conditions like grazing flow speed, microphone distances, standing wave pattern, and temperature. This study aims to provide the experimental analysis on the effect these factors have on the calculated perforate properties, namely the transfer impedance and the scattering matrix. Global sensitivity analysis is performed to study the individual effect of these factors in the post-processing, as well as the overall effect of error in the determination of these operating conditions.

Aeroacoustic and Aerodynamic Investigation of a Wingtip-Mounted Tractor Propeller

The wingtip-mounted propeller configuration is widely used in many aircraft, particularly owing to the emergence of urban air mobility. This paper focuses on analyzing the aeroacoustic properties of a wingtip-mounted tractor propeller by understanding the aerodynamic interaction between the two geometric components. Computational fluid dynamics based on Reynolds-averaged Navier–Stokes equations were used to calculate the flow field and aerodynamic loads on a wingtip-mounted propeller model, and the results were compared with experimental data for validation. The resulting surface pressure was fed as input data for the acoustic analysis performed using a Ffowcs Williams–Hawkings equationbased acoustic solver at multiple angles of attack. The presence of the wing significantly increased the propeller noise along the rotational axis. With increasing angle of attack, the propeller noise increased further. By contrast, the wing noise decreased with the angle of attack due to propeller wake dissipation, leading to only a small increase in the maximum total noise.

Control of High-Temperature Impinging Jet Issued from Overexpanded Rocket Nozzle

High-speed impinging jets issued from the exhaust of a launch vehicle are highly oscillatory and create hazardous conditions for the vehicle structure, affecting the sensitive payloads and personnel due to intense acoustic loads. Therefore, to reduce the unsteadiness of a jet and mitigate its adverse effects, a fundamental understanding of the associated flow and acoustic field is of utmost importance. The present experimental study is one such step to characterize the aeroacoustic properties of a high-temperature jet issued from an overexpanded rocket nozzle. Further, the effectiveness of microjet-based active flow control in reducing the flow unsteadiness and near-field noise levels is also explored. The experiments were performed at different jet stagnation temperatures, and the results were quantified at different impingement heights simulating the takeoff and landing environment of reusable launch vehicles. Mean pressure measurements were mapped on the ground plane using discrete pressure taps. The time-varying pressure component was measured using an unsteady pressure transducer located at the impingement point, and near-field acoustic measurements were carried out using a microphone. In addition, the velocity field was mapped using planar particle image velocimetry. The near-field pressure spectra are broadband, and the magnitude of noise content is a function of jet stagnation temperature. Furthermore, the injection of microjets resulted in a significant attenuation of near-field noise, impingement point pressure fluctuations, and turbulent kinetic energy.

Enhancing the Vortex Whistle for Measures of Respiratory Capacity Via Computational Fluid Dynamics and Computational Aero-Acoustic Analysis.

A vortex whistle produces a fundamental frequency proportional to the inlet flowrate. Recent investigations using vortex whistles have focused on the use of this relationship to quantify aspects of respiratory function. Despite promising results, there is a lack of understanding of the physical mechanisms underlying vortex whistle function. This paper begins with a principled study of the aero-acoustic properties of the vortex whistle. First, a high-fidelity computational fluid dynamics (CFD) simulation was developed to predict the unsteady flow field induced by the vortex whistle when the expiratory flow is applied. A computational aero-acoustic analysis (CAA) was applied to predict the acoustic response of the vortex whistle and to capture the frequency and level of the signature spectral peaks. The CFD is validated against prior experimental data on the vortex whistle. The CFD was used to: (a) determine the source of the vortex whistle harmonics and (b) investigate the effect of an outlet tube terminator, proposed by Awan and Awan (2020, "Use of a Vortex Whistle for Measures of Respiratory Capacity," J. Voice). The CFD and CAA indicated that the harmonics are generated by the cylindrical cavity of the vortex whistle, and the outlet terminator increases harmonic signal-to-noise ratio by increasing the pressure fluctuation within the cylindrical cavity. These results support the addition of the outlet tube terminator and provide insight into future design modifications that will enhance the reliability of the vortex whistle analyses and enable additional measures of respiratory capacity.

An experimental system to acquire aeroacoustic properties on wind turbine blades

Wind turbine noise is a key issue preventing the successful exploitation of the full potential of wind energy throughout the world, especially in urban areas. To better assess and predict wind turbine noise, several aeroacoustic simulations and models have been developed over the past. Many semi-empirical models for noise emission and propagation rely on aeroacoustic properties at the blade level, including the pressure gradient, the spectrum of the pressure fluctuations, the convection velocity and the coherence lengths. Field measurements of these local quantities on operating wind turbines are valuable to improve the accuracy of the models. In the Aerosense project, a cost-effective smart measurement system is being developed that is thin, easy to install without damaging the blade, low power, self-sustaining and wirelessly transmitting. This measurement system uses MEMS sensors, which require some calibrations and corrections to obtain sufficiently accurate data. This paper describes the experimental system and its workflow, which has been developed within the Aerosense project to obtain sufficiently accurate measurements for semi-empirical noise emission and propagation models. The experimental system and its workflow are then validated in an anechoic wind tunnel on a NACA63-418 airfoil. The results show that this experimental system is able to acquire relevant aeroacoustic properties on operating wind turbines.

Expert decision support system for aeroacoustic source type identification using clustering.

This paper presents an Expert Decision Support System for the identification of time-invariant, aeroacoustic source types. The system comprises two steps: first, acoustic properties are calculated based on spectral and spatial information. Second, clustering is performed based on these properties. The clustering aims at helping and guiding an expert for quick identification of different source types, providing an understanding of how sources differ. This supports the expert in determining similar or atypical behavior. A variety of features are proposed for capturing the characteristics of the sources. These features represent aeroacoustic properties that can be interpreted by both the machine and by experts. The features are independent of the absolute Mach number, which enables the proposed method to cluster data measured at different flow configurations. The method is evaluated on deconvolved beamforming data from two scaled airframe half-model measurements. For this exemplary data, the proposed support system method results in clusters that mostly correspond to the source types identified by the authors. The clustering also provides the mean feature values and the cluster hierarchy for each cluster, and for each cluster member, a clustering confidence. This additional information makes the results transparent and allows the expert to understand the clustering choices.

Investigating the Aeroacoustic Properties of Porous Fabrics

The aeroacoustic properties of porous fabrics are investigated experimentally with the goal of finding a fabric that serves as an improved interface between wind tunnel flow and quiescent conditions. A total number of eight porous fabrics were investigated, namely, four glass fiber fabrics, two plain-weave Kevlar fabrics, and two modified plain Kevlar fabrics with their pores irregularly clogged. Two custom-made rigs were used to quantify the transmission loss (TL) and self-noise of all fabrics. The pores were found to serve as a low-resistance gateway for sound to pass through, hence enabling a low TL. The TL was found to increase with decreasing open area ratio (OAR), whereas other fabric properties had a minor impact on TL. The thread density was found to be a primary factor in determining the frequency range of porous fabrics’ self-noise, with the OAR potentially playing a secondary role in the self-noise levels. Fabrics with irregular pore distribution showed a more broadband self-noise signature associated with lower frequencies compared to fabrics with periodic pore patterns. Overall, fabrics with an irregular pore distribution or fabrics with increased thread density were identified as two potential ways to obtain superior aeroacoustic behavior compared to commonly used Kevlar fabrics.

Aerodynamic and aeroacoustic properties of axial fan blades with slitted leading edges

A detailed experimental analysis of the aerodynamic and aeroacoustic properties of flat-plate axial fans with slitted leading edges is performed. The sound emissions of five slitted leading edge designs are measured at a constant rotational speed and at a constant total-to-static pressure rise of the fans. For both cases, the fan blades with slitted leading edges reduce the turbulence interaction noise and lead to a reduction of the overall sound pressure level for volume flow rates above 0.6 m3 s−1 compared to an axial fan with solid leading edges. The far-field noise analysis shows that the slits result in a noise reduction for frequencies below 2 kHz and a noise increase above 2 kHz. In addition, sound source localization is conducted with a microphone array and rotating beamforming methods are applied. The identified sound source distributions prove that slitted leading edges reduce turbulence interaction noise, but generate broadband noise in the fan blades’ trailing edge regions. The maximum sound reduction due to the slits could be detected at a dimensionless frequency of $ fh/\bar{w}\approx 0.5$, where f is the frequency, h is the height of the slit and $ \bar{w}$ is the mean relative inflow velocity. The noise reduction mechanism on axial fan blades corresponds well to previous investigations on flat-plate airfoils with slits.

Production and characterization of experimental low noise trailing edges made of cold rolled porous aluminum with special attention to the influence of cold rolling on the mechanical stability of the investigated materials

In the framework of the CRC 880 “Fundamentals of high-lift for future civil air craft” methods for the reduction of aircraft noise are investigated. An important method for this noise reduction is the usage of porous material as low noise trailing edges. To improve the aeroacoustic properties of porous materials, an innovative rolling process was established by Tychsen et al. (Metals 8:598, 2018). Here, the rolling process is described as it is used as an important method for the production of samples. The influence of cold rolling on two different porous materials namely porous aluminum 80–110 (PA 80–110) and PA 120–150 is investigated. Important characteristics studied are the porosity, mechanical properties and the dependence of flow resistivity from the degree of deformation. The flow resistivity is of particular interest as the aeroacoustic performance is significantly influenced by it. The results are then compared to the findings for PA 200–250, which was investigated in Tychsen et al. (Metals 8:598, 2018). Lastly, experimental trailing edges made out of cold rolled porous aluminum with a gradient in thickness reduction are shown. The characterization of the aeroacoustic behavior is not part of this study. Reference is made to Rossignol et al. (Int J Aeroacoust 19:365–384, 2020), where trailing edges shown here are characterized aeroacoustically. The findings shown here demonstrate that different porous materials can be tailored by cold rolling without negative impact on the mechanical behavior. It is proven that the new rolling process is a versatile tool for the production of gradient porous material.

Experimental characterization of aero-acoustic performance of micro-perforated materials in a wind tunnel

Mitigating the propagation of low frequency noise in ducted flows represents a challenging task since wall treatments have often limited dimensions. Micro-perforated panels have been successfully used backed by a cavity constituting a bulk-reacting resonator or in combination with honeycomb for a locally reacting system. In this work, a cost-efficient methodology for the study and characterization of the aero-acoustic properties of flush-mounted micro-perforated resonators has been developed. Although Laser Doppler Velocimetry is a non-intrusive technique, it requires delicate instrumentation to obtain an estimation of the acoustic velocity and pressure fluctuations at several points over a wall liner. Here, the attenuation has been estimated from sound pressure level measurements performed with two nosecone microphones positioned along a vertical line centered on the resonator axis. Experiments have been performed in a wind tunnel in presence of a low-speed turbulent boundary layer of air fully developed over different samples made up of micro-perforated sheets or porous materials with surface roughness. The micro-perforates have been flush-mounted over a cylindrical cavity of depth 30 mm situated on the floor of the test section. The ability of bulk-reacting resonators for reducing the acoustic or flow-induced noise has been assessed in comparison with locally reacting treatments.

Effect of trailing-edge bevel on the aeroacoustics of a flat-plate

An experimental study was conducted to investigate the effect of the bevel angle on the radiated noise and the associated flow-field over the trailing-edge. The bevel angle of the trailing-edge was adjusted over a broad range of angles to demonstrate the aeroacoustic properties of the attached and separated boundary layers. The far-field noise results obtained from the beamforming array were assessed in conjunction with the associated flow-field. Far-field measurement results show an increased level of radiated noise for a range of bevel angles where the flow remains attached compared to the baseline flat-plate case without a bevel angle. Near-field measurements are presented in terms of pressure coefficients, unsteady surface pressure results, turbulence properties of the boundary layer, and corresponding spectral properties of these quantities. The near-field measurement results indicate that introducing a bevel angle yields a region of favorable pressure gradient on the flat-plate and bevel, which accelerates the flow and reduces the size of structures in the boundary layer up until the mid-bevel region, which then decelerates toward the trailing-edge. This change increases the energy content of surface pressure fluctuations as well as the energy content of the velocity field over the trailing-edge and wake, which, in turn, results in an increased radiated far-field noise as supported by beamforming results. Furthermore, at a sufficiently high bevel angle, where pronounced flow separation occurs, the lack of interaction between trailing-edge and shear layer leads to a significant reduction in the radiated far-field noise compared to that of the baseline flat-plate.

On the Possibility of Using a Single Time Realization for Investigating Noise from Vortex Rings

The possibility of using a single time realization of a vortex ring to study its aeroacoustic properties is considered. For each realization, averaging is performed over six microphones located around the path of the ring at the same distance from its axis. Spectra obtained as a result of such averaging are presented. It is shown that the specific features of noise from an individual vortex ring agree well with earlier results when averaging over an ensemble of realizations. It is proposed for the first time to localize the sources of noise produced by a vortex ring during its motion using a multimicrophone array with a beamforming algorithm adapted for the considered problem. The position of the noise sources on the localization map is in good agreement with the trajectory measurements of the vortex ring. A beamforming array was used to obtain the averaged spectra of single realizations corresponding to localization of sources for different delays from the moment of vortex launch.

Investigation of Aeroacoustic Properties of Low-Pressure Axial Fans with Different Blade Stacking

Several blade-stacking strategies exist for low-pressure axial fans with potential benefits with respect to the aeroacoustic sound emission. In this study, the impact of such different blade-stacking strategies (including single-type and combined-skewed fan blades) on the fan characteristic curves, aerodynamic properties, and aeroacoustic properties was investigated. It was found that the fan aerodynamic and acoustic characteristic curves are strongly governed by the type of fan blade skew in the outer part of the fan blade. This was also observed for the flow-field on the suction side and on the pressure side. Sound pressure spectra and beamforming sound maps of the fans were further studied for one representative point in the typical fan operating range. The spectra showed similar tendencies as the fan characteristic curves, with spectra progressions being mainly dependent on the type of fan blade skew in the outer part of the fan blade. The sound maps revealed that, for the most part, sound sources are located in the outer fan region but that source characteristics may still be different for single-type and combined-skewed fans. These findings can be further used for reducing the sound emission of low-pressure axial fans with a focus on the fan blade shape in the outer part of the fan blade.

The effect of low lobe count chevron nozzles on supersonic jet screech

An experimental study was conducted into the fluid and aeroacoustic properties of jets issuing from nozzles with two or four chevrons. Flow visualisation in the form of schlieren imaging was utilised to investigate the general change in jet structure. Schlieren imaging revealed a highly asymmetric flow field produced by the azimuthal asymmetry at the nozzle lip. The shock structures within the flow were significantly altered in both symmetry and spacing by the introduction of chevrons. A spatial two-point correlation was used to classify the dominant azimuthal instability mode produced by each of the respective nozzles. It was found that the two-lobe nozzle favoured a lateral flapping mode towards the chevron sides of the nozzle. The four-lobe nozzle, when coherent structures were visible, produced a mode somewhat more difficult to classify, being either a helical mode or a multi-axial flapping mode. A complex relationship was observed between the range of pressure ratios over which strong screech tones are evident, and the number of lobes. The two-lobe nozzle provided screech suppression below a nozzle pressure ratio of 2.7, but directional screech amplification at higher pressures. The four-lobe nozzle provided less screech amelioration at low pressures, but eliminated the screech tone entirely for most other pressures studied. The flapping oscillation of the two-lobe nozzle produced a highly asymmetric acoustic field. The four-lobe nozzle produced a relatively symmetric acoustic field.

On the aeroacoustic properties of a beveled plate

The flow around a beveled flat plate model with an asymmetric 25 degrees trailing edge with three rounding radii is analyzed using a Navier-Stokes based open source software package OpenFOAM in order to predict the aeroacoustic properties of the models. A Large Eddy Simulation with a dynamic Smagorinsky and implicit model are used as closure model for the flow solver, and are compared regarding their aeroacoustic performance. Velocity coherence and pressure correlation is determined in spanwise direction. The acoustic far field spectrum is obtained by solving Curle’s analogy in frequency domain as a post-processing step.