Overall Sound Pressure Level Research Articles

Acoustic characteristics of pulse detonation engine sound propagating in enclosed space**Project supported by the National Natural Science Foundation of China (Grant Nos. 11372141 and 11472138), the Fundamental Research Funds for the Central Universities, China (Grant No. 30919011258), and the Young Scientists Fund of the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20190439).

Acoustic characteristics of pulse detonation engine (PDE) sound propagating in enclosed space are numerically and experimentally investigated. The finite element software LS-DYNA is utilized to numerically simulate the PDE sound propagating in enclosed space. Acoustic measurement systems are established for testing the PDE sound in enclosed space, and the time-frequency characteristics of PDE sound in enclosed space are reported in detail. The experimental results show that the sound waveform of PDE sound in enclosed space are quite different from those in open space, and the reflection and superposition of PDE sound on the walls of enclosed space results in the sound pressure oscillating obviously. It is found that the peak sound pressure level (PSPL) and overall sound pressure level (OASPL) of PDE sound in enclosed space are higher than those in open space and their difference increases with the rise of propagation distance. The results of the duration of PDE sound indicate that the A duration of PDE sound in enclosed space is higher than that in open space except at measuring points located at 2-m and 5-m while the B duration is higher at each of all measuring points. Results show that the enclosed space has a great influence on the acoustic characteristic of PDE sound. This research is helpful in performing PDE experiments in enclosed laboratories to prevent the PDE sound from affecting the safety of laboratory environment, equipment, and staffs.

Effect of non-parallel mean flow on the acoustic spectrum of heated supersonic jets: Explanation of “jet quietening”

Noise measurements of heated axisymmetric jets at a fixed supersonic acoustic Mach number indicate that the acoustic spectrum reduces when the temperature ratio increases. The “spectral quietening” effect has been observed both experimentally and computationally using large Eddy simulations. It was explained by Afsar, Goldstein, and Fagan [AIAA J. 49, 2522 (2011)] through the cancellation introduced by the enthalpy flux/momentum flux coupling term using the generalized acoustic analogy formulation. But the parallel flow assumption is known to give inaccurate predictions at high jet speeds. In this paper, we therefore extend the nonparallel flow asymptotic theory of Goldstein, Sescu, and Afsar [J. Fluid Mech. 695, 199 (2012)] for the vector Green’s function of the adjoint linearized Euler equations in the analogy. Using the steady Reynolds averaged Navier Stokes calculation for the jet mean flow, we find that the coupling term propagator is positive-definite and asymptotically subdominant at low frequencies corresponding to the peak jet noise when nonparallel flow effects are taken into account and self-consistent approximations for the turbulence structure are made. We then assess the validity of the non-parallel flow-based acoustic analogy model by computing the overall sound pressure level (OASPL) at various observation angles. Interpretation of the latter allows a more rational explanation of the quietening effect. In general, our noise predictions are in very good agreement with acoustic data beyond the peak frequency.

Experimental study of over-expanded jets with centered thermal non-uniformity

Recently, there has been increased concern in reducing supersonic jet noise, due in part to stricter community noise standards, renewed interest in supersonic transport over land, and the hearing damaged sustained by Navy personnel operating on the decks of aircraft carriers. In support of this, we propose an examination of the flow and acoustic fields of a supersonic jet with nozzle boundary conditions tailored to produce thermal non-uniformity. Specifically, we aim to study a heated jet with a centered thermal non-uniformity operating at over-expanded conditions. It will be shown through the course of this study that thermal non-uniformity reduces the overall sound pressure level (OASPL) by a maximum of 2.5 dB near the peak noise direction. We hypothesize the reduction in the radiated sound field is related to perturbations which persist beyond the near nozzle region and disrupt the organized structure of acoustically efficient turbulence over large axial distances. This hypothesis is supported by experimental evidence showing a reduced peak Reynolds shear stress, a decoherence in the hydrodynamic field, and a decorrelation of Mach wave structures. These effects are captured using PIV and high-speed schlieren imaging of the density near-field.

Numerical analysis of transport in porous media to reduce aerodynamic noise past a circular cylinder by application of porous foam

Metal foams as porous media have very high porosity and good strength. These materials could easily be applied to exterior surfaces of metals used in different industries. Porous media have been used less in the external flows to control the wake region or reduce the generated sound in high-speed flows. In this study, a numerical investigation is done to explore the effect of different characteristics of a porous medium—such as PPI and porosity or porous coating thickness—on the sound generated by a porous-coated cylinder. The governing transport equations in the clear and porous regions are solved using 3D Navier–Stokes and non-Darcy equations, respectively, and employing the large eddy simulation turbulence model. The generated noise is estimated using the Ffowcs-Williams and Hawkings model at the observer location. The results show that use of porous media could considerably reduce the aerodynamic noise provided that its characteristics are selected precisely. For example, results show that a porous-coated cylinder with PPI = 40 has about 30 dB higher overall sound pressure level (OASPL) than one with PPI = 10 with the same porosity and thickness. Same conditions are also observed about the thickness of the coating. The results indicate that an optimal thickness exists for which the OASPL is minimal. For example, porous-coated cylinders with coating thickness of 5 and 15 mm have about 25 dB higher OASPL compared to a cylinder with the coating thickness of 10 mm.

Leakage Noise and Related Flow Pattern in a Low-Speed Axial Fan with Rotating Shroud

The effect of rotational speed and pressure rise on the leakage flow noise radiated by a low-speed axial fan, provided with rotating shroud, has been systematically investigated. The flow in the gap region has been studied by means of particle image velocimetry (PIV) measurements taken in the meridional plane. At low blade loading, the leakage flow is restrained close to the rotor ring and, at higher loading, it forms a wide recirculation zone. In the latter conditions, an unsteady flow separation likely takes place in the blade tip region which may be observed in the instantaneous flow field only. The leakage flow noise generally increases with the blade loading, but is non-monotonic, as the overall sound pressure level (OASPL) growth is interrupted by local minima; such a trend is qualitatively independent of the rotational speed. As the loading increases, the sound pressure level (SPL) spectrum shows important modifications, since the characteristic frequency of the subharmonic narrowband humps related to the leakage noise decreases; furthermore, height and width of the humps vary non-monotonically. Such a complicated behavior is likely related to the modifications in the leakage flow pattern and also to the appearance of the flow separation at the blade tip.

Investigation of Flow Induced Structural and Acoustic Behaviour of a Flat Solar Panel Mounted on a Mini Bus Roof Top

External fitting of a flat solar panel on a vehicle at its roof top changes the aerodynamics of flow around it. A local variation of pressure and velocity parameters around the solar panel at increasing speed and yaw angle may lead to structural instability and acoustic performance deterioration of the vehicle. Present paper addresses this issue numerically using ANSYS Fluent software. The investigation consists of three steps - flow analysis, structural analysis and acoustic analysis which focus more on flat solar panel and its supporting pillars. The vehicle speed is varied up to 100 kmph and yaw angle varied from 0ï‚° to 18ï‚°. The flow analysis reports pressure and velocity distribution, Structural analysis shows the deformation and stresses induced in panel and supporting pillars and Acoustic analysis captures the change in overall sound pressure level (OASPL) at front and rear location of the vehicle. The result of investigation shows no significant effect of flow induced effect on solar panel structure. Change in OASPL at front and rear end is observed to be marginal only.

An aeroacoustic investigation into the effect of self-oscillating trailing edge flaplets

The aeroacoustics of a NACA 0012 aerofoil with an array of self-oscillating flexible flaplets attached to the trailing edge has been investigated at low to moderate chord based Reynolds number (50,000–350,000) and at geometric angles of attack from αg=0∘ – 20°. Two distinct situations were tested: one in which the flaplets were attached to the pressure side, tangentially extending the surface down the trailing edge; the other is with the flaplets attached to the suction side and extending this surface, respectively. For the reference aerofoil, strong tonal peaks are observed. When the passive flaplets are attached to the pressure side, these tonal peaks are removed and the overall sound pressure level (OSPL) is reduced by up to 20 dB. If the flaplets are placed on the suction side, the noise reduction is still present but not as strong as compared to the other case. It is concluded that the case with flaplets on the pressure side is more beneficial in interrupting the feedback loop between the boundary layer instabilities and noise sources in the wake as it is seemingly modifying the laminar separation bubble situated on the pressure side of the aerofoil, the major tonal noise source. The somewhat lower overall noise reduction obtained by the case with flaplets attached on the suction side is suggested to result from the upstream stabilisation in the boundary layer on the suction side by the mechanism of lock-in as show in a recent paper by Talboys and Brücker (2018).

Cone angle effects on noise characteristics of jets impinging on conical obstacles

This paper presents an acoustic measurement experiment to investigate noise characteristics of jets impinging on conical obstacles of different cone angles. Besides cone angle, effects of impinging velocity and impinging distance are also studied. Overall sound pressure level (OASPL) and noise spectrum are analyzed. The results suggest that effects of cone angle on noise characteristics of the impinging jet are mainly related to the impinging distance. The main spectrum characteristic for impinging jet cases is of broadband characteristic in the middle and high frequency bands. Further, compared to the free jet case, main acoustic sources of the impinging jet for three cone angle cases are discussed under different impinging distances, combined with flow field analysis and spectrum analysis. Finally, a detailed noise directivity study is carried out. The OASPL of impinging jet for different cone angle cases is presented under various impinging distances and monitoring point positions.

Prediction of high-speed train full-spectrum interior noise using statistical vibration and acoustic energy flow

The high-speed train (HST) interior noise in low, middle and high frequency bands is predicted in different simulations due to the limitations of current numerical methods. In this paper, a new method named statistical vibration and acoustic energy flow (SVAEF) is proposed to predict full-spectrum HST interior noise directly. In the SVAEF theory, interior noise is solved in the form of energy flow, and panel vibration responses caused by mechanical and acoustic excitations are assumed to satisfy the linear superposition principle. The acoustic energy of exterior sound sources is mainly attenuated by the carriage structure in the equivalent form of sound transmission loss, while the mechanical vibration energy flows inside carriage structure and interior acoustic cavities. The interior acoustic responses are calculated under a dynamic equilibrium of the whole energy flow. Initially, a HST carriage model is established based on SVAEF theories. And all the subsystem parameters and exterior excitations are acquired properly through the measurements, simulations and empirical formulas. Among them, inverse-fast Fourier transform (IFFT) and rigid multi-body dynamics are used for track irregularity excitations and contact force, while the wheel-rail rolling noise is extracted and verified with the sound scattering effect of carriage surfaces considered. The distributions of aerodynamic noise are calculated by the hybrid method of Reynolds-averaged Navier-stokes (RANS) and non-linear acoustic solver (NLAS) approach with the pantograph system, the bogie and windshield completely considered. In addition, the equipment noise under the carriage is measured as one of acoustic excitations and the vibration of carriage panels is measured to extract mechanical excitations. Finally, the interior noise is predicted through SVAEF and compared with the measurements and other prediction methods. The results indicate that the trend of SVAEF predictions is in better agreement with measurements in the whole frequency bands, and the deviations of overall sound pressure level (OASPL) are much smaller. In conclusion, the effectiveness and accuracy of SVAEF method have been verified in the full-spectrum prediction of HST interior noise.

Acoustic test facility at Tianjin spacecraft AIT center of China

The China Academy of Space Technology (CAST) designs and builds a new world-class Assembly, Integration and Test Center (AITC) at Tianjin, China. Modal, vibration and acoustic test facilities are built to support the future environmental testing needs of China space station program. The Reverberation Acoustic Test Facility (RATF) is about 4000 m3 in volume and can achieved an empty chamber acoustic overall sound pressure level (OASPL) higher than 156 dB. It is the largest acoustic facility in Asia. Beijing Institute of spacecraft Environment Engineering (BISEE) started its design and construction work from 2012 and put it into use at October of 2015. The RATF will provide an efficient support for environment testing needs of China and world’s space program in the future.

Acoustic characteristics of pulse detonation engine with ellipsoidal reflector**Project supported by the National Natural Science Foundation of China (Grant Nos. 11372141 and 11472138) and the National Defense Pre-Research Foundation of China (Grant No. 61426040201162604002).

Acoustic characteristics of a pulse detonation engine (PDE) with and without an ellipsoidal reflector are numerically and experimentally investigated. A two-dimensional (2D) non-splitting unstructured triangular mesh Euler solver based on the space-time conservation element and solution element (CE/SE) method is employed to simulate the flow field of a PDE. The numerical results clearly demonstrate the external flow field of the PDE. The effect of an ellipsoidal reflector on the flow field characteristic near the PDE exit is investigated. The formation process of reflected shock wave and reflected jet shock are reported in detail. An acoustic measurement system is established for the PDE acoustic testing. The experimental results show that the ellipsoidal reflector changes the sound waveform and directivity of PDE sound. The reflected shock wave and reflected jet shock result in two more positive pressure peaks in the sound waveform. The ellipsoidal reflector changes the directivity of PDE sound from 20° to 0°. It is found that the peak sound pressure level (PSPL) and overall sound pressure level (OASPL) each obtain an increment when the PDE is installed with a reflector. The maximum relative increase ratio of PSPL and OASPL are obtained at the focus point F2, whose values are 6.1% and 6.84% respectively. The results of the duration of the PDE sound indicate that the reflecting and focusing wave generated by the reflector result in the increment of A duration and B duration before and near focus point F2. Results show that the ellipsoidal reflector has a great influence on the acoustic characteristic of PDE sound. The research is helpful for understanding the influence of an ellipsoidal reflector on the formation and propagation process of PDE sound.

Sensitivity of acoustic predictions on the mesh size of large-eddy simulation of supersonic jet

Intense noise produced by the supersonic jet plume from a rocket can cause damage to its structural system and even a failure during launch. In order to predict the acoustic loading by rocket jet noise, large-eddy simulation (LES) is commonly employed for analysis of the jet flow field that is subsequently used as a source condition for predicting the acoustic field. Although desirable from the accuracy standpoint, LES of a supersonic jet is often burdened by an overwhelming computational cost in both runtime and memory. In this talk, we discuss the sensitivity of acoustic predictions on the spatial resolution of LES, and propose a guideline for reducing the mesh requirement. . Here, the flow field of a supersonic jet is calculated using a hybrid RANS/LES scheme on meshes of different sizes, and is fed into the Helmholtz-Kirchhoff integral for prediction of the acoustic farfield. Changes in overall sound pressure level (OASPL) and directivity of the Mach radiation are monitored as the LES mesh varies in size. The study suggests that (a) the accuracy of the acoustic prediction is not much affected by the use of a coarser mesh to a certain extent and (b) the resulting improvement in computation speed and economy thus outweighs the loss of details in LES, given the prediction of acoustic loading as the ultimate goal. (This work was conducted at High-Speed Vehicle Research Center of KAIST with the support of the Defense Acquisition Program Administration and the Agency for Defense Development under Contract UD170018CD.)Intense noise produced by the supersonic jet plume from a rocket can cause damage to its structural system and even a failure during launch. In order to predict the acoustic loading by rocket jet noise, large-eddy simulation (LES) is commonly employed for analysis of the jet flow field that is subsequently used as a source condition for predicting the acoustic field. Although desirable from the accuracy standpoint, LES of a supersonic jet is often burdened by an overwhelming computational cost in both runtime and memory. In this talk, we discuss the sensitivity of acoustic predictions on the spatial resolution of LES, and propose a guideline for reducing the mesh requirement. . Here, the flow field of a supersonic jet is calculated using a hybrid RANS/LES scheme on meshes of different sizes, and is fed into the Helmholtz-Kirchhoff integral for prediction of the acoustic farfield. Changes in overall sound pressure level (OASPL) and directivity of the Mach radiation are monitored as the LES mesh varies in s...

Numerical Study on Wind‐Induced Noise of High‐Rise Building Curtain Wall with Outside Shading Devices

Perforation metal plates with dense holes are often used as external curtain walls of high‐rise buildings. When air flow passes through these holes at a high speed, complex vortex is generated and causes a significant issue of wind‐induced noise. In this study, both Reynolds‐averaged Navier–Stokes (RANS) simulations and large‐eddy simulations (LES) were conducted to study flow around high‐rise buildings with an external sunshade curtain wall. First, wind speed distributions at the height of a typical level under 16 wind directions were acquired. Then, the maximum wind speed ratio and its corresponding azimuth were identified. Second, the sound pressure levels in the vicinity of the shading devices with two types of perforation plate schemes were calculated to evaluate the acoustic characteristics by using the FW‐H equation to simulate sound generation and propagation. The results indicate that the maximum wind speed around the buildings exists at the building corners, and the maximum wind speed ratio is 2.8 observed at 0‐degree wind direction. Under two different wind conditions, the aeroacoustic performance of perforation plate is enhanced with reducing end plate size and increasing aperture size. The overall sound pressure level (OSPL) and A‐weighted sound pressure level (ASPL) around the shading devices are 80 dB and 68 dB(A), respectively, for the improved perforation plate scheme under the 1‐year return period maximum speed, which are changed to 58dB and 45dB(A) under the annual average speed. Therefore, it is believed that perforation plates with small end plate size and large aperture size are desirable for the noise prevention design of shading devices.

Noise characteristics and flow field of an impinging jet on a conical obstacle

The noise characteristics and flow field of a low-speed impinging jet on a conical obstacle have been numerically simulated using the kinetic energy transport subgrid-scale model of the large-eddy simulation method. Noise measurement is carried out to validate the proposed simulation method. The effects of the impinging distance on the development, separation and diffusion of vortices on the back of the conical obstacle are investigated. The jet structure is better preserved and the vorticity value becomes larger as the impinging distance increases. Simulation results of the noise spectrums and overall sound pressure level (OASPL) agree well with the experimental data. The noise spectrums are analyzed and combined with simulation results for the flow field. When the impinging distance is small, the main acoustic sources with the broadband characteristic consist of the dipole source produced by pressure fluctuation on the surface of the obstacle, and the quadrupole source produced by vortices. As the impinging distance increases, the quadrupole source becomes the major acoustic source with low-frequency characteristics. In addition, the OASPL of the impinging jet is obtained at different impinging distances.

Glycemic variability and oxidative stress in patients with type 2 diabetes mellitus during combined glucose-lowering therapy

To evaluate the impact of intensified glucose-lowering therapy on carbohydrate metabolic indicator, such as glycated hemoglobin, fasting blood glucose level (BGL) (FBGL), postprandial BGL (PBGL), and glycemic variability (GV) in patients with type 2 diabetes mellitus (T2DM) during metformin monotherapy before and 3 months after therapy intensification. The investigation enrolled 51 patients with T2DM treated with metformin 1000 mg twice daily, who failed to achieve satisfactory glycemic control. During randomization, the treatment was intensified by addition of sitagliptin 100 mg/day in Group 1 (n=25) or gliclazide MB 60 mg/day in Group 2 (n=26). Before and 3 months after the treatment, carbohydrate metabolic indicators were investigated, 24-hour BGL monitoring (continuous glucose monitoring system (GMS)) was performed, and the body's antioxidant status was examined by determining the total antioxidant capacity of blood plasma (overall sound pressure levels (OASPL)). During 3-month treatment, Group 1 had a significantly reduced FBGL compared to that before the therapy; in Group 2 this index did not change significantly. Both study groups showed a significant decrease in PBGL and glycated hemoglobin (HbA1c). The mean amplitude of glycemic excursion (MAGE) was significantly decreased in the sitagliptin intensification group. In both groups, the standard deviation (SD) reduced significantly by 26% in Group 1 and by 38% in Group 2. Both groups also displayed a significant increase in blood OASPL (p<0.05). The addition of sitagliptin significantly affected the change in the indicators of both the standard carbohydrate metabolism (FBGL, PBGL, and HbA1c) and GV (MAGE, SD), whereas that of gliclazide MV altered some studied parameters. OASPL significantly increased in both groups.

Improved empirical prediction method for impinging acoustic sources distributed around interactive structures

An improved empirical prediction method has been developed to predict the noise generated by impinging acoustic sources of flight vehicles. A representative empirical prediction method is the second distributed source method (DSM-II) in the report NASA SP-8072. However, since DSM-II is based on specific experimental data, it has limitations of not being able to perfectly reflect some acoustic phenomena such as impingement, reflection, and diffraction effects induced by interactive structures. In this study, we introduce some additional experimental data and a simple theory to the conventional method, and propose an improved empirical method that could reflect the above acoustic phenomena also. First, we describe the improved method that includes the impingement effect and validate the results for the Korean Sounding Rocket-III (KSR-III) in terms of overall sound pressure level (OASPL) and sound spectrum. Next, we expand this method to the case of a high speed vehicle surrounded by a canister that generates additional acoustic phenomena such as reflection and diffraction effects. We inspect the canister effects on the vehicle as the height increases, from the view point of OASPL and spectrum. Finally, the results are discussed.

UH-1H와 UH-60 헬리콥터 블레이드의 제자리 비행 시공력 성능 및 소음 특성

Aerodynamic characteristics and noise fields of main rotors in hovering motion for UH-1H and UH-60 operated in Korean army were analyzed numerically. Unlike UH-1H blade, UH-60 blade has a 20˚ swept tip, which can decrease the wave drag at high speed range. A density-based compressible and incompressible solver of CFD++ was used for flow analysis. The Farassat formula 1A as a way of acoustic analogy was adopted for noise fields. Rotational speed of the blade set to maximum of design value, and any local shocks or delocalization of shocks from the blade were not observed at given rotational speed. FFT analysis was performed for timewise noise data, and overall sound pressure level (OASPL) was obtained based on the A-weighted scale. It turns out that the OASPL of UH-60 rotor at the designed tip speed shows 12 dB (18 dBA) lower than that of UH-1H one.

Experimental study of supersonic flow over cavity with aft wall offset and cavity floor injection

Experimental studies were conducted at Mach 1.71 supersonic flow over open cavity to find the best configuration of combined passive (aft wall offset) and active methods (cavity floor injection) to optimize the noise suppression. Cavity of length to depth ratio 3 with 0%, 5% and 10% aft wall offset were analyzed with the combined effect of 2 bar, 4 bar and 6 bar injection pressures, injected from locations of 25%, 50% and 75% of cavity length from the front wall. Experimental methodology includes instantaneous schlieren visualization and unsteady pressure measurements. In all the schlieren images, the presence of shear layer and various cavity flow features are clearly visible. Suppression in tonal amplitude for increase in offset and injection pressure is observed for various injection locations. Injection location at 75% cavity length from front wall was found to provide the maximum suppression. Spectrogram plots clearly indicate the redistribution of energy among tones and broadband noises. Increase in the number of tones is observed for higher injection pressure and aft wall offset combination. Nature of acoustic wave is confirmed from correlation and coherence plot. Over All Sound Pressure Level (OASPL) and normalized mean pressure plot are used to perform comparative study for various configurations.

Using steady flow analysis for noise predictions

Abstract Three different methods based on steady Reynolds-averaged Navier–Stokes (RANS) solutions are used to calculate the noise emitted from airfoils. Their results are compared to the ones obtained from experiments, a semi-empirical airfoil self-noise prediction code called NAFNoise developed by NREL, and large eddy simulations (LES). The three methods considered are a noise metric developed by Hosder et al. which can only predict overall sound pressure levels (OASPL) but no frequency spectra and two different statistical models developed by Doolan et al. and Remmler et al. The method by Doolan et al. employs a Green’s function solution for airfoil trailing-edge far-field noise whereas the method by Remmler et al. predicts the pressure spectrum on the airfoil surface which is then used to compute the far-field sound by means of a hybrid noise prediction. All noise predictions were made at low speed and moderate Reynolds number similar to the environment of a small unmanned aerial system and involved different NACA airfoils as well as the SD 7003 airfoil.

Numerical study on reduction of aerodynamic noise around an airfoil with biomimetic structures

A biomimetic airfoil featuring leading edge waves, trailing edge serrations and surface ridges is proposed in this study, based on flow control with each section meeting the NACA 0012 airfoil profile. Numerical simulations have been conducted to compare aerodynamic and acoustic performances between the NACA 0012 and biomimetic airfoils. These simulations utilize the large eddy simulation (LES) method and aeroacoustic analogy at an angle of attack of 0° and a Reynolds number of 1.0×105, based on using the airfoil chord as the characteristic length. The simulation results reveal the overall sound pressure levels (OASPLs) for all frequencies and at the seven observer points around the biomimetic airfoil, and a decrease of 13.1–13.9dB is observed, whereas the drag coefficient is almost unchanged. The biomimetic structures can transform the shedding vortices in laminar mode for the NACA 0012 airfoil to regular horseshoe-type vortices in the wake, and reduce the spanwise correlation of the large-scale vortices, thereby restrain the vortex shedding noise around the biomimetic airfoil.