Noise Reduction Mechanism Research Articles

Hybrid acousto-elastic metamaterials for simultaneous control of low-frequency sound and vibration

Metamaterials have shown great potential for controlling acoustic waves and structural dynamics. Although various types of metamaterials have been developed, simultaneous control of low-frequency sound in air and vibration in solids is less investigated. This paper presents hybrid acousto-elastic metamaterials that enable simultaneous control of low-frequency sound in air and vibration in solids. For the first time, this novel metamaterial adds a compound of membrane and silicone rubber to cladding. The membrane moves the resonance frequency to a low frequency, and the silicone rubber makes the cladding layer rigid enough to support the mass. Bandgap, sound transmission loss (STL), and vibration transmission loss (VTL) were calculated by using the finite element method. Combining modal vibration mode and sound intensity streamline, the mechanisms of vibration isolation and noise reduction were analyzed and then verified through the equivalent mass–spring model. This novel metamaterial combines acoustic metamaterials and mechanical metamaterials to achieve the collaborative control of elastic waves and acoustic waves. At the same time, the peak frequencies of both STL and VTL are lower than those of the traditional metamaterials of the same size, which provides a theoretical basis and method guidance for the next step of collaborative control research of mechanical metamaterials and acoustic metamaterials. It has potential application value in the field of low-frequency vibration and noise control engineering.

Single-trap phenomena stochastic switching for noise suppression in nanowire FET biosensors

With the fast-shrinking of the transistor dimensions, the low-frequency noise level considerably increases emerging as an important parameter for the design of advanced devices for information technologies. Single-trap phenomena (STP) is a promising approach for the low-frequency noise suppression technique in nanotransistor biosensors by considering trapping/detrapping noise as a signal. We show a noise reduction mechanism offered by STP in nanoscale devices making the analogy with stochastic resonance effect found in biological systems by considering a single trap as a bistable stochastically driven nonlinear system which transmits and amplifies the weak signals. The STP noise suppression effect is experimentally demonstrated for the fabricated liquid-gated nanosensors exploiting STP. We found the optimal conditions and parameters including optimized gate voltages to implement a stochastic switching effect for the extraction of useful signals from the background noise level. These results should be considered for the development of reliable and highly sensitive nanoscale biosensors.

Leading edge serrations for the reduction of aerofoil self-noise at low angle of attack, pre-stall and post-stall conditions

This paper addresses the usefulness of leading edge serrations for reducing aerofoil self-noise over a wide range of angles of attack. Different serration geometries are studied over a range of Reynolds number [Formula: see text]. Design guidelines are proposed that permit noise reductions over most angles of attack. It is shown that serration geometries reduces the noise but adversely effect the aerodynamic performance suggesting that a trade-off should be sought between these two considerations. The self-noise performance of leading edge serrations has been shown to fall into three angle of attack (AoA) regimes: low angles where the flow is mostly attached, moderate angles where the flow is partially to fully separated, and high angles of attack where the flow is fully separated. Leading edge serrations have been demonstrated to be effective in reducing noise at low and high angles of attack but ineffective at moderate angles. The noise reduction mechanisms are explored in each of three angle regimes.

A novel image segmentation approach using fcm and whale optimization algorithm

The segmentation of images is considered a significant step in the processing of images. Due to its simplicity and efficiency, Fuzzy c-means (FCM) is most commonly employed clustering approach for image segmentation. FCM, however, has the drawbacks of sensitiveness to the prior values and local optimum solution and also, it is very sensitive to the effect of noises. In the literature survey, several optimization-based fuzzy clustering approaches were proposed to counter these drawbacks. Whale Optimization Algorithm (WOA) has a strong capability for global optimization and a combination of FCM and WOA has enhanced efficiency over conventional FCM clustering. A new approach to segmentation of image which is based on the WOA and FCM Algorithm is proposed in this paper along with the noise detection and reduction mechanism. Since exploration and exploitation phases are performed in nearly equal numbers of iterations separately, the WOA simultaneously shows better avoidance from local optima and superior convergence speed. In our experiment, we have used synthetic images and Medical Resonance Imaging (MRI) Images to validate the performance of the proposed system by taking various types of noise and the findings indicate that the proposed method is more efficient and effectively reduce the impact of noise. We compared the proposed method with other existing clustering-based segmentation techniques and then measured their efficiency using different evaluation indices, and the findings demonstrate the efficacy of the methodology proposed.

SEPARATION OF THE CONTRIBUTIONS OF "OWN" AND "SHIFT" NOISE AND DETERMINATION OF GAS-DYNAMIC AND ACOUSTIC PARAMETERS OF SILENCERS

This article presents the results of calculations of acoustic and gas-dynamic parameters of silencers of various design designs. The calculations are based on the developed methods [1, 2]. 8 types of exhaust noise mufflers of various design designs were selected for the calculations. The article contains calculated values of the following parameters: the range of gas flow velocities at the muffler outlet pipe, m/s; pressure range, Pa; temperature range, K and acoustic power values, dB. Radiation patterns with the values of the "own" and "shift" noise parameters are presented. The separation of "own" and "shift" noise is performed. The data of comparative results of experimental and numerical parametric studies are presented; satisfactory comparative results are obtained. The influence of individual design parameters (the volume of the silencer, the presence of perforation) on the acoustic efficiency of silencers is analyzed. The mechanisms of noise reduction and ways to improve their designs are shown.

Design Analysis and performance evaluation of 3D printed baffle path diverters in hydraulic noise suppressor

The pump in the hydraulic systems is the main source of noise in form of vibrations or pulsations while noise reduction is an important part of pump design still some ripples are produced by the pump termed as pulsations. Pulsations result in line vibrations which cause additional noise. How much ripple the pump produces cannot be changed, so other ways to an average of 50 dB(A) to 60 dB(A) level of noise is often considered to safe, although the noise levels resulting due to pulsations are seen to be in ranges as high as 130 t 0 150 dB(A) that result in hose and piping failures and also results in damage to components like seals and valves. Through the introduction of a compressible membrane in the path with additional baffles for muffling of the noise the proposed solution is in the form of hydraulic noise reducer that incorporates both the compressible membrane as well as the baffle path diverter. Objective of paper is carried out the performance evaluation of the hydraulic noise reducer with elastomer membrane for noise reduction mechanism in conjunction to baffle type path diverters to even out the fluid pulsations or ripple to minimal without much pressure drop across the device to achieve minim; pumping losses. The paper discusses the performance evaluation of the diverter. The has been tested to determine mass flow rate and pressure drop. Experimental has been manufactured and test has been conducted to evaluate reduction in noiselevelinwithandwithoutthe3-dprintedbaffle arrangement and comparison has been carried out.

Noise reduction mechanism of airfoils with leading-edge serrations and surface ridges inspired by owl wings

The remarkable characteristics of the silent flight of owls provide infinite inspiration for the low-noise operation of the rotating impeller machinery. However, how the characteristics of owl wings, including the leading-edge serrations, trailing-edge serrations, and plumed surfaces, affect the aerodynamic noise has not been studied comprehensively. According to previous research, the noise reduction level of an airfoil with an extensive sinusoidal profile is limited to a certain degree. In this paper, as a new coupling element, the surface ridge of owl wings is added to the airfoils with leading-edge serrations. Based on the NACA0012 (National Advisory Committee for Aeronautics) airfoil, the bionic airfoils with sinusoidal, serrated, and iron-shaped leading-edge serrations and surface ridges are reconstructed and studied to reveal the noise reduction mechanism of the coupled elements. The hybrid numerical method of large eddy simulation combined with the acoustic analog equations is adopted to predict the far-field acoustic characteristics. The vortex dynamic method is used to exposit the noise reduction mechanism of biomimetic flow control. The results demonstrate that the airfoil with iron-shaped leading-edge serrations has the best effect of noise reduction. Relative to the original airfoil, the sound pressure level is reduced by 14.3 dB. The change of streamwise vortices caused by the biomimetic structures leads the regular large-scale tubular vortices to separate into smaller horseshoe vortices. In addition, the correlation coefficient of spanwise is reduced, and the change of time-averaged vorticity in the space field promotes the attenuation effect of sound source caused by sound pressure fluctuation radiation.

A lowlight image enhancement method learning from both paired and unpaired data by adversarial training

In recent years, deep learning has been widely used in the field of lowlight image enhancement. In this paper, we propose a novel deep-learning-based lowlight image enhancement method, which could learn from both paired and unpaired data, and perform end-to-end contrast enhancement and noise reduction simultaneously. Concretely, we first employ a multi-level content loss on paired synthesized data for training enhancer to recover detail better. Then, we propose a GAN-based domain adaptation mechanism, which applies an adversarial training strategy and a novel gamma-correction-based self-supervised content loss on abundant unpaired real data, for training the enhancer to perform better on real lowlight images. Furthermore, a Patch-GAN-based noise reduction mechanism is proposed to adversarially train the enhancer to better reduce noise in real lowlight images. Finally, we improve the enhancer by introduce attention mechanism and global feature to original U-net, make it more suitable for lowlight image enhancement task. We conduct experiments on several common datasets and the results show that our method outperforms other state-of-the-arts under a variety of image quality assessment metrics. And when applied as pre-processing module, our method can improve the classification accuracy on lowlight dataset by 1.7%, outperforming other methods too.

Noise control of subsonic flow past open cavities based on porous floors

In implicit large eddy simulations, a porous material was used to suppress the noise produced by a flow past an open cavity. The base case has a Mach number and Reynolds number based on the cavity depth ReD = 105. Strong pressure fluctuations were produced. The solid cavity floor was replaced by a porous material to suppress high-pressure oscillations. With a porous floor, both the pressure fluctuations inside the cavity and noise in the near field were substantially suppressed. Four controlled cases with different porosities were considered. For low porosities, the control was better with increasing porosity. The control was best when the porosity was about 11.2%, and the maximum noise reduction was more than 10 dB. As the porosity was further increased from 11.2% to 19.27%, the control effect was decreased slightly. A porous floor can produce effects of suction and injection, which alter the structures of the recirculation and the shear layer. The control is mainly influenced by the strength of the suction effect. With control, the shear layer has less energetic smaller structures, and the interactions between the shear layer and recirculation inside the cavity are weakened. The vortex–edge impingements are mitigated, and thus, the acoustic feedback is lower, which decreases the self-sustained oscillations and the noise. Basically, the noise reduction mechanisms of the four controlled cases are similar. Our results suggest that a porous cavity floor is an effective method of noise control. However, an appropriate porosity must be chosen.

Numerical investigation of leading edge noise reduction on a rod-airfoil configuration using porous materials and serrations

A lattice-Boltzmann method has been employed to study the aeroacoustics and aerodynamics of airfoils equipped with leading edge treatments, namely the porous leading edge and leading edge serrations. The present study aims to identify the differences in noise reduction mechanisms between the two treatments. Within the context of turbomachinery applications, the airfoils undergo aerodynamic excitation due to the impingement of turbulent wake shed by an upstream rod. Two airfoil profiles are considered: NACA 0012 and NACA 5406; the latter mimics geometrical features and aerodynamic loading distribution of the outlet-guide vane in a turbofan test rig. Simulations are carried out at a freestream Mach number of 0.22, corresponding to Reynolds number based on the rod diameter of 48 000. The serrations are designed to follow a sinusoidal planform shape, whereas the porous leading edge is based on a Ni-Cr-Al metal-foam with homogeneous and isotropic properties. It is found that the porous leading edge attenuates noise by dampening surface pressure fluctuations due to the reduced blockage effect compared to the solid one. Differently, the leading edge serrations promote destructive interference of noise sources along the span. When applied against turbulent inflow with tonal characteristic, such as that induced by the impingement of Kàrmàn vortex street in the rod wake, the latter is more effective. On the other hand, both treatments are found to produce similar broadband noise reduction. When comparing aerodynamic performances, it is found that under a lifting condition, cross-flow is present through the porous material which results in lift reduction and drag increase. A serrated porous leading edge is then proposed to combine the benefits of the two leading edge treatments. This results in optimal noise reduction performances and lower aerodynamic penalty with respect to the fully porous leading edge.

Passive airfoil tonal noise reduction by localized flow-induced vibration of an elastic panel

In this paper a novel passive control method is explored for airfoil tonal noise reduction using localized flow-induced vibration of a short elastic panel flush mounted on suction surface of a NACA 0012 airfoil at low Reynolds number. The key idea is to provide local absorption of energy of natural instabilities evolving in the laminar boundary layer by self-sustained flow-induced vibration of a properly designed panel. The panel ultimately acts to weaken the aeroacoustic feedback loop responsible for airfoil tonal noise radiation. Perturbation evolution method with acoustic broadband excitation is utilized to assess the tonal noise reduction potential with various combinations of elastic panel design parameters and optimal resonating and non-resonating elastic panel configurations are determined. Subsequently, direct aeroacoustic simulation of airfoil flow with optimal panel configurations are carried out to uncover the mechanism of tonal noise reduction using localized flow-induced vibration in a quantitative manner. Extensive analysis of numerical results reveals that a resonating panel located just ahead of the sharp growth of boundary layer instability within the airfoil separation bubble provides the strongest reduction of flow instabilities and an overall tonal noise reduction up to 3 dB. Such significant noise reduction is achieved without any sacrifice in the original aerodynamic characteristics of the airfoil. The outcomes of the study evidently suggest that the proposed passive control method with a localized flow-induced panel vibration is effective in suppressing the fundamental mechanism responsible for tonal noise generation of airfoil flow, making it a promising approach for modifying the acoustics of existing aerodynamic or wing profile operating at low Reynolds numbers.

Effect of serrated trailing edges on aerofoil tonal noise

Abstract

Noise reduction of automobile cooling fan based on bio-inspired design

This study aims to minimize the noise generated by automobile cooling fans. Fan blade structures with ridged surfaces based on bio-inspired principles are 3D printed and used to replace the conventional fan blades. The effect of the bio-inspired ridge structures on the noise reduction of the cooling fan is demonstrated by orthogonal experiments in a semi-anechoic chamber. Experimental results show that with an increase in the rotational speed, the effect of the surface textures on the acoustic performance of the cooling fan becomes more significant. For example, at a fan speed of 1750 r/min, all the bio-inspired blade designs reduce noise compared with the original fan and, in particular, the sound pressure level is reduced by 3.83 dB(A) for the design with a ridge width of 4 mm and a ridge pitch of 15 mm. Through variance analysis of the measured noise, the ridge pitch distance has the most significant impact on noise reduction under low speed conditions whilst, under high speed conditions, the ridge width has the most significant influence. In addition to the experimental studies, computational fluid dynamics (CFD) simulations of the cooling fan are carried out to explain the mechanism of noise reduction for the ridged fan blades. When the fan runs, the horseshoe vortexes generated by the ridge structures disturb the flow of the boundary layer, reduce the influence of the fluid flow on the boundary layer, and delay the transition of the fan blade laminar flow to turbulence. It is also seen that there is a reduction of the intensity of the fan blade trailing edge vortices and the scale of the secondary vortices, thereby achieving the overall aim of noise reduction. This research has significance in the noise reduction design of automobile cooling fans.

Vibro-acoustic analysis of parallel barriers integrated with flexible panels

In urban communities, parallel barriers are commonly erected for controlling environmental noise such as traffic and construction noise. However, owing to the multiple reflections between the parallel barriers, their performance may be worse than that of a single barrier. To improve the performance of parallel barriers, a small piece of flush-mounted panel backed by a slender cavity in an otherwise rigid wall of barriers is proposed. With the excitation of the incident wave from a sound source inside the parallel barriers, the flexible panel vibrates, and sound is radiated out to undergo acoustic interference with the sound field bounded by the parallel barriers. Consequently, the sound energy in this space and diffraction at the barrier top edge are reduced over a broad band in the low-frequency regime. A theoretical model for tackling the vibro-acoustic coupling between the sound field of the open cavity and the vibrating panel is established to investigate the noise reduction mechanism in the shadow zone. With optimal structural properties of the panel, an additional averaged insertion loss of approximately 5 dB can be achieved at 80–1000 Hz. The theorical results, which are experimentally validated, pave the way for the potential applications of the flexible panel devices (FPDs) for improving the noise reduction of parallel barriers.

Flow and aeroacoustic characteristics evaluation of microjet noise reduction concept in the nozzle design for minimum quantity lubrication

Minimum quantity lubrication (MQL) is an efficient and sustainable technology, which has been considered to be an option of conventional flood cutting process. Even though MQL provides green solution for the cooling and lubrication function during machining, it brings about unendurable aeroacoustic jet noise from the high velocity spray. The acoustic pollution has seriously affected the operator's health and efficiency so that it should be paid much attention to. In this paper, with the utilization of microjets noise reduction concept, a noise cancellation accessory for MQL nozzle has been developed. The flow field and aeroacoustic characteristics of the original nozzle and microjets have been investigated numerically to help understand the noise reduction mechanism. Variables including the main jet flow rates, the air flow rate ratios of the microjets to the main jet and the measuring azimuths on noise directivities and sound pressure level (SPL) have been analysed. It has found the flow ratio is a critical parameter, which has an optimal value in regard to the main jet flux. The simulation results have been partially verified through OASPL measurement. The overall aeroacoustic noise reduction varied from 0.70 to 4.88 dB has been observed experimentally. Finally, the noise cancellation mechanism has been discussed from the flow field and the frequency characteristics points of view. The shrinking effect of microjet injection at appropriate air flow ratios has significantly narrowed the distribution of the turbulent intensity and reduced the SPL of low and medium frequency while increased that of the high frequency in specific directions. The overall amplitude of OASPL has been reduced. However, inappropriate air flow ratios will generate additional jet noise.

On the noise reduction mechanisms of porous aerofoil leading edges

This paper is predominantly an experimental study into the reduction of turbulence - aerofoil interaction noise by the introduction of aerofoil porosity. In this paper we study three scenarios applied to flat plates: (a) when the flat plate is fully porous, (b) when the flat plate is partially porous from the leading edge and (c) when porosity is introduced downstream of the leading edge. This paper shows that the noise reduction spectra collapse when plotted against non-dimensional frequency fl/U, where l is the length of porous section and U is the flow velocity. Narrow band measurements on a partially porous aerofoil have shown that its noise reduction spectra is characterised by a number of narrow peaks. This paper proposes two main mechanisms for explaining this behaviour. The noise reduction mechanisms are validated against noise reductions measured on a realistic aerofoil at relatively low angles of attack. One of the key findings of this paper is that, by using only a single row of holes downstream of the aerofoil leading edge one can obtain significant levels of noise reduction. This use of downstream porosity is specifically shown to be capable of providing low-frequency noise reductions without increasing the radiated noise at higher frequencies.

Noise reduction mechanisms of an open-cell metal-foam trailing edge

Open-cell porous materials have been reported as a promising concept for mitigating turbulent boundary-layer trailing-edge noise. This manuscript examines the aeroacoustics of a porous trailing edge to study its noise reduction mechanisms. Numerical investigations have been carried out for a NACA 0018 aerofoil with three different types of trailing edge: a baseline solid trailing edge, a fully porous trailing edge and a blocked-porous variant in which a solid core is added at the symmetry plane. The latter prevents flow interaction between the two sides of the aerofoil. Flow-field solutions are obtained by solving the explicit, transient and compressible lattice-Boltzmann equation, while the Ffowcs-Williams and Hawkings acoustic analogy has been used to compute far-field noise. The porous material is modelled using an equivalent fluid region governed by Darcy's law, in which the properties of a Ni-Cr-Al open-cell metal foam are applied. The simulation results are validated against reference data from experiments. The regular porous trailing edge reduces noise substantially, particularly at low frequency, whereas the blocked variant retains similar noise characteristics as the solid one. By employing a beamforming technique, the dominant source is found at the trailing edge for the solid and blocked trailing edges, while for the fully porous one, the dominant source is located near the solid-porous junction. The analysis of the scattered sound suggests that the permeability of the porous trailing edge allows for acoustic scattering along the porous medium surface that promotes destructive interference, and in turn, attenuates far-field noise intensity. The spectra and spanwise coherence of surface pressure fluctuations at the trailing edge are hardly affected by the presence of the porous material, which are found to be insufficient to justify the noise reduction. The flow field inside the porous medium is also examined to explain the differences between the fully porous and blocked-porous trailing edges. While the mean velocity components are similar for both, substantial difference is found for the velocity fluctuations. The impedance of the porous medium is computed as the ratio of velocity and pressure fluctuations. Unlike the blocked variant, the impedance in the fully porous trailing edge gradually decreases along the downstream direction, which leads to the distributed noise scattering along the porous medium surface. Additionally, the scattering efficiency at the actual trailing edge location is reduced due to the smaller impedance discontinuity.

On the use of leading-edge serrations for noise control in a tandem airfoil configuration

Passive noise control for a tandem NACA 65-710 airfoil configuration is experimentally investigated by applying leading-edge serrations on the rear airfoil. With a sliding side-plate mechanism that allows the rear airfoil to move in the vertical direction relative to the front airfoil, the position of maximum turbulence interaction noise is first identified from the far-field noise measurements. Subsequently, detailed static surface pressure distribution and unsteady surface pressure fluctuations are acquired to shed more light on the physical phenomenon and underlying noise-reduction mechanism of the leading-edge serrations. The far-field noise measurements confirm that a notable turbulence interaction noise reduction can be achieved from 600 Hz < f < 3000 Hz, agreeing well with the previous literature on the effectiveness of the leading-edge serrations. The near-field hydrodynamic analyses obtained using remote-sensing techniques of the fluctuating pressure fields over the airfoil show that a significant reduction in the surface pressure fluctuation levels up to 20 dB/Hz can be observed at the serrated-tip plane of the rear serrated airfoil close to the leading-edge regions, over the range of frequencies investigated. Although reduction can also be observed on the serrated-root plane, the magnitude is much less significant. The present results suggest that the modification of the unsteady loading on the rear airfoil by the leading-edge serrations plays a crucial role in the reduction of turbulence interaction noise in the tandem airfoil configuration, which may find practical application for noise reduction in aerodynamic systems involving rows of airfoils, such as contra-rotating open rotors and outlet guide vanes.

Control effect on deep cavity noise by slanted walls at low Mach numbers

Experimental and numerical studies on noise radiated by flow past a rectangular two-dimensional deep cavity with passive control are conducted to research the mechanism of cavity noise reduction at low Mach numbers. The clean cavity has a depth-to-length ratio of 1.5 and a width-to-length ratio of 3. The passive control method is used by slanting the front and rear walls. Using acoustic microphones, both the surface noise and far-field noise are tested in an aeroacoustic wind tunnel. It is observed that the slanted rear wall can suppress the noise effectively, but for the slanted front wall, the tones will be enhanced at some velocities. Numerical simulation is conducted to reveal the mechanism. The results reveal that the slanted rear wall can reflect the unsteadiness back to the shear layer and break up the vortices in it. These vortexes will diffuse after impacting the rear wall and prevent the perturbation from moving deeper, which brings a stable flow field into the cavity. As for the slanted front wall, the vortices will be enlarged and become accelerated in the shear layer, which makes the impingement of it to the rear wall more intense, thus leading to an increase in the noise level.

Theoretical and Experimental Study on the Transmission Loss of a Side Outlet Muffler

Vibration and noise reduction methods are important research directions in many industries. Mufflers are widely used in pipelines to reduce noise and vibration transmission. A theoretical analysis is conducted to determine the noise reduction mechanism of a muffler with a side outlet, and the results are verified with experiments. The quadrupole parameters of the muffler are obtained using transfer matrix analysis, and sensitivity analysis of the quadrupole parameters is conducted. The influence of the side outlet location on the transmission loss of the muffler is investigated experimentally using an impedance tube. The key factors affecting the transmission loss are analyzed. The theoretical calculation results of the transmission loss of the side outlet muffler are in good agreement with the experimental results.