Suppression of flow-acoustic coupling in sidebranch ducts by interface modification

The flow-acoustic coupling of shear layer instabilities with the acoustic resonances at the interface of a closed sidebranch and main duct can produce high-amplitude pure-tone noise, known as “whistle”. This study investigates experimentally the effect of various interface geometry modifications on whistles. The objective of the modifications is to suppress the noise by redirecting the shear layer at the main duct-sidebranch interface. Interchangeable suppressor blocks of varying shapes and sizes mounted upstream and downstream of the sidebranch opening are used to change the geometry. The block shapes include those with square edges, ramps, bevelled edges, and curved (radiused) edges. The experiments are conducted in a flow facility at conditions that include certain ranges of Strouhal numbers known to coincide with significant noise generation. The effectiveness of various suppressors in reducing the noise is assessed by analyzing the measured sound pressure levels.     

WHISTLES WITH A GENERIC SIDEBRANCH: PRODUCTION AND SUPPRESSION

The present study investigates experimentally the production and suppression of whistle noise resulting from the shear layer instabilities coupled with the acoustic resonances at the interface of two ducts, a main duct and connecting sidebranch. A generic sidebranch adapter is built to allow for mounting downstream of the throttle body in the induction system of a production engine, and the adjustment of sidebranch length. The adapter has also the provision to investigate a number of suppression methods such as: (1) ramps mounted in the main duct right upstream of the sidebranch opening; (2) a spacer to increase the distance between the throttle plate and sidebranch opening; and (3) the rotation of the throttle body from its original position. Experiments with the same hardware are conducted in both a flow laboratory and an engine dynamometer facility. The effectiveness of these suppression techniques is examined experimentally along with the correlation between the two facilities.     

Time-domain simulation of acoustic wave propagation and interaction with flexible structures using Chebyshev collocation method

A time-domain Chebyshev collocation (ChC) method is used to simulate acoustic wave propagation and its interaction with flexible structures in ducts. The numerical formulation is described using a two-dimensional duct noise control system, which consists of an expansion chamber and a tensioned membrane covering the side-branch cavity. Full coupling between the acoustic wave and the structural vibration of the tensioned membrane is considered in the modelling. A systematic method of solution is developed for the discretized differential equations over multiple physical domains. The time-domain ChC model is tested against analytical solutions under two conditions: one with an initial state of wave motion; the other with a time-dependent acoustic source. Comparisons with the finite-difference time-domain (FDTD) method are also made. Results show that the time-domain ChC method is highly accurate and computationally efficient for the time-dependent solution of duct acoustic problems. For illustrative purposes, the time-domain ChC method is applied to investigate the acoustic performance of three typical duct noise control devices: the expansion chamber, the quarter wavelength resonator and the drum silencer. The time-dependent simulation of the sound-structure interaction in the drum silencer reveals the delicate role of the membrane mass and tension in its sound reflection capability.     

Flow-resonant sound interaction in a duct containing a plate,part I: Semi-circular leading edge

The interaction between flow and flow-induced acoustic resonances near rigid plates with semi-circular leading edges located in a hard-walled duct is described. These plates generate acoustic resonances over flow velocity ranges depending on thickness, chord and trailing edge geometry, together with rigidity, internal dimensions, length of the working section and shape of the terminations of the working section. A potential flow model for the plate with a smooth leading edge is developed, and the acoustic power generated by vortices growing and shedding from the trailing edge is calculated. The rate of growth of the vortices is determined by an instantaneous Kutta condition applied over part of the cycle. This technique simulates the influence of the sound field on vortex growth.     

Effects of the pump-circuit acoustic coupling on the blade-passing frequency perturbations

Centrifugal pumps are a source of pressure and flow rate perturbations in hydraulic pumping systems. In particular, a significant excitation is usually induced at the blade-passing frequency and harmonics as a consequence of the fluid-dynamic interaction between the rotor and the stator. The magnitude of this excitation is very dependent on the internal geometry of the pump and on its point of operation, but it depends also on the acoustic response of the hydraulic network to the perturbations. The induced and transmitted perturbations can be either amplified or reduced depending on the pump-circuit acoustic coupling, and thus they can lead to excessive levels of noise and vibration under certain conditions. The purpose of the present investigation is the theoretical and experimental characterization of the perturbations induced in a laboratory pumping system, as a function of the acoustic impedance of the pipelines. For different points of operation, the blade-passing frequency impedance is changed by varying the speed of rotation and, additionally, by modifying a closed side branch of the hydraulic system (that is, in the absence of net flow through it). For the theoretical calculations an acoustic model, based on matrix formulation, is applied to obtain the influence of different acoustic impedances of the suction side on the pressure fluctuations at the pump. Test measurements with a fast-response piezoelectric pressure transducer situated at the tongue region of the pump under the same system configurations confirm the significant effect of the pump-circuit acoustic coupling on the pressure perturbations.     

PREDICTION OF NON-CAVITATING UNDERWATER PROPELLER NOISE

Non-cavitation noise of underwater propeller is numerically investigated. The main purpose is to analyze non-cavitation noise in various operating conditions with different configurations. The noise is predicted using time-domain acoustic analogy and boundary element method. The flow field is analyzed with potential-based panel method, and then the time-dependent pressure data are used as the input for Ffowcs Williams-Hawkings formulation to predict the farfield acoustics. Boundary integral equation method is also considered to investigate the effect of ducted propeller. Sound deflection and scattering effect on the duct is considered with the BEM. The governing equations are based on the assumption that all acoustic pressure is linear. A scattering approach is applied in which the acoustic pressure field is split into the known incident component and the unknown scattered component. Noise prediction results are presented for single propeller and ducted propeller in non-uniform flow conditions similar to real situation. The investigation reveals that the effect of a duct on the acoustic performance propeller is small in the far field under non-cavitating situations since the noise directivities of single and ducted propellers are almost the same. Only the high order BPFs are influenced by the existence of the duct.     

Passive noise control by enhancing aeroacoustic interference due to structural discontinuities in close proximity

In-duct devices are commonly installed in flow ducts for various flow management purposes. The structural construction of these devices indispensably creates disruption to smooth flow through duct passages so they exist as structural discontinuities in duct flow. The presence of these discontinuities provides additional possibility of noise generation. In real practice, in-duct devices do not exist alone in any duct system. Even though each in-duct device would generate its own noise, it might be possible that these devices could be properly arranged so as to strengthen the interference between individual noise; thus giving rise to an overall reduction of noise radiation in the in-duct far field. This concept of passive noise control is investigated by considering different configurations of two structural discontinuities of simple form (i.e., a cavity) in tandem in an unconfined flow and in opposing setting within a flow duct. It is known that noise generated by a cavity in unconfined domain (unconfined cavity) is strongly dependent on flow-resonant behavior within the cavity so the interference it produces is merely aeroacoustic. The objective of the present study is to verify the concept of passive noise reduction through enhancement of aeroacoustic interference due to two cavities by considering laminar flow only. A two-dimensional approach is adopted for the direct aeroacoustic calculations using a direct numerical simulation (DNS) technique. The position and geometries of the cavities and the Mach number are varied; the resultant aeroacoustic behavior and acoustic power are calculated. The numerical results are compared with a single cavity case to highlight the effect of introducing additional cavities to the aeroacoustic problem. Resonant flow oscillations occur when two unconfined cavities are very close and the associated acoustic field is very intense with no noise reduction possible. However, for duct aeroacoustics, it is found that a 7.9 db reduction of acoustic power in the downstream side of the duct or a total reduction of ∼6 db is possible with opposing cavities having an offset of half a cavity length. In addition, the reduction is shown to be free from lock-on with trapped modes of the ducts with cavities.     

Multi-mode sound transmission in ducts with flow

Exhaust mufflers, large exhaust stacks, and turbofan engines are common examples of ducted noise. The most useful measure of the sound produced by these noise sources is the sound power transmitted along the duct. When airflow is present, sound power flow can no longer be uniquely determined from the usual measurements of acoustic pressure and particle velocity.One approach to sound power determination from in-duct pressure measurement, and the one discussed in this paper, is to predict the relationship between the sound power and pressure based upon an assumed mode amplitude distribution. This paper investigates the relationship between acoustic pressure and power for a family of idealized source distributions of arbitrary temporal and spatial order. Incoherent monopole and dipole sources uniformly distributed over a duct cross-section can be obtained as special cases. This paper covers the sensitivity of the pressure-power relationship to source multipole order, frequency and, in particular, flow speed. It is shown that the introduction of flow in a hard-walled duct can have a substantial effect on the behavior of the pressure-power relationship for certain source distributions. Preliminary experimental results in a no-flow facility are presented in order to verify some of the main results.     

Time-domain characterization of the acoustic damping of a perforated liner with bias flow

Combustion instabilities are caused by the interaction of unsteady heat releases and acoustic waves. To mitigate combustion instabilities, perforated liners, typically subjected to a low Mach number bias flow (a cooling flow through perforated holes), are fitted along the bounding walls of a combustor. They dissipate the acoustic waves by generating vorticity at the rims of perforated apertures. To investigate the absorption of plane waves by a perforated liner with bias flow, a time-domain numerical model of a cylindrical lined duct is developed. The liners' damping mechanism is characterized by using a time-domain "compliance." The development of such time-domain compliance is based on simplified or unsimplified Rayleigh conductivity. Numerical simulations of two different configurations of lined duct systems are performed by combining a 1D acoustic wave model with the compliance model. Comparison is then made between the results from the present models, and those from the experiment and the frequency-domain model of previous investigation [Eldredge and Dowling, J. Fluid Mech. 485, 307-335(2003)]. Good agreement is observed. This confirms that the present model can be used to simulate the propagation and dissipation of acoustic plane waves in a lined duct in real-time.     

Voice production model integrating boundary-layer analysis of glottal flow and source-filter coupling

A voice production model is created in this work by considering essential aerodynamic and acoustic phenomena in human voice production. A precise flow analysis is performed based on a boundary-layer approximation and the viscous-inviscid interaction between the boundary layer and the core flow. This flow analysis can supply information on the separation point of the glottal flow and the thickness of the boundary layer, both of which strongly depend on the glottal configuration and yield an effective prediction of the flow behavior. When the flow analysis is combined with the modified two-mass model of the vocal fold [Pelorson et al. (1994). J. Acoust. Soc. Am. 96, 3416-3431], the resulting acoustic wave travels through the vocal tract and a pressure change develops in the vicinity of the glottis. This change can affect the glottal flow and the motion of the vocal folds, causing source-filter coupling. The property of the acoustic feedback is explicitly expressed in the frequency domain by using an acoustic tube model, allowing a clear interpretation of the coupling. Numerical experiments show that the vocal-tract input impedance and frequency responses representing the source-filter coupling have dominant peaks corresponding to the fourth and fifth formants. Results of time-domain simulations also suggest the importance of these high-frequency peaks in voice production.     

Prediction of flow-generated noise produced by acoustic and aerodynamic interactions of multiple in-duct elements

Flow-generated noise problem caused by in-duct elements is due to the complicated acoustic and turbulent interactions of multiple in-duct flow noise sources. The approach of partially coherent sound fields used previously by Mak and Yang [C.M. Mak, J. Yang, Flow-generated noise radiated by the interaction of two strip spoilers in a low speed flow ducts, Acta Acust united with Acustica 88 (2002) 861–868] and Mak [C.M. Mak, A prediction method for aerodynamic sound produced by multiple elements in air ducts, J Sound Vib 287 (2005) 395–403] is adopted to formulate the sound powers produced by interactions of multiple elements at frequencies below and above the cut-on frequency of the lowest transverse duct mode. The study indicates that the level and spectral distribution of the additional acoustic energy produced by the interactions of multiple elements can be predicted based on the measured data with respect to the interactions. The proposed method can form a basis of a generalized prediction method for flow-generated noise produced by multiple elements. The application of the proposed method is supported by two engineering examples.     

Simulations of the scattering of sound waves at a sudden area expansion

The scattering of acoustic plane waves at a sudden area expansion in a flow duct is simulated using the linearized Navier–Stokes equations. The aim is to validate the numerical methodology for the flow duct area expansion, and to investigate the influence of the downstream mean flow on the acoustic scattering properties. A comparison of results from numerical simulations, analytical theory and experiments is presented. It is shown that the results for the acoustic scattering obtained by the different methods gives excellent agreement. For the end correction, the numerical approach is found superior to the analytical model at frequencies where coupling of acoustic and hydrodynamic waves is significant. A study with two additional flow profiles, representing a non-expanding jet with an infinitely thin shear layer, and an immediate expansion, shows that a realistic jet is needed to accurately capture the acoustic–hydrodynamic interaction. A study with several different artificial jet expansions concluded that the acoustic scattering is not significantly dependent on the mean flow profile below the area expansion. The constructed flow profiles give reasonable results although the reflection and transmission coefficients are underestimated, and this deviation seems to be rather independent of frequency for the parameter regime studied. The prediction of the end correction for the constructed mean flow profiles deviates significantly from that for the realistic profile in a Strouhal number regime representing strong coupling between acoustic and hydrodynamic waves. It is concluded that the constructed flow profiles lack the ability to predict the loss of energy to hydrodynamic waves, and that this effect increases with increasing Mach number.     

Noise reduction using a quarter wave tube with different orifice geometries

It is well known that the acoustic performance of silencing elements decreases with an increase in exhaust gas flow. Tests were conducted on three orifice geometries of side-branches on an adaptive quarter-wave tube to determine which was the least compromised by the high-speed exhaust gas passing over the side-branch. The side-branch geometries that were tested were a sharp edge, a backward inclined branch, and a bell mouth. The experimental results show that the side-branch with a bell-mouth geometry resulted in the greatest noise reduction by an adaptive quarter-wave tube.     

空腔非局域反应声衬传播特性研究

本文对声波在空腔非局域反应声衬管道内的传播进行了研究。特征方程通过声衬内和管道内两部分声场的耦合求解得到,并采用积分方法对特征方程积分求解,通过模态匹配的方法建立并求解了有限长管道非局域反应声衬的声辐射数值模型,展示了在管道消声主动控制方面的应用潜力。     

Active noise control in finite length ducts

A simple technique for the active control of sound in ducts, initially suggested by Olson and May [1], is investigated in detail. A simple, “virtual earth” principle, feedback loop is used to drive the sound pressure to a minimum at a microphone placed close to a loudspeaker in the duct wall. This produces a reflection of downstream travelling plane waves. A detailed investigation of the loudspeaker near field has enabled the optimum position of the microphone to be identified. The system is shown to be especially effective at the frequencies of the longitudinal duct resonances, where the acoustic response of the duct produces a high loop gain. Results are presented which show a reduction of up to 20 dB in the amplitude of low frequency broadband noise at a position downstream of the cancelling source.     

一种基于声相似律的旋转流体机械噪声声源分离方法

针对管道系统中的流体机械发声问题,提出了一种声源函数与声响应函数的频谱分离方法。该方法以声相似律为基础,分别采用最小均方值多重线性回归方法和高斯滤波获得声源函数和声响应函数。通过算例验证了该方法的有效性,辨识得到的声源函数和声响应函数的相对误差均远小于现有方法。利用该方法分析了自由空间和带管道两种安装条件下轴流风扇的辐射噪声,辨识得到了相应的声源函数、声响应函数和叶尖速度律指数,并对风扇的主要噪声成分进行了分析。      

Fan interaction noise reduction using a wake generator: experiments and computational aeroacoustics

A control grid (wake generator) aimed at reducing rotor-stator interaction modes in fan engines when mounted upstream of the rotor has been studied here. This device complements other active noise control systems currently proposed. The compressor model of the instrumented ONERA CERF-rig is used to simulate suitable conditions. The design of the grid is drafted out using semi-empirical models for wake and potential flow, and experimentally achieved. Cylindrical rods are able to generate a spinning mode of the same order and similar level as the interaction mode. Mounting the rods on a rotating ring allows for adjusting the phase of the control mode so that an 8 dB sound pressure level (SPL) reduction at the blade passing frequency is achieved when the two modes are out of phase. Experimental results are assessed by a numerical approach using computational fluid dynamics (CFD). A Reynolds averaged Navier-Stokes 2-D solver, developed at ONERA, is used to provide the unsteady force components on blades and vanes required for acoustics. The loading noise source term of the Ffowcs Williams and Hawkings equation is used to model the interaction noise between the sources, and an original coupling to a boundary element method (BEM) code is realized to take account of the inlet geometry effects on acoustic in-duct propagation. Calculations using the classical analytical the Green function of an infinite annular duct are also addressed. Simple formulations written in the frequency domain and expanded into modes are addressed and used to compute an in-duct interaction mode and to compare with the noise reduction obtained during the tests. A fairly good agreement between predicted and measured SPL is found when the inlet geometry effects are part of the solution (by coupling with the BEM). Furthermore, computed aerodynamic penalties due to the rods are found to be negligible. These results partly validate the computation chain and highlight the potential of the wake generator system proposed.     

The acoustic reflex threshold in aging ears

This study investigates the controversy regarding the influence of age on the acoustic reflex threshold for broadband noise, 500-, 1000-, 2000-, and 4000-Hz activators between Jerger et al. [Mono. Contemp. Audiol. 1 (1978)] and Jerger [J. Acoust. Soc. Am. 66 (1979)] on the one hand and Silman [J. Acoust. Soc. Am. 66 (1979)] and others on the other. The acoustic reflex thresholds for broadband noise, 500-, 1000-, 2000-, and 4000-Hz activators were evaluated under two measurement conditions. Seventy-two normal-hearing ears were drawn from 72 subjects ranging in age from 20-69 years. The results revealed that age was correlated with the acoustic reflex threshold for BBN activator but not for any of the tonal activators; the correlation was stronger under the 1-dB than under the 5-dB measurement condition. Also, the mean acoustic reflex thresholds for broadband noise activator were essentially similar to those reported by Jerger et al. (1978) but differed from those obtained in this study under the 1-dB measurement condition.     

电子器件散热风扇气动噪声管道声学模态截止控制技术

深度学习输入特征的选择直接影响其分类性能,为了进一步提高基于深度学习的鸟类物种识别模型的分类性能,该文提出一种多特征融合识别方法。该方法首先通过短时傅里叶变换、梅尔倒谱变换和线性调频小波变换分别计算得到鸣声信号的3种语图样本集,然后分别利用3种语图样本集训练3个基于VGG16迁移的单一特征模型,将3个模型的输出进行自适应加权求和实现融合,并修正了加权交叉熵函数以克服样本不平衡的问题,最后对语图进行分类实现鸟类物种的识别。以ICML4B鸣声库的35种鸟类为研究对象,对比了4种模型的平均识别准确率(MAP),结果表明特征融合模型较单一特征模型的MAP最大提高了0.307;选择输入语图的持续时间分别为100 ms、300 ms以及500 ms,对比不同持续时间下4种模型的测试MAP值,结果表明持续时间为300 ms时4种模型的MAP值均为最高;对比了不同信噪比下4种模型的识别效果,多特征融合模型的识别准确率随着信噪比的下降降低最少。说明在选择合适的语图持续时间后,该文提出的特征融合模型能得到更高的识别准确率,具有一定的抗噪能力,且训练参数少,更适合于少样本鸟类的识别。