开孔结构流致噪声的数值模拟和机理分析

飞机机体表面的开孔设计会形成空腔结构,产生空腔流致噪声。空腔噪声的控制需要彻底认识其流动和噪声机理。以飞机的功能性开孔为例,通过半经验公式分析了其空腔噪声频率随速度的变化规律,预测了出现流声共振的工况。空腔发生流声共振时,特定频率的纯音噪声会被放大。为此,采用脱体涡模拟方法开展了开孔结构流声共振的三维非定常数值计算,分析了其流场和声场特性。其中,数值方法的准确性通过圆形空腔标模计算进行验证。结果表明,在一定速度下剪切层内的扰动将诱发空腔深度方向声模态,出现流声共振现象。此时,剪切层表现为强烈的周期性上下拍动,空腔底部和后缘区域的局部压力脉动幅值较大,声波主要由空腔后缘向上游方向辐射,上游噪声大于下游。     

Scattering and absorption of sound at flow duct expansions

The scattering of plane acoustic waves at area expansions in flow ducts is analysed using the vortex sheet model where the flow at the expansion is modelled as a jet, with a vortex sheet emanating from the edge. Of particular interest is the influence of the flow field on acoustic scattering and absorption.First, it is demonstrated that the stability properties of the shear layer can be simulated by modifying the edge condition within the vortex sheet model. To this end the accuracy for the region where the shear layer is changing from unstable to stable is improved by introducing a gradually relaxed Kutta edge condition with empirically derived coefficients. For low Strouhal numbers the vortex sheet model applies and for higher Strouhal numbers the two models converge.Second, it is demonstrated that the acoustic transmission through the jet expansion region can be determined by neglecting the scattering there. Incorporating this assumption, the vortex sheet model reproduces well the experimental results on transmission and absorption for an area expansion. This result supports the assumption that the main part of the scattering occurs at the area expansion at least for the low-frequency range. Furthermore, the influence of the flow field is particularly strong for small Strouhal numbers.     

Procedures for determining the acoustic efficiency of edge-modified noise barriers

This paper describes the noise shielding efficiency of barriers with an acoustic device mounted on their top edge for reducing sound diffraction. Diffraction behind the edge-modified barrier is investigated by scale model experiments in which the positions of a source and a receiver are aligned along a circular arc around the barrier top. The result indicates that the acoustic efficiency of the edge device is a function of the angles of the source and receiver and independent of their radii. Based on this finding, a novel procedure for determining the efficiency of manufactured edge devices is established. This procedure is very beneficial for estimating the edge device efficiency by eliminating ground and meteorological effects. The measured efficiency of the device will be quite useful for the prediction of noise propagation behind the edge-modified barriers.     

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.     

Flap-edge aeroacoustic measurements and predictions

An aeroacoustic model test has been conducted to investigate the mechanisms of sound generation on high-lift wing configurations. This paper presents an analysis of flap side-edge noise, which is often the most dominant source. A model of a main element wing section with a half-span flap was tested at low speeds of up to a Mach number of 0.17, corresponding to a wing chord Reynolds number of approximately 1.7 million. Results are presented for flat (or blunt), flanged, and round flap-edge geometries, with and without boundary-layer tripping, deployed at both moderate and high flap angles. The acoustic database is obtained from a small aperture directional array (SADA) of microphones, which was constructed to electronically steer to different regions of the model and to obtain farfield noise spectra and directivity from these regions. The basic flap-edge aerodynamics is established by static surface pressure data, as well as by computational fluid dynamics (CFD) calculations and simplified edge flow analyses. Distributions of unsteady pressure sensors over the flap allow the noise source regions to be defined and quantified via cross-spectral diagnostics using the SADA output. It is found that shear layer instability and related pressure scatter is the primary noise mechanism. For the flat edge flap, two noise prediction methods based on unsteady-surface-pressure measurements are evaluated and compared to measured noise. One is a new causality spectral approach developed here. The other is a new application of an edge-noise scatter prediction method. The good comparisons for both approaches suggest that the prediction models capture much of the physics. Areas of disagreement appear to reveal when the assumed edge noise mechanism does not fully define the noise production. For the different edge conditions, extensive spectra and directivity are presented. The complexity of the directivity results demonstrate the strong role of edge source geometry and frequency in the noise radiation. Significantly, for each edge configuration, the spectra for different flow speeds, flap angles, and surface roughness were successfully scaled by utilizing aerodynamic performance and boundary-layer scaling methods developed herein.     

The rôle of surface shear stress fluctuations in the generation of boundary layer noise

The influence of unsteady wall shear stress on boundary layer noise and wall pressure fluctuations is discussed. It is argued that in the acoustic analogy theory of boundary layer noise the surface shear stress “dipole” characterizes acoustic propagation and not generation. Analytical results are presented in support of this view which, in addition, indicate that the effect of the surface dipole is to dininish rather than enhance boundary layer radiation at low Mach numbers.     

The displacement-thickness theory of trailing edge noise

The problem of estimating the sound generated by turbulent boundary layer flow over the edge of a rigid half-plane is re-examined. A theory is proposed which is strictly valid at low Strouhal numbers based on boundary layer width, wherein the flow inhomogeneities are specified in terms of the fluctuations in the boundary layer displacement thickness. This enables account to be taken of changes in the properties of the turbulence as it translates past the edge, which are shown to result in the appearance of an acoustic dipole whose axis is aligned with the mean flow, and which supplements the radiation field predicted by conventional methods [1,2]. Detailed comparison is made with acoustic and surface pressures which are calculated according to the evanescent wave theory of edge noise [3–5].     

The effect of forward flight on the diffraction radiation of a high speed jet

This paper describes a theoretical modelling of the effect of aircraft flight on the diffractional generation of sound which occurs when shear layer turbulence convects at high speed past a trailing edge. This is relevant to the study of noise problems associated with blown flaps, powered lift and aerodynamic shielding devices. The analysis is conducted for a two-dimensional configuration at arbitrary subsonic flight velocity. It is concluded that in the absence of a Kutta condition at the trailing edge, the effect of flight results in a forward arc amplification of the diffraction radiation through a single Doppler factor on the linear acoustic pressure field. The forward arc lift in the field shape disappears when a Kutta condition is imposed. In all cases the intensity of the diffraction radiation at 90° to the flight path is diminished by forward motion of the aircraft.     

An aeroacoustically driven thermoacoustic heat pump

The mean flow of gas in a pipe past a cavity can excite the resonant acoustic modes of the cavity--much like blowing across the top of a bottle. The periodic shedding of vortices from the leading edge of the mouth of the cavity feeds energy into the acoustic modes which, in turn, affect the shedding of the next vortex. This so-called aeroacoustic whistle can excite very high amplitude acoustic standing waves within a cavity defined by coaxial side branches closed at their ends. The amplitude of these standing waves can easily be 20% of the ambient pressure at optimal gas flow rates and ambient pressures within the main pipe. A standing wave thermoacoustic heat pump is a device which utilizes the in-phase pressure and displacement oscillations to pump heat across a porous medium thereby establishing, or maintaining, a temperature gradient. Experimental results of a combined system of aeroacoustic sound source and a simple thermoacoustic stack will be presented.     

Radiation efficiency of a guitar top plate linked with edge or corner modes and intercell cancellation

This paper was based on a theoretical framework to determine strong and weak radiation by a guitar top plate, vibrating through deflections hard to analyze: multipolar mode shapes. The air-structure interaction was examined in terms of edge modes or corner modes, and considering even or odd number modes. A numerical model was implemented and experimentally calibrated, exhibiting several advantages exploring the coupling between vibratory and acoustic waves in a top plate. Two analyses were applied detecting high or low radiation efficiency for the structure. First, the addition of volume velocity for odd numbers of poles and cancellation for even numbers were examined. In fact, both behaviors can happen at the same time, as it was shown for a corner radiator case used as an example. Second, the ratio between bending and acoustic wavenumbers was explored. To illustrate the importance of this ratio, some theoretical features of a more efficient radiator than the corner mode were exposed in an edge mode example. Labeling multipolar mode shapes as efficient or inefficient radiators showed to be a useful alternative analyzing the top plate behavior. It can be applied knowing the nodal lines of the vibration pattern and estimating the bending and acoustic wavelengths.     

Acoustic properties of heated twin jets

A thorough experimental study of the noise characteristics of twin jets is presented in this paper. Twin round jets are investigated at typical jet engine conditions: that is, with heated high velocity flow. By varying the nozzle to nozzle spacing, it is possible to discriminate between the effects of turbulent mixing and acoustic shielding. As a result of this investigation, it was established that the turbulent mixing effects (both interaction noise generation and mixing suppression) occur for closely spaced nozzles. While acoustic shielding occurs at all nozzle spacings, it plays the dominant role at wide nozzle spacings. The levels of this acoustic shielding afforded by an adjacent jet can be sufficient to cause a nearly complete masking of the noise of the shielded jet. A significant discovery of this investigation was the importance of the layer of cooler, slower moving ambient air that exists between the twin jet plumes. This inter-jet layer causes acoustic refraction and reflection, and as the nozzle separation increases, the layer extends to shield more of the jet noise sources.     

Application of frequency-domain linearized Euler solutions to the prediction of aft fan tones and comparison with experimental measurements on model scale turbofan exhaust nozzles

Although it is widely accepted that aircraft noise needs to be further reduced, there is an equally important, on-going requirement to accurately predict the strengths of all the different aircraft noise sources, not only to ensure that a new aircraft is certifiable and can meet the ever more stringent local airport noise rules but also to prioritize and apply appropriate noise source reduction technologies at the design stage. As the bypass ratio of aircraft engines is increased - in order to reduce fuel consumption, emissions and jet mixing noise - the fan noise that radiates from the bypass exhaust nozzle is becoming one of the loudest engine sources, despite the large areas of acoustically absorptive treatment in the bypass duct. This paper addresses this ‘aft fan’ noise source, in particular the prediction of the propagation of fan noise through the bypass exhaust nozzle/jet exhaust flow and radiation out to the far-field observer. The proposed prediction method is equally applicable to fan tone and fan broadband noise (and also turbine and core noise) but here the method is validated with measured test data using simulated fan tones. The measured data had been previously acquired on two model scale turbofan engine exhausts with bypass and heated core flows typical of those found in a modern high bypass engine, but under static conditions (i.e. no flight simulation). The prediction method is based on frequency-domain solutions of the linearized Euler equations in conjunction with perfectly matched layer equations at the inlet and far-field boundaries using high-order finite differences. The discrete system of equations is inverted by the parallel sparse solver MUMPS. Far-field predictions are carried out by integrating Kirchhoff's formula in frequency domain. In addition to the acoustic modes excited and radiated, some non-acoustic waves within the cold stream-ambient shear layer are also captured by the computations at some flow and excitation frequencies. By extracting phase speed information from the near-field pressure solution, these non-acoustic waves are shown to be convective Kelvin-Helmholtz instability waves. Strouhal numbers computed along the shear layer, based on the local momentum thickness also confirm this in accordance with Michalke's instability criterion for incompressible round jets with a similar shear layer profile. Comparisons of the computed far-field results with the measured acoustic data reveal that, in general, the solver predicts the peak sound levels well when the farfield is dominated by the in-duct target mode (the target mode being the one specified to the in-duct mode generator). Calculations also show that the agreement can be considerably improved when the non-target modes are also included, despite their low in-duct levels. This is due to the fact that each duct mode has its own distinct directionality and a non-target low level mode may become dominant at angles where the higher-level target mode is directionally weak. The overall agreement between the computations and experiment strongly suggests that, at least for the range of mean flows and acoustic conditions considered, the physical aeroacoustic radiation processes are fully captured through the frequency-domain solutions to the linearized Euler equations and hence this could form the basis of a reliable aircraft noise prediction method.     

High amplitude acoustic transmission through duct terminations: Theory

Recent experimental measurements have demonstrated that net acoustic energy dissipation can occur when sound waves interact with free shear layers, which are produced either by boundary layer separation in mean fluid flow at sharp edges, or by separation of the boundary layer in the acoustic flow at an edge in the absence of mean flow. This paper presents theoretical results which are offered in an attempt to explain these observations quantitatively. Comparison is made between the predicted and measured net energy loss which occurs upon transmission of high amplitude impulsive acoustic waves through various duct terminations, and also between calculated and measured reflection coefficients in the duct. The agreement is generally at least qualitatively good, and would appear to justify the physical assumptions on which the theoretical arguments are based.     

Measurement of aerodynamic noise and unsteady flow field around a symmetrical airfoil

The purpose of this paper is to study the physics of aerodynamic noise generation from the symmetrical airfoil of NACA 0018 in a uniform flow. The relationship between the noise spectrum and the unsteady flow field around the airfoil is studied in an acoustic wind tunnel using flow visualization and PIV analysis. The discrete frequency noise was generated from the airfoil inclined at small angle of attack to the free stream. The flow visualization result indicates the presence of attached boundary layer over the suction side and the separated shear layer over the rear pressure side of the airfoil, when the discrete frequency noise is observed. It is found from the PIV analysis that a large magnitude of vorticity is generated periodically from the pressure side of the trailing edge and it develops into an asymmetrical vortex street in the wake of the airfoil. The periodicity of the shedding vortices was found to agree with that of the frequency of the generated noise.     

Investigation of flow features and acoustic radiation of a round cavity

In this work, the interaction between a boundary layer and a circular cylindrical cavity is studied. Experimental pressure and velocity results for a cavity of diameter 10 cm and depth ranging from 10 to 15 cm are described, for flow velocities between 50 and 110 m s^?1. This flow configuration is found to generate intense discrete depth- and flow-dependent tones, resulting in modes similar in appearance to Rossiter modes found in shallow rectangular cavities. Differences between the cylindrical cavity's mean flow and that of a similarly sized rectangular cavity are highlighted. The development of the shear layer is quantified, in terms of thickening and of velocity statistics profiles. Radial and azimuthal acoustic modes are observed in the acoustic field inside the cavity. A feedback model based on the coupled behaviour of the fundamental acoustic depth mode of the cavity and the large scale dynamics of the shear layer is constructed, and its response is compared to experimental data. A good qualitative agreement between available data and modeled behaviour is observed, allowing the two acoustic modes found in this work to be attributed to the interaction of the shear layer with the cavity's fundamental depth mode.     

Flexural effects of sandwich beam with a plate insert under in-plane bending

The local stress concentrations in sandwich beam with a plate insert under in-plane bending are concerned in the study. An improved six-step phase shifting method in digital photoelasticity is employed to calculate the whole-field shear stress.The shear load transfer is realized by shear bands which connect the top and bottom sheet faces through adhesively-bonded interfaces. The plate insert plays a role in load transfer in the sandwich structure, and the fact that debonding might occur at more sites of the interfaces may also leads to the failure of the structure. The local stress concentrations at the insert end change with the load under three-point bending loads, while they remain as the initial residual shear stress under four-point bending loads. The local stress concentration effects generated by the plate insert is essentially caused by the mismatch of elastic properties of the core materials and the irrational geometry of the insert.     

Azimuthal behaviour of flow-excited diametral modes of internal shallow cavities

The aerodynamic excitation of ducted cavity diametral modes gives rise to complex flow-sound interaction mechanisms, in which the axisymmetric free shear layer interacts with the asymmetric acoustic modes. This results in various azimuthal patterns and behaviours depending on different flow and geometrical parameters. The azimuthal behaviour of this self-excitation mechanism is investigated experimentally. Axisymmetric shallow cavities in a duct have been tested over the range of cavity length to depth ratio from 1 to 6 and at Mach numbers up to 0.4. A set of pressure transducers flush mounted to the cavity floor is used to determine the acoustic mode amplitude and orientation. The excited acoustic modes are classified into spinning, partially spinning, and stationary diametral modes. An analytical representation based on the duct acoustics theory is used to analyse the measurements and provides a physical explanation of the observed behaviour of the diametral modes. Splitter plates are installed inside the cavity to form a geometrical preference. The acoustic response of this geometrically altered case show that pressure oscillations at different azimuthal angles along the cavity circumference can be uncorrelated, or even oscillate at different frequencies, while the diametral modes are still strongly excited. Two hot-wire probes are also used in a separate set of measurements to investigate the azimuthal behaviour of the shear layer oscillation. The results show that the shear layer oscillation has the same azimuthal distribution as that of the excited acoustic modes, indicating that the shear layer oscillation at different azimuthal angles can be not only uncorrelated but also occur at different frequencies.     

NOISE GENERATED BY A COANDA WALL JET CIRCULATION CONTROL DEVICE

An analysis is made of the production of sound by a hydrofoil with a Coanda wall jet circulation control (CC-) device. Three principal sources are identified in the vicinity of the trailing edge of the hydrofoil. The radiation at very low frequencies is dominated by “curvature noise” generated by the interaction of boundary layer turbulence with the rounded trailing edge of the CC-hydrofoil; this is similar in character and magnitude to the low-frequency component of the conventional trailing edge noise produced by a hydrofoil of the same chord, but with a sharp trailing edge. Higher frequency sound is produced principally at the Coanda jet slot. “Passive slot noise” is caused by the “scattering” by the slot lip of nearfield pressure fluctuations in the turbulent boundary layer of the exterior mean flow past the slot. This is of comparable intensity to high frequency, sharp-edged trailing edge noise. However, the acoustic spectrum is greatly extended to much higher frequencies if the Coanda jet is turbulent; the sound produced by the interaction of this turbulence with the lip tends to dominate the spectrum at frequencies f (Hz) greater than about U_j/h, where h is the slot width and U_jthe Coanda jet speed. Sample numerical results are presented for a typical underwater application that indicate that at this and higher frequencies the slot noise can be 20 dB or more greater than conventional trailing edge noise, although the differences become smaller as the thickness of the slot lip increases.     

冲击射流激波振荡与抑制

欠膨胀冲击射流具有复杂的激波结构,并伴随产生高幅值的离散频率单音.通过高速摄像获取的纹影图像并结合噪声测量,对欠膨胀冲击射流激波振荡过程、剪切层不稳定波的模态和离散频率单音的产生进行了系列研究.给出了冲击距离为5倍喷嘴出口直径的复杂流动实验结果分析,射流剪切层不稳定波有对称和非对称两种模态,发现不同模态下的离散频率单音...