The research on semi-active muffler device of controlling the exhaust pipe’s low-frequency noise

In order to control low frequency noise in exhaust pipe, this paper puts forward a new concept of H-Q tube based semi-active muffler device. The semi-active muffler device and bench testing system have been designed and operated. Finite element simulation study on semi-active muffler and experimental study on semi-active muffler and passive muffler have been carried on. The effect of simulation and experiment are consistent. The semi-active muffler device acts well in low frequency band, especially between 50 Hz and 150 Hz. The average level of noise reduction is around 35 dB, which is much better than passive muffler. Between 150 Hz and 350 Hz, semi-active muffler has a better performance than passive muffler; above 350 Hz, it has worse performance compared with the passive muffler.     

Expansion-chamber muffler for impulse noise of pneumatic frictional clutch and brake in mechanical presses

A compound expansion-chamber muffler, which consists of a sound absorbing chamber and a switch valve, the chamber integrating structural features of impedance muffler and micropunch plate muffler, is proposed to diminish impulse exhaust noise of pneumatic friction clutch and pneumatic friction brake (PFC/B) in mechanical presses. The structure decreases the impulse exhaust noise of PFC/B over 30 dB(A). A one-dimensional flow model is applied to study the aerodynamic characteristics of compound exhaust process of the single acting cylinder and muffler because the exhaust time is a critical factor for application of muffler in PFC/B. The volume of sound absorbing chamber is found to be an important design parameter to minimize the exhaust resistance of pneumatic cylinder. Experiments are also conducted to validate analytical results. Then the effects of diameter of exhaust ducts and volume of muffler on the exhaust time are discussed in detail. The proposed one-dimensional computational method, which considers the coupling of air-flow field and sound field, gives satisfactory results for the preliminary design of an expansion-chamber muffler. This method has been applied to an existing model HKM3-40MN to reduce its impulse noise.     

LES-FEM coupled analysis and experimental research on aerodynamic noise of the vehicle intake system

To thoroughly explore the aerodynamic noise in order to achieve a more efficient engineering application for a vehicle intake system, the large eddy simulation and the finite element method were employed in numerical simulations, and the aeroacoustic characteristics were validated through the experimental data. In this research, the k-ε model was adopted to simulate the steady state fluid dynamic, and the static pressure loss was consistent with the bench test data, indicating the computational fluid dynamics model was valid. After acquiring the data from the steady state simulation, the fluctuating pressure of the inner wall was calculated based on the transient state calculation results from the large eddy simulation. Thereafter, the finite element method was used to determine the acoustic performance of the intake system. By comparing the experiment data, the noise reduction indicated that the intake system performed well at various frequencies, e.g. 320 Hz, 520 Hz and 770 Hz, but poorly at 140 Hz, 210 Hz, 420 Hz and 600 Hz. Finally, the far-field aerodynamic noise was calculated based on FW-H equation, and the output showed that the noise of each measuring point agreed well with the test results in trend. In particular, the inlet sound pressure spectrum almost fit the test data with the airflow of 300 m³/h, and several amplitude peaks appeared at 210 Hz, 420 Hz and 600 Hz, corresponding to the low-attenuation region of the noise reduction curve. Moreover, the specific frequencies were not shifted with the airflow changing. In conclusion, the numerical simulation method proves to be effective in calculating the aerodynamic noise accurately.     

Simulative and experimental investigations on pressure-induced structural vibrations of a rear muffler

The periodically blown out exhaust gas of a combustion engine may excite structural vibrations of the exhaust system. In addition to the noise of the orifice, these vibrations contribute to the overall noise radiation of the exhaust system. In this work, the excitation of structural vibrations of a rear muffler via the acoustic path is investigated both in experiments and simulations. In both cases transfer functions from the acoustic pressure at the inlet to the structural deflection on the surface of the rear muffler are determined and compared to each other. For the simulation an FE-FE (finite element) coupling is applied to account for the fluid-structure interaction. To efficiently predict the fluid-structure coupled behavior, a model reduction technique for the finite element method based on the Craig-Bampton method and the Rubin method is presented. In a last step, the sound radiation is evaluated by solving the exterior acoustic problem with the fast multipole boundary element method. For this purpose, the results of the FE computation are used as boundary datum.     

Experimental and numerical study on aeroacoustic sound of axial flow fan in room air conditioner

Fan is one of the main noise sources of the room air-conditioners. Axial flow fans are widely used in the outdoor unit of split type air-conditioners. The interaction between the fan and the heat exchanger should be taken into consideration. However, only a few researches have been carried out on predicting the aeroacoustic noise because of the difficulty in obtaining detailed information of the flow field. This paper is to understand the generation mechanism of sound and to develop a prediction method for the flow field and the acoustic pressure field of the outdoor unit. Acoustic measurement is performed in a semi-anechoic chamber. Effects of each components is analyzed. Based on commercial computational fluid dynamics (CFD) code, Fluent, Fukano’s model is used to predict the overall sound pressure level of broadband noise. The predicted sound pressure levels based on original Fukano’s model are 7.66 dB and 7.42 dB lower than measurement results at 780 rpm and 684 rpm, respectively. And the errors are about 13%. However, when wake width and relative velocity are both calculated by numerical simulations and the distance to blade trailing edge is taken into consideration, the difference of sound pressure level between measurement and prediction is less than 3.4 dB and errors less than 5.5% while the distance is less than 10 mm. Thus, the distance to blade trailing edge should also be an important parameter for Fukano’s model. In comparison with experimental results, it is clearly shown that the Fukano method based on numerical simulation can provide more accuracy than the original Fukano model and numerical results are in a reliable level.     

A hybrid approach for aeroacoustic analysis of the engine exhaust system

This paper presents a new hybrid approach for prediction of noise radiation from engine exhaust systems. It couples the time domain analysis of the engine and the frequency domain analysis of the muffler, and has the advantages of both. In this approach, cylinder/cavity is analyzed in the time domain to calculate the exhaust mass flux history at the exhaust valve by means of the method of characteristics, avoiding the tedious procedure of interpolation at every mesh point and solving a number of equations simultaneously at every junction. This is done by making use of an interrelationship between progressive wave variables of the linear acoustic theory and those of the method of characteristics. In this approach, nonlinear propagation in the exhaust pipe is neglected and free expansion is assumed at the radiation end of the exhaust pipe. In the case of a muffler proper, expansion from the exhaust pipe into the first chamber is assumed to be a free expansion. Various results of this approach are compared with those of the method of characteristics and the classical acoustic theory, and various peaks and troughs in insertion loss curves are analytically validated.     

微穿孔板抑制不稳定燃烧的声学数值仿真

为了研究微穿孔板吸声结构对不稳定燃烧的抑制作用,采用高精度的计算气动声学(Computational Aeroacoustics,CAA)方法开展时域下的数值仿真。首先对带有压力时滞模型的三维声学扰动方程进行求解,给出发动机不稳定燃烧的频率信息。然后通过解析模型分析微穿孔板吸声结构的阻抗特性,并由多自由度宽频阻抗模型模拟微穿孔板对该不稳定频率的抑制作用。仿真捕捉到的不稳定燃烧频率与地面试车测得的频率相一致。表明采用的计算气动声学方法及相应模型可以准确地捕捉不稳定燃烧的频率信息,并分析微穿孔板对不稳定燃烧的抑制作用,对于工程上快速预测不稳定燃烧具有一定意义。      

Transverse plane wave analysis of short elliptical chamber mufflers: An analytical approach

Short elliptical chamber mufflers are used often in the modern day automotive exhaust systems. The acoustic analysis of such short chamber mufflers is facilitated by considering a transverse plane wave propagation model along the major axis up to the low frequency limit. The one dimensional differential equation governing the transverse plane wave propagation in such short chambers is solved using the segmentation approaches which are inherently numerical schemes, wherein the transfer matrix relating the upstream state variables to the downstream variables is obtained. Analytical solution of the transverse plane wave model used to analyze such short chambers has not been reported in the literature so far. This present work is thus an attempt to fill up this lacuna, whereby Frobenius solution of the differential equation governing the transverse plane wave propagation is obtained. By taking a sufficient number of terms of the infinite series, an approximate analytical solution so obtained shows good convergence up to about 1300 Hz and also covers most of the range of muffler dimensions used in practice. The transmission loss (TL) performance of the muffler configurations computed by this analytical approach agrees excellently with that computed by the Matrizant approach used earlier by the authors, thereby offering a faster and more elegant alternate method to analyze short elliptical muffler configurations.     

Acoustical source characterization studies on a multi-cylinder engine exhaust system

It is well known that the characterization of the acoustic source in an exhaust muffler system is of utmost importance in the proper evaluation of the acoustic performance of the muffler. However, in the literature, there are very few experimental studies on source characterization of a multi-cylinder internal combustion engine. This paper describes the use of a transfer function method (with a random excitation source) for measurement of the internal source impedance of an eight-cylinder engine under running conditions. The results obtained agree well with those obtained by the standing wave method by earlier investigators. The studies include the effect on the measured internal source impedance caused by variation of engine speed and load. The source impedance results obtained for the engine in operation are compared with those for the engine not in operation. The use of these results in the overall modeling of the exhaust system is described in an accompanying paper.     

Acoustic analysis of a liquefied petroleum gas-fired pulse combustor

Experimental investigation of acoustic characteristics of a Helmholtz type liquefied petroleum gas-fired pulse combustor is presented. In the experiments, the length of the tail pipe was changed from 1.9 m to 1.3 m by 10 cm intervals. Sound level measurements were taken from the exhaust side (outlet) and air flapper side (inlet) at a distance of 1 m from both sides. With decreasing lengths of the tail pipe, the sound pressure level increased. At the measurements related to the exhaust side, the maximum value of equivalent continuous sound pressure level, LEQ was 96.6 dB when the length of the tailpipe and fundamental frequency were 1.3 m and 63 Hz, respectively. Same kinds of measurements were performed at the air flapper side, but the LEQ value was stronger at the exhaust side than the one at the air flapper side. It was also observed that the effect of the type of gaseous fuel on the acoustic efficiency of the pulse combustor can be neglected when the results of the acoustic efficiencies were compared to those in the literature. In order to compare the accuracy of frequencies measured by the sound level meter, a suitable dynamic pressure transducer and a spectrum analyzer were used to perform amplitude and frequency measurements. The average deviation between the measurements performed by the sound level meter and dynamic pressure transducer was 2.4 Hz (3.8% errors) while the average deviation was 3.8 Hz (6% errors) between the sound level meter and spectrum analyzer.     

Acoustic noise simulation and measurement of a gradient insert in a 4 T MRI

High speed switching of current in gradient coils within high magnetic field strength magnetic resonance imaging (MRI) scanners results in high acoustic sound pressure levels (SPL) in and around these machines. Many studies have already been conducted to characterize the sound field in and around MRIs and various methods have been investigated to attenuate the noise generated. In the work presented here a computational vibro-acoustic model was developed based on an iteratively modified and validated finite element (FE) model to characterize the acoustic noise properties of the gradient coil. The simulation results from the computational model were verified through experimental noise measurement for the gradient coil insert in a 4 T MRI scanner by using swept sinusoidal time waveform inputs. Comparisons show that the computational model predicts the noise characteristic properties extremely accurately. There are three dominant frequency bands where the SPL is much higher than those at other frequencies. The SPL in the horizontal direction is much higher than that in the vertical direction due to the excitation to the horizontally placed X coil. The SPL to the inner surface of the coil is higher than far from the inner surface, which proves that the acoustic noise is radiated from the inner surface and primarily caused by the normal vibration of the inner surface. Further verification was conducted by using two types of trapezoidal sequence inputs usually used, which is to simulate real scanning sequences for small animals. Again the accuracy of the developed model is verified. The validated acoustic computational model could be used as an effective method to predict the noise that would be produced by a coil in the design stage. Modification of the structural design or the excitation pulse could be performed to reduce the acoustic noise when the gradient coil is in scanning.     

Laser Doppler velocimetry measurement at a hard-to-reach intake port of a two-stroke engine

To understand better the complex scavenging process in ported two-stroke engines, optical set-ups were designed for velocity measurements at the exit of an intake port of a motoring and firing single-cylinder propane-fueled two-stroke engine by laser Doppler velocimetry. The radial velocity component was measured at the center of one port at engine speeds of 600, 900, and 1200 rpm. Laser beams entered the engine from the top through a quartz window and the light scattered by the seed particles was collected at 90° through the exhaust ports and a side window. The noise produced by the piston head was analyzed and separated from the signal generated by the seed particles. Analysis of the results from this engine showed that, in general, differences in both magnitude and the shape of the plotted results were observed when the engine was firing. A backflow into the intake system was observed at 600 rpm; this backflow decreased in strength, shifted in crank angle at 900 rpm, and eventually was eliminated at an engine speed of 1200 rpm.     

柴油机排气过程流动的多维瞬态数值模拟研究

柴油机排气过程气体的流动直接影响到排气管换热、发动机噪音、废气再循环系统设计等,对柴油机整体性能起着重要作用.因此,本文利用大型通用CFD软件STAR-CD及ES-ICE,在进气压缩、喷雾燃烧过程多维瞬态数值模拟基础上,对柴油机排气过程流动进行多维瞬态数值模拟研究,通过计算给出排气过程中气体的流场分布,为柴油机排气系统的优化设计提供重要的理论指导.     

Simulations and experiments for hybrid noise control systems

The purpose of this study is to explore the effects of sound elimination in a cylindrical duct by combining a reactive muffler and active noise control (ANC) system. Besides the exploration via experiment of the combined noise control system, a Grey prediction based on Grey theory is also applied to ANC for this hybrid system.In the experiment for this system, a combined adaptive algorithm is adopted. The results of sound elimination are compared between cases with ANC systems installed before the muffler and after the muffler. The results indicate that the sequence of arrangement of muffler can influence the results of active noise control. According to the results of experiment and simulation, the effect of noise reduction in ANC system is influenced extremely by reference signal received. The transmission loss and insertion loss in this system are also discussed in details. Besides, the experimental results indicate that the hybrid system has the advantages over a traditional muffler when the muffler is not designed for the frequency of the noise. Furthermore, the mathematic simulation for acoustic field in a cylindrical duct with a muffler is performed in order to verify the experiment results. Finally, Grey theory is applied to estimate the expected signals in order to perform a computer simulation of Grey prediction to explore effects of the ANC system. The results indicate that application of Grey theory gives a good control for the hybrid system.     

Correction of fan noise for effects of forward flight

When acoustic measurements are made on a static engine test stand, the data must be corrected for the effects of forward flight to predict correctly the noise characteristics of the engine in flight. A ray tracing approach is used here to relate the static test case to the flight case. The assumptions of isentropic irrotational flow into the fan inlet and a cylindrical shear layer at the fan exhaust lead to slightly different methods for correcting inlet noise and exhaust noise. The forward flight correction method generally involves both an angle and an amplitude correction. The amplitude correction factors for inlet and exhaust noise are the same as that for a dipole and can be as much as 6 dB for a flight Mach number of 0·3. The angle correction for the inlet noise differs from that of the exhaust noise, and both differ from the generally used correction to retarded angle.     

A study of the influence of cooling water on the structural modes and vibro-acoustic characteristics of a gasoline engine

The influence of the cooling water on the vibration mode and damping characteristics of an engine was evaluated through experimental and simulation methods. The wet mode method of the fluid–structure interaction field was applied in numerical analysis, which led to several significant conclusions. Several additional fluid related modes were present when the effect of cooling water was considered. Meanwhile, the frequencies of other modes were almost the same. On the other hand, the damping characteristics of an engine were changed while the cooling water was used. For Rayleigh damping, the cooling water effect has made the mass matrix scale factor α nearly double and the stiffness matrix scale factor β almost unchanged. The engine surface vibration acceleration magnitude was calculated by means of the multi-body dynamic analysis. It was shown that the calculated vibration acceleration magnitude was close to the measured one when the cooling water was taken into account. Furthermore, acoustic simulation results were estimated by Boundary Element Method, which revealed that the sound power level of the radiation noise of most engine components was lower than that with the fluid structure interaction model being applied in the engine block modeling. The overall sound power level of the whole engine radiation noise was increased by 3.6 dB when the effect of the cooling water was considered, which demonstrated that the effect of the cooling water on the engine vibro-acoustic characteristics should not be neglected during the computation. This paper has disclosed the influences of the cooling water on the engine structural modes, damping, sound power level of the engine radiation noise and a systematic approach for evaluation of the influences.     

Broadband noise reduction by circular multi-cavity mufflers operating in multimodal propagation conditions

In this study, sound propagation through a circular duct with non-locally lining is investigated both numerically and experimentally. The liner concept is based on perforated screens backed by air cavities. Dimensions of the cavity are chosen to be of the order or bigger than the wavelength so acoustic waves within the liner can propagate parallel to the duct surface. This gives rise to complex scattering mechanisms among duct modes which renders the muffler more effective over a broader frequency range. This work emanates from the Cleansky European HEXENOR project which aim is to identify the best multi-cavity muffler configuration for reduction of exhaust noise from helicopter turboshaft engines. Here, design parameters are the cavity dimensions in both longitudinal and azimuthal directions. The best cavity configuration must in addition fit weight specifications which implies that the number of walls separating each cavity should be chosen as small as possible. To achieve these objectives, the scattering matrix of the lined duct section is obtained experimentally for two specific muffler configurations operating in multimodal propagation conditions. The good agreement with numerical predictions serves to validate the perforate plate impedance model used in our calculation. Finally, given an incident acoustic pressure which is representative of typical combustion noise spectrum, the best cavity configuration achieving the maximum overall acoustic Transmission Loss is selected numerically. The study also illustrates how the acoustic performances are dependent on the nature of the incident field.     

Numerical investigation on radiation characteristics of discrete-frequency noise from scarf and scoop aero-intakes

Numerical studies have been carried out to investigate the detailed geometrical effects of full three-dimensional aero-intakes on the radiation of the discrete-frequency rotor-stator interaction noise. The near-field acoustic radiation characteristics and the far-field directivity patterns from the scarf and scoop aero-intakes with three different scarf/scoop angles are quantitatively analyzed and compared. The near-field predictions were obtained by solving the linearized Euler equations with computational aeroacoustic techniques consisting of high-order finite difference scheme, non-reflecting boundary conditions, overset grids and parallel computational methods. For the prediction of far-field directivity pattern, the Kirchhoff integral method was applied. By comparing the directivities of discrete-frequency noise radiating from the scarf and the scoop aero-intakes with that from an axisymmetric aero-intake, it is shown that the 7 dB noise reduction at downward peak radiation angle can be achieved by using the scoop aero-intake with scoop angle of 15°, and the 5 dB noise reduction by the scarf aero-intake with the scarf angle of 15°. The scattering of the radiating acoustic wave by the background mean flow around the aero-intakes shifts the peak lobe radiation angle toward ground and increases the amplitude of the acoustic pressure compared with the cases without mean flow effect. Overall, the scoop aero-intake was found to be more effective than the scarf and the axisymmetric aero-intakes in view of the lower noise radiation toward ground.     

时均流诱导声振荡的数值计算研究

时均流诱导的声振荡可以为热声制冷提供驱动源或驱动发电机和换能器发电,为风能利用提供了新思路,是热声领域的最新研究方向之一。本文基于计算流体动力学(CFD)方法,建立了正十字型时均流激声发动机的三维数学模型,采用大涡模拟湍流模型计算。计算结果验证了时均流诱导声振荡效应,揭示出谐振管内声场分布和谐振管内部压力与开口处涡的关系,为后续的实验研究奠定了理论基础。     

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.