Methods for predicting the influence of intense sound, steady flow, and elevated temperature on the acoustic performance of automotive exhaust system components

The principal objective of this investigation was to study the feasibility of developing laboratory techniques for automobile-exhaust muffler design in order to reduce the amount of on-vehicle trial-and-error testing currently required. The investigation included (1) improving the capability of existing laboratory equipment to simulate the conditions in automobile exhaust (i.e., high-amplitude pressure waves, steady flow, and elevated temperature), and (2) theoretical and experimental studies of typical acoustic elements (side-branch resonators, expansion chambers, louvered tubes) used in muffler design. The sound fields used for these studies included pure tones, single pulses (tone bursts), random noise, and simulated automobile exhaust noise. Empirical correction factors, which adequately accounted for the effects of both finite-amplitude waves and flow on the impedance of side-branch resonators under pure-tone excitation, were obtained, Using these empirical correction factors, the theoretical response characteristics (in terms of transmission loss and insertion loss) were calculated; these results were in good agreement with the measured responses under anechoic conditions. Single-pulse excitation was found to substantially increase the resistive portion of the impedance of side-branch resonators. Empirical correction factors, applicable to side-branch resonators under single-pulse excitation were also obtained. Theoretical calculations of the reflection and transmission characteristics of plane discontinuities in the presence of flow were made using small perturbation theory. The use of these reflection and transmission characteristics gave theoretical expansion chamber response characteristics which were in good agreement with measured values. The response of expansion chambers was found to be independent of pressure amplitude. Theoretical modeling of acoustic filters terminated with finite tailpipes is presented. Good agreement was observed between theoretical and measured responses. Sound generation by flow past a side-branch resonator showed a complicated dependence on the tailpipe length and flow Mach number, with regard to both magnitude and frequency. This self-noise of resonators, in the presence of flow, was effectively eliminated by placing a wire screen at the resonator neck. A simple prototype muffler showed some deviations from theoretical predictions when tested with simulated exhaust noise but did show good agreement with theoretical predictions when tested with both pure-tones and random noise. Improvement of the muffler performance was achieved by the use of a dissipative element in the muffler; an insertion loss of 20.5 dBA was obtained. A practical muffler design and testing procedure, using simulated automobile exhaust noise in the laboratory, is described --Abstract, pages ii-iii

Noise suppression of a dipole source by tensioned membrane with side-branch cavities

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DEVELOPMENT OF A MUFFLER INSERTION LOSS FLOW RIG

Mufflers and silencers are commonly used to attenuate noise sources such as internal combustion engines and HVAC systems. Typically, these environments contain mean flow that can affect the acoustic properties of the muffler components and may produce flow generated noise. To characterize the muffler performance, common metrics such as insertion and transmission loss and noise reduction are used in industry. Though transmission loss without flow is often measured and is a relatively simple bench top experiment and useful for model validation purposes, mean flow can significantly affect the muffler performance. There are a few existing and commercial transmission loss rigs that incorporate flow into the measurement procedure. These rigs are useful for model verification including flow but do not predict how the muffler will perform in the system since the source, termination, and pipe lengths significantly impact performance. In this research, the development of an insertion loss test rig is detailed. This testing strategy has the advantage of being simpler, quantifying the self-generated noise due to flow, and taking into account the effect of tailpipe length and a realistic termination. However, the test does not include the actual source and is not as useful for model validation. An electric blower produces the flow and a silencer quiets the flow. Loudspeakers are positioned just downstream of the flow silencer and they are used as the sound source. The low frequency source is a subwoofer installed in a cylindrical enclosure that includes a conical transition from speaker to pipe. Special care is taken to reduce any flow generated noise. Qualification of the system is detailed by comparing the measured transmission loss, noise reduction, and insertion loss to one-dimensional plane wave models. The results demonstrate that the developed rig should be useful as a muffler evaluation tool after a prototype has been constructed. The rig can also be used for transmission loss and noise reduction determination which will prove beneficial for laboratory testing

Effect of Mean Flow on the Transmission Loss of a Doubly Tuned Flow Reversal Muffler

The same-end-inlet-outlet (SEIO) muffler, also referred to as a flow reversal muffler under flow conditions, features inlet and outlet pipes positioned on the same side of the chamber. Recently, a parametric expression has been developed to determine the end correction for double tuning of the SEIO muffler. This study extends the development of the SEIO muffler by experimentally validating the derived end correction expressions. Additionally, the tuning of the muffler is assessed with a mean flow using 3-D computational fluid dynamics, solving the linearized Navier-Stokes equation. This investigation explores the impact of flow conditions (Mach number 0.05 and 0.1) and temperature conditions (T = 733 K and 953 K) on the transmission loss (TL) of a doubly tuned muffler. The findings reveal that the muffler maintains its double tuning, even in the presence of mean flow at elevated temperatures, albeit with somewhat of a reduction in performance

An investigation of the adjustable element concept for design of automotive exhaust mufflers

The concept of the adjustable muffler is felt to be a step in the direction of easing the present tedium of constructing and testing prototype mufflers. The evaluation of such a muffler also furthers the knowledge of the interaction of acoustic elements. A review of past and present efforts in the design and evaluation of acoustic filters and filter elements is herein presented. Some consideration is given to phenomena, such as tube attenuation and transverse modes of vibration, which are encountered in muffler evaluation techniques. The characteristics of wave guides are discussed at length. The solution of the plane wave equation is presented and its applicability to standing wave tube measurements is scrutinized. The derivation of reflection factor and transmission factor equations is reviewed, and these equations are related to standing wave tube measurements. Furthermore, the standing wave tube which was used in this investigation is described. Methods of sound filtration are presented, and a theoretical foundation is constructed with a discussion of acoustic elements and their analogous mechanical and electrical components. A lumped parameter means of predicting muffler performance is also delineated, and the filtering characteristics of simple elements such as orifices and chambers are discussed. Also described are the design and evaluation of an adjustable muffler. The experimental means of evaluating the reflection and transmission characteristics are presented, and tables and graphs are incorporated to portray the effect of acoustic element adjustment on muffler performance. Furthermore, the effect of adjustment on automotive exhaust spectrums is presented and comparison of pure tone and spectral analysis is given. Inclusive in the conclusions of this investigation is the conviction that construction and evaluation of adjustable element mufflers lead to a greater understanding of the action and interaction of acoustic elements, and that this knowledge, in turn, will greatly simplify the design of silencers for specific applications --Abstract, pages ii-iii

STUDIES TO IMPROVE EXHAUST SYSTEM ACOUSTIC PERFORMANCE BY DETERMINATION AND ASSESSMENT OF THE SOURCE CHARACTERISTICS AND IMPEDANCE OPTIMIZATION

It is shown that the relationship between an impedance change and the dynamic response of a linear system is in the form of the Moebius transformation. The Moebius transformation is a conformal complex transformation that maps straight lines and circles in one complex plane into straight lines and circles in another complex plane. The center and radius of the mapped circle can be predicted provided that all the complex coefficients are known. This feature enables rapid determination of the optimal impedance change to achieve desired performance. This dissertation is primarily focused on the application of the Moebius transformation to enhance vibro-acoustic performance of exhaust systems and expedite the assessment due to modifications. It is shown that an optimal acoustic impedance change can be made to improve both structural and acoustic performance, without increasing the overall dimension and mass of the exhaust system. Application examples include mufflers and enclosures. In addition, it is demonstrated that the approach can be used to assess vibration isolators. In many instances, the source properties (source strength and source impedance) will also greatly influence exhaust system performance through sound reflections and resonances. Thus it is of interest to acoustically characterize the sources and assess the sensitivity of performance towards source impedance. In this dissertation, the experimental characterization of source properties is demonstrated for a diesel engine. Moreover, the same approach can be utilized to characterize other sources like refrigeration systems. It is also shown that the range of variation of performance can be effectively determined given the range of source impedance using the Moebius transformation. This optimization approach is first applied on conventional single-inlet single-outlet exhaust systems and is later applied to multi-inlet multi-outlet (MIMO) systems as well, with proper adjustment. The analytic model for MIMO systems is explained in details and validated experimentally. The sensitivity of MIMO system performance due to source properties is also investigated using the Moebius transformation

One-dimensional numerical approach for predicting attenuation performance of stand-alone silencers of internal combustion engine

LAUREA MAGISTRALELa progettazione corretta del silenziatore è importante per ottenere la riduzione del rumore dovuto alla dinamica dei gas nei veicoli. L'approccio numerico 3D tradizionale è complicato poichè la performance del silenziatore è sempre collegata a quella del motore a combustione interna che è un sistema molto complesso da modellare. In questa tesi, il codice sperimentale 1D Gasdyn, sviluppato dal Dipartimento di Energia del Politecnico di Milano, è stato usato per modellare il comportamento acustico dei silenziatori. Lo schema numerico adottato per la soluzione è il metodo di Lax-Wendroff, cioè un metodo simmetrico di secondo ordine a due fasi. Il trattamento delle condizioni agli estremi si basa sulla assunzione classica di flusso quasi-stazionario, quindi comprendendo la conservazione stazionaria di massa, quantità di moto ed energia, risolte con il mesh-method of characteristics (MOCs). La fonte di eccitazione imposta è un rumore bianco e ciò permette di determinare la Transmission Loss (TL) e la Transfer Function (TF) del sistema attraverso una analisi spettrale con Fast Fourier Transform (FFT) e un tempo computazionale molto ridotto.Proper silencer design is important to achieve the reduction of gas dynamic noise of the vehicles. The regular three-dimensional numerical approach is complicated since the performance of silencer is always related to the internal combustion engine which is a highly complicated system to be modeled. In this thesis, the one-dimensional research code Gasdyn, developed by Energy Department of Politecnico Di Milano, has been adopted to model the acoustic behavior of the silencers. The numerical scheme adopted for the solution is the symmetric second order two-step Lax-Wendroff Method. The treatment of the boundary condition relies on the basis of the classic assumption of quasi-steady flow, involving the steady conservation equation of mass, momentum, and energy, solved by the mesh-method of characteristics (MOCs). The excitation source is set as a white noise which allows determining the Transmission Loss (TL) and transfer function (TF) of the system by a Fast Fourier Transform (FFT) spectral analysis with an extremely short computational time. Several types of silencers will be simulated, starting from the basic reactive and dissipative configurations, until the complex configurations. For each of them, the result will be compare with those obtained experimentally to determine the accuracy of the numerical approach

Investigation of Performance Evaluation and Design Techniques for Large Industrial Mufflers

This document is the culmination of an investigation into the techniques used to design and evaluate the acoustic performance of large industrial mufflers using three-dimensional acoustic theory. Three performance criteria were considered. These criteria were noise reduction (NR), insertion loss (IL), and transmission loss (TL). Three different large sized mufflers were considered in this study. Each muffler was experimentally evaluated through field measurements. These field measurements were then used to validate finite element analysis (FEA) software that is designed to predict muffler performance. A method to corroborate in situ measurements with idealistic FEA simulations was developed. Once the software was validated, it was used to perform parametric studies that investigated the effects of certain muffler design elements on the TL performance of the muffler. The parametric studies yielded a significantly better understanding of the effects of individual muffler features, and their role in a complex geometry