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Modelling of fluid machines as sources of sound in duct and pipe systems
... The starting point is the assumption that we have a linear and time-invariant acoustic source or system. In the frequency domain such a system can be described as an acoustic multi-port or blackbox model [3]: , ss =+ y K x y (1) where x, y are column state vectors containing complex (Fourier) amplitudes at a certain frequency, K is a matrix relating in-and output and the sub-script s denotes the source strength. For air-borne sound which is the main interest here one can select acoustic pressure (p) and volume flow (Q) as the state variables defining the state vectors. ...
... , ss =− p p Z Q (2) . ss =− Q Q M p (3) Here the usual convention that the volume flow is positive out from the source regions is assumed. ...
One problem for railway noise predictions is to characterize noise from various auxiliary equipment, e.g., fans, compressors, transformers. The noise from such sources can be a dominating contribution under low-speed operation or stand still. To better handle this problem the EU-project TRANSIT investigates improved methods for acoustic source characterization. As a starting point it is assumed that an acoustic source is enclosed by a control surface. The surface is sub-divided into smaller areas and each area is assumed to act as an acoustic one-port coupled to all the other areas. The properties of each area can then be described by its volume flow and internal impedance. The resulting acoustic pressure at a receiving point, can finally be expressed as a product of the source volume flows and a matrix representing the acoustic installation effects ("source+radiation impedances"). To simplify the method one can assume uncorrelated sources and use an ISO procedure for sound power to determine the volume flows. The acoustic installation effects can be obtained using a monopole point source to measure or calculate the pressure at selected receiving positions.
... For the acoustic characterization the silencer unit was treated as an acoustic two-port [9]. The acoustic power was determined at the inlet and outlet cross-section of the silencer by using the classical two-microphone wave decomposition method [10]. ...
...Micro perforated elements are innovative acoustic solutions,which silencing effect is based on the dissipation of the acoustic wave energy in a pattern of sub-millimeter apertures. Similarly to fibrous materials the micro-perforated materials have been proved to provide effective sound absorption in a wide frequency range. Additionally, the silencer is designed as a two-stage system that provides an optimal solution for a variety of exploitation conditions. In this paper a novel design for a cruiser type motorcycle silencer, based on micro-perforated elements, is presented. It has been demonstrated that the micro-perforated elements can successfully be used to achieve high attenuation of IC-engine noise in strictly limited circumstances. A technical description of the design and manufacturing of the prototype silencer is given and technological issues are discussed. The acoustical and fluid-dynamical performance of the silencer is characterized by transmission loss and pressure drop data. The influence of the two-stage system valve operation has been analyzed by studying the acoustics data and engine output characteristics. In addition to the experimental investigations, numerical 1-D models were developed for the optimization of the silencer geometry and the results are compared in a number of operating conditions. The studies have resulted in development of a silencer system for a small series cruiser type motorcycle. The first silencer prototypes have been tested on the motorcycle. While maintaining acceptable pressure drop characteristics, it has proven to comply with standard noise criteria without incorporating fibrous materials....
... The accuracy depends on the load combinations selected and whether modifying the load changes the source [5][6][7][8][9][10]. There is a wave decomposition variation of the approach that assumes the forward wave is a sum of the outgoing and reflected waves, requiring two sound pressure measurements per load [11]. This research introduces an alternative approach using an electroacoustic analogy to find acoustic free velocity (source strength) and impedance inversely. ...
A common method to characterize linear time invariant ducted acoustic sources is the electro-acoustic analogy. The sources are typically defined by their internal source strength and source impedance. Various methods have emerged to determine the source characteristics, broadly categorized into direct and indirect methods. Direct methods involve using a secondary source with significantly higher amplitude to measure source impedance but cannot determine source strength. Indirect methods vary acoustic loads to obtain both source strength and source impedance, accuracy depends on appropriate load combinations. In this research, we propose an inverse method to determine the acoustic source strength from the measured acoustic transfer function and the operational acoustic pressure. This approach yields what we term as the "acoustic free velocity," corresponding to a plane monopole source that characterizes the original sound source. Subsequently, the source impedance is obtained using the acoustic free velocity in conjunction with the load impedance and acoustic pressure in the acoustic circuit. The method is demonstrated using a compressor driver source attached to a 1 3/8" impedance tube. Validation of the measured source strength and source impedance is conducted using a two-load method. Additionally, prediction of muffler insertion loss shows a good correlation with experimental measurements, affirming the efficacy of the proposed methodology.
... Some relevant studies have been conducted (Alenius et al., 2015;Å bom et al., 2006), in which the authors have studied the aeroacoustics of an orifice plate in a duct using the two-port scattering method. This method, however, targets the characterization of the scattering matrix of the system for an already existing acoustic signal (Boden and Å bom, 1995). Moreover, the system's geometry under analysis does not reflect the 3D behavior of a human vessel. ...
Collapsible tubes can be employed to study the sound generation mechanism in the human respiratory system. The goals of this work are (a) to determine the airflow characteristics connected to three different collapse states of a physiological tube and (b) to find a relation between the sound power radiated by the tube and its collapse state. The methodology is based on the implementation of computational fluid dynamics simulation on experimentally validated geometries. The flow is characterized by a radical change of behavior before and after the contact of the lumen. The maximum of the sound power radiated corresponds to the post-buckling configuration. The idea of an acoustic tube law is proposed. The presented results are relevant to the study of self-excited oscillations and wheezing sounds in the lungs.
... The objective of this study is to aid in predicting the thermoacoustic instability in a generic burner configuration by a system model approach. In order to comprehend and predict the thermoacoustic instability in a system, a linear stability analysis (LSA) of network models is performed [8,23]. The network modelling is achieved by deconstructing a complete combustion system into smaller acoustic elements that interact via a single interface, and this framework defines the one-dimensional low-order acoustic network models of acoustic elements, enabling quick analysis of the system's stability [47]. ...
Thermoacoustic instabilities induced by the coupling of acoustic modes and unsteady heat release are a matter of concern in a wide variety of high-performance systems, including rocket engines and gas turbine combustors. They can result in severe noise emissions, operational difficulties, and even catastrophic damage to the system. The Linear Stability Analysis (LSA) of low-order network models is a reliable method for identifying and predicting the thermoacoustic instability in a system by approximation of complex configuration as a network of acoustic elements. Mathematically these elements are described by an acoustic scattering or transfer matrix, defining the dynamic relation of acoustic elements travelling within the configuration. Deriving the transfer matrix coefficient enables quick evaluation of the system’s acoustic behaviour under various operating conditions. The present thesis describes a procedure for the characterization of the acoustic transfer matrix, which predicts the aeroacoustic properties of the narrow ducts and perforated plates in a burner configuration. The transfer matrix coefficients are determined by combining modern compressible computational fluid dynamics based on direct numerical simulation (DNS) and empirical modelling via system identification (SI).
... In general, such assumption is invalid, especially in the low frequency range, and can lead to a significant error. In the present work this assumption is eliminated by implementing a complete acoustic two-port model [39] in the scattering matrix [25] and source strength vector [33] form: ...
The challenging problem of noise generation and propagation in automotive turbocharging systems is of real interest from both scientific and practical points of view. Robust and fast steady-state fluid flow calculations, complemented by acoustic analogies can represent valuable tools to be used for a quick assessment of the problem during e.g. design phase, and a starting point for more in-depth future unsteady calculations. Thus, as a part of the initial phase of a long-term project, a steady-state Reynolds Averaged Navier-Stokes (RANS) flow analysis is carried out for a specific automotive turbocharger compressor geometry. Acoustic data are extracted by means of aeroacoustics models available within the framework of the STAR-CCM+ solver (i.e. Curle and Proudman acoustic analogies, respectively). This part of the work focuses on the discussion and comparison of the aeroacoustic models, and their suitability towards predicting flow and acoustics trends corresponding to the operating conditions investigated. However, given the unsteady nature of acoustics, the project will have to develop towards an investigation of the problem using more expensive, but more accurate, Large Eddy Simulation (LES) calculations. An entire compressor map with 80 operating conditions was simulated, yielding trends in the behaviour of the performance parameters for the analysed compressor. Detailed results calculated on the same compressor speed-line for one design and one off-design operating conditions are presented in terms of time-averaged pressure coefficient, Mach number, and acoustic power distributions. A total acoustic power map has been generated based on the outcome from the Curle and Proudman acoustic models, giving an indication of the noisiest operating conditions.
... Alternatively, source impedance can be measured using wave decomposition 16,17 . The complex wave amplitude propagating downstream (A) in the exhaust system is divided into two parts consisting of the direct outgoing wave from the source (P S+ ) and reflected wave from the source interface (B • R S ) as shown in Fig. 2. ...
Knowledge of the strength and internal impedance of the sound source is essential to predict the overall acoustic performance of an exhaust or intake system. Source impedance is sometimes measured directly using a strong external source. At other times, a suitable external source is difficult to identify and an indirect multi-load method is used to determine source impedance and strength. Data can be processed using the commonly used circuit analogy or via a similar wave decomposition model. In this work, the two processing schemes are used to determine the source impedance and strength of a diesel engine. Results are compared to one another and to assumed source impedances and equations in the literature for source strength. The sound pressure in the exhaust is predicted from the measured source strength and impedance and compared with measurement. There is good agreement especially at the first few harmonics of the firing frequency. It is then observed that the equation relating the source impedance to the acoustic response is in the form of the Moebius transformation, which maps straight lines or circles in one complex domain into straight lines or circles in another complex domain. It is demonstrated that the range of source impedance, defined as an outline in the complex plane, can be transformed to a corresponding outline of the muffler insertion loss complex amplitude, and the minimum and maximum insertion loss can be computed at a given frequency. This permits the expected insertion loss range to be plotted as a function of frequency. � 2018 Institute of Noise Control Engineering.
The control of noise propagating along ventilation system ducts has always been an important issue in the building and vehicle sectors. This problem is generally tackled by selecting noise-reducing components with a suitable transmission loss, possibly verifying their effectiveness at a later time. The aim of this article is to characterize the nature of the problem and propose a design approach focusing directly on the perceived effect, that is, on the sound pressure level downstream of the outlet. Because the nature of the noise emission depends on various generation mechanisms, different methods can be applied. Usually, it is more difficult to realize good attenuations at low frequencies because of the limits of sound absorbing materials in such frequency range. For this reason, the ability of reactive components to attenuate the noise below the cut-on frequency will be investigated. This goal is reached by applying the transfer matrix approach to a duct system, with the implementation of the transfer matrices of each single element, and then assembling a system capable of acoustically describing the source and the duct structure. The coupling between the duct system with source and receiver impedances allows one to predict the sound pressure level at a given distance from the outlet. The proposed methodology is implemented in a user-friendly calculation tool with possible academic and professional application. Predictive capability, usability, and intuitiveness of the proposed design procedure are validated against experimental results by real potential users, who express positive feedback.
This chapter discusses the designs of both reactive mufflers and passive mufflers and combined types. The acoustical performance of a system with a muffler as a path element is described in terms of the transmission loss (TL), insertion loss (IL), and radiated sound pressure. The chapter presents a system for reactive muffler modeling based on electrical analogies that has been found useful in predicting the acoustical performance of such systems. The various methods for determining the impedance of a ducted source are described. Theoretical models for the acoustical performance of dissipative mufflers are complicated, particularly above the cutoff frequency, and often practitioners must rely on design charts or mufflers designed and tested by manufacturers. Different theoretical models are reviewed, and some empirical methods are described for simple predictions of the IL of lined ducts. The TL can be measured for an absorbent silencer.
12:th European Turbomachinery Conference
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