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  • 1.
    Allam, S.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Cooling fan noise control using micro-perforates2012In: Int. Congr. Expos. Noise Control Eng., INTER-NOISE, 2012, p. 10434-10445Conference paper (Refereed)
    Abstract [en]

    Baffle or split silencers are commonly used, e.g., in HVAC systems and as inlet/outlet silencers on gas turbines. Another application is to reduce noise from the cooling fan inlet for large IC-engines. A baffle silencer can be seen as a periodic arrangement of parallel rectangular absorbers which can be placed in a rectangular duct. The noise reduction afforded by parallel baffles depends not only on the physical properties of the lining, but also upon the angle of incidence of the sound waves impinging and the baffle length. In this paper the potential of using baffles made of Micro-Perforated Panels is investigated in particular with the cooling fan inlet application in mind. Theoretical models for the damping is derived and used to design optimum configurations. The models are based on the wave propagation in a periodic array of baffles so that only one period can be investigated in order to find the different modes. In particular the least attenuated mode is important to find in order to optimize the behavior. An important aspect is the inner structure of the MPP baffle, i.e., can it just be an empty air volume or to what extent must internal waves be prevented by putting in walls. From a stiffness point of view some inner walls might also be needed to avoid vibration problems. Due to these complexities the theoretical models are only presented for the simplest cases. In order to validate the models and to get a more complete test of different designs experiments were also carried out. During these experiments the effect of flow was also tested.

  • 2.
    Allam, Sabry
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Acoustic modelling and testing of advanced exhaust system components for automotive engines2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The increased use of the diesel engine in the passenger car, truck and bus market is due to high efficiency and lower fuel costs. This growing market share has brought with it several environmental issues for instance soot particle emission. Different technologies to remove the soot have been developed and are normally based on some kind of soot trap. In particular for automobiles the use of diesel particulate traps or filters (DPF:s) based on ceramic monolithic honeycombs are becoming a standard. This new exhaust system component will affect the acoustics and also work as a muffler. To properly design exhaust systems acoustic models for diesel particulate traps are needed. The first part of this thesis considers the modelling of sound transmission and attenuation for traps that consist of narrow channels separated by porous walls. This work has resulted in two new models an approximate 1-D model and a more complete model based on the governing equations for a visco-thermal fluid. Both models are expressed as acoustic 2-ports which makes them suitable for implementation in acoustic software for exhaust systems analysis. The models have been validated by experiments on clean filters at room temperature with flow and the agreement is good. In addition the developed filter models have been used to set up a model for a complete After Treatment Device (ATD) for a passenger car. The unit consisted of a chamber which contained both a diesel trap and a Catalytic Converter (CC). This complete model was also validated by experiments at room temperature. The second part of the thesis focuses on experimental techniques for plane wave decomposition in ducts with flow. Measurements in ducts with flow are difficult since flow noise (turbulence) can strongly influence the data. The difficulties are also evident from the lack of good published in-duct measurement data, e.g., muffler transmission loss data, for Mach-numbers above 0.1-0.2. The first paper in this part of the thesis investigates the effect of different microphone mountings and signal processing techniques for suppressing flow noise. The second paper investigates in particular flow noise suppression techniques in connection with the measurement of acoustic 2-ports. Finally, the third paper suggests a general wave decomposition procedure using microphone arrays and over-determination. This procedure can be used to determine the full plane wave data, e.g., the wave amplitudes and complex wave numbers k+ and k-. The new procedure has been applied to accurately measure the sound radiation from an unflanged pipe with flow. This problem is of interest for correctly determining the radiated power from an engine exhaust outlet. The measured data for the reflection coefficient and end correction have been compared with the theory of Munt [33] and the agreement is excellent. The measurements also produced data for the damping value (imaginary part of the wavenumber) which were compared to a model suggested by Howe [13]. The agreement is good for a normalized boundary layer thickness less than 30-40

  • 3.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Helwan University, Egypt.
    Bodén, Hans
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Methods for Accurate Determination of Acoustic Two-Port Data in Flow Ducts2005In: 12th International Congress on Sound and Vibration 2005: ICSV 2005, 2005Conference paper (Other academic)
    Abstract [en]

    Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications. It is an important tool for instance in the development of mufflers for IC-engines. Accurate measurement of the acoustic two port data can be used not only to determine the transmission loss but also to determine physical properties like flow resistivty as well as speed of sound and impedance. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. Techniques to improve the acoustic two-port determination have been tested in this paper. A number of possible configurations for connecting loudspeakers to the flow duct have been investigated. It was found that using a perforate pipe section with about 50% porosity between the loudspeaker side branch and the duct gave the best signal-to-noise ratio out of the studied configurations. Different signal processing techniques have been tested for reducing the adverse effects of flow noise at the microphones. The most successful techniques require a reference signal which can be either the electric signal being input to the loudspeakers or one of the microphone signals. As a reference technique stepped sine excitation with cross-spectrum based frequency domain averaging was used. This technique could give good results for most cases. Using a periodic signal (saw-tooth) and synchronised time domain averaging good results could be obtained if a sufficient number of averages was used. At flow velocities higher than M=0.2 about 10000 averages were needed. Random excitation together with cross-spectrum based frequency domain averaging also gave good result if the same number of averages was used. Ordinary frequency domain averaging is not sufficient at high flow velocities. It was also shown that using cross-spectrum based frequency domain averaging an improvement could be obtained if the microphone with the highest signal-to-noise ratio at each frequency was used as the reference microphone rather than a fixed microphone.

  • 4.
    Allam, Sabry
    et al.
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Bodén, Hans
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Signal to noise ratio enhancement methods in acoustic flow duct measurements2004In: ICSV12-St Petersburg, 2004Conference paper (Other academic)
  • 5.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Bodén, Hans
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Over-determination in acoustic two-port data measurement2006In: ICSV13-Vienna / [ed] J. Eberhardsteiner, H.A. Mang, H. Waubke, 2006Conference paper (Other academic)
    Abstract [en]

    Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications. It is an important tool for instance in the development of mufflers for IC-engines. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. The plane wave acoustic two-port is a 2x2 matrix containing 4 complex quantities at each frequency. To experimentally determine these unknowns the acoustic state variables on the inlet and outlet side must be measured for two independent test cases. The two independent test cases can be created by: changing the acoustic load on the outlet side leading to the so-called two-load technique or by using one acoustic source on the inlet side and one acoustic source on the outlet side leading to the so-called two-source technique. In the latter case the independent test cases are created by first using the source on the inlet side and then the source on the outlet side. As pointed out by Åbom it is also possible to run both sources simultaneously to create more than two independent test cases. This over-determination could be used to improve the measurement results for instance if the data is contaminated by flow-noise. In this paper over-determination is tested by applying up to 5 different test cases. This procedure has been applied to a single orifice test object.

  • 6.
    Allam, Sabry
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Knutsson, M.
    Bodén, Hans
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Development of acoustic models for high frequency resonators for turbocharged IC-engines2012In: SAE Technical Paper 2012-01-1559, 2012, 2012Conference paper (Refereed)
    Abstract [en]

    Automotive turbo compressors generate high frequency noise in the air intake system. This sound generation is of importance for the perceived sound quality of luxury cars and may need to be controlled by the use of silencers. The silencers usually contain resonators with slits, perforates and cavities. The purpose of the present work is to develop acoustic models for these resonators where relevant effects such as the effect of a realistic mean flow on losses and 3D effects are considered. An experimental campaign has been performed where the two-port matrices and transmission loss of sample resonators have been measured without flow and for two different mean flow speeds. Models for two resonators have been developed using 1D linear acoustic theory and a FEM code (COMSOL Multi-physics). For some resonators a separate linear 1D Matlab code has also been developed. Different models, from the literature, for including the effect of mean flow on the acoustic losses at slits and perforates have been implemented in the codes and compared to the experimental data. Correct modeling of acoustic losses for resonators with complicated geometry is important for the simulation and development of new and improved silencers, and the present work contributes to this understanding. The developed models give acceptable agreement with the measured results even with flow but can be improved for 3D FEM if correct CAD data is available. The 1D linear theory can be used for simple geometries and to get a general overview related to the resonance frequencies and damping level.

  • 7. Allam, Sabry
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    A New Type of Muffler Based on Microperforated Tubes2011In: Journal of Vibration and Acoustics-Transactions of the ASME, ISSN 1048-9002, E-ISSN 1528-8927, Vol. 133, no 3, p. 031005-Article in journal (Refereed)
    Abstract [en]

    Microperforated plate (MPP) absorbers are perforated plates with holes typically in the submillimeter range and perforation ratios around 1%. The values are typical for applications in air at standard temperature and pressure (STP). The underlying acoustic principle is simple: It is to create a surface with a built in damping, which effectively absorbs sound waves. To achieve this, the specific acoustic impedance of a MPP absorber is normally tuned to be of the order of the characteristic wave impedance in the medium (similar to 400 Pa s/m in air at STP). The traditional application for MPP absorbers has been building acoustics often combined with a so called panel absorber to create an absorption peak at a selected frequency. However, MPP absorbers made of metal could also be used for noise control close to or at the source for noise control in ducts. In this paper, the possibility to build dissipative silencers, e. g., for use in automotive exhaust or ventilation systems, is investigated.

  • 8.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Acoustic modeling and testing of a complex car muffler2006In: International Congress on Sound and Vibration 2006, 2006, p. 1119-1126Conference paper (Refereed)
    Abstract [en]

    Perforated mufflers are used by exhaust system manufacturers to improve the broadband attenuation at low frequencies, with the drawback that this normally also implies an increased pressure drop. The detailed modelling of this type of muffler depends on knowledge of the perforate impedance which is influenced by hole geometry as well as the details of the flow distribution. The existing formulas for calculation of perforate impedance are semi-empirical and a number of alternatives have been published. One motivation behind this work was to review the existing formulas for perforate impedance using accurate measured data for perforated mufflers. A modified model presented by Bauer 1977 was found to be the best. A second motivation was to show that for a detailed analysis, using 3D acoustic FEM, the mean flow can be neglected except for calculating the perforate impedances.

  • 9.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Acoustic modelling and characterization of plate heat exchangers2012In: SAE Technical Paper 2012-01-1562, Society of Automotive Engineers, 2012Conference paper (Refereed)
    Abstract [en]

    There is increased concern about the noise emission from cooling systems. This is mainly due to an increased need for cooling needs due to turbo-charging and EGR systems, which tend to increase the fan power and thereby the noise. An important issue in this context is the behavior of the heat-exchanger and its acoustic transmission and absorption properties. In this paper an acoustic model to evaluate such data for a common type of heat exchanger, the parallel plate type, is presented. The basic configuration is assumed to be a matrix of parallel, narrow channels. The developed model is based on a so called equivalent fluid for an anisotropic medium. It is mainly dependent on the heat exchanger geometry combined with the Kirchhoff model for thermo-viscous wave propagation in narrow tubes. The proposed model can be used to predict the sound transmission and absorption for an entire heat exchanger for incident plane waves. This model is validated by comparison with measurement results for seven different heat exchangers used in vehicle and train cooling units at different flow speeds

  • 10. Allam, Sabry
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Acoustic modelling and testing of diesel particulate filters2005In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 288, no 02-jan, p. 255-273Article in journal (Refereed)
    Abstract [en]

    The use of Diesel Particulate Filters (DPFs) on automobiles to reduce the harmful effects of diesel exhaust gases is becoming a standard in many countries. Although the main purpose of a DPF is to reduce harmful emission of soot particles it also affects the acoustic emission. This paper presents a first attempt to describe the acoustic behavior of DPFs and to present models which allow the acoustic two-port to be calculated. The simplest model neglects wave propagation and treats the filter as an equivalent acoustic resistance modeled via a lumped impedance element. This simple model gives a constant frequency-independent transmission loss and agrees within I dB with measured data on a typical filter (length 250 mm) up to 200-300 Hz (at 20 degrees C). In the second model, the ceramic filter monolith is described as a system of coupled porous channels carrying plane waves. The coupling between the channels through the porous walls is described via Darcy's law. This model gives a frequency-dependent transmission loss and agrees well with measured data in the entire plane wave range.

  • 11.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Advanced experimental procedure for in-duct aero-acoustics2006In: 13th International Congress on Sound and Vibration 2006, ICSV 2006, 2006, p. 1185-1192Conference paper (Refereed)
    Abstract [en]

    The purpose of this paper is to present a method for characterization of in-duct aero-acoustic sources that can be described as active acoustic two-ports. The method is applied to investigate the sound produced from an orifice plate. The motivation is to obtain better data for the development of improved prediction methods for noise from flow singularities, e.g., in HVAC systems on aircrafts. Most of the earlier works fall into two categories; papers modeling the scattering of acoustic waves and papers modeling the sound generation. Concerning the scattering it is possible to obtain estimates of the low frequency behavior from linear perturbations of the steady state equations for the flow. Concerning the sound generation most of the presented work is experimental and follows a paper by Nelson&Morfey, which present a scaling law procedure for the in-duct sound power based on a dipole model of the source. One limitation with the earlier works is that the sound power only was measured on the downstream side. Also data was only obtained in 1/3-octave bands, by measuring the sound radiated from an open duct termination. Assuming plane waves and linear acoustics the flow duct singularity can be completely modeled as an active 2-port. The experimental determination of its properties is done in a two steps procedure. In the first step the passive data, i.e., the scattering matrix S, is determined using external (independent) sources. In the second step the S matrix is used and the source vector is determined by testing the system with known acoustic terminations.

  • 12.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Aeroacoustic investigation of diaphragm orifices in ducts2007In: Turkish Acoustical Society - 36th International Congress and Exhibition on Noise Control Engineering, INTER-NOISE 2007 ISTANBUL, 2007, p. 292-301Conference paper (Refereed)
    Abstract [en]

    Diaphragm orifices are used in duct systems to control or measure the flow rate. Such components generate complex flows and aeroacoustic phenomena, e.g., dissipation via forced vortex shedding, sound generation from eddy structures (broadband noise) and non-linear whistling. In this paper the acoustic properties (passive and active) of single and double diaphragm orifices are investigated experimentally for small Mach-numbers and low frequencies (plane waves). Using microphone arrays and wave decomposition the induct sound fields are resolved and used as input to determine the active acoustic 2-port. The work represents one of the first efforts to apply 2-port methods to characterize flow generated noise in-ducts. The motivation of this work is to obtain better understanding for noise from flow singularities in ducts, e.g., in HVAC systems on vehicles, develop and improve prediction methods and produce data for validation of CFD and other models. First the single orifice case is investigated and the 2-port data is obtained. The active (source) strength part represents a dipole type of source for which a scaling law is derived. For the passive part (the scattering matrix) a simple quasi-stationary model is tested and works well up to a few hundred Hz. Secondly the double orifice configuration is investigated and again the 2-port data is measured. To investigate the presence of orifice interaction and non-linear aeroacoustic effects, such as whistling, the double orifice data is reduced to two identical single orifices. The equivalent source data for this reduced case is then compared with the single orifice scaling law. It is found that if the separation is larger than 10 orifice diameters then orifice interaction can be neglected. Non-linear effects and tendencies for whistling were found for separations less than 3-4 duct diameters.

  • 13.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Diesel engines after treatment devices: Acoustic modeling2005In: 12th International Congress on Sound and Vibration 2005: ICSV 2005, 2005, p. 2358-2365Conference paper (Refereed)
    Abstract [en]

    To reduce exhaust pollutants from diesel engines a Diesel Particulate Filter (DPF) is normally fixed after a Catalytic Converter (CC) in an expansion chamber to create a complete After-Treatment Device (ATD). As part of the work in the EC-project ARTEMIS the authors have published a series of papers on the modeling of DPF units. Here the final and complete DPF model is presented. The model calculates the acoustic 2-port by solving the convective acoustic wave equations for two neighboring cells simplified in the manner of the Zwikker and Kosten theory. A segmentation approach has been employed to handle the actual flow, density, pressure, and temperature distribution inside the monoliths at each frequency. The theoretical results were compared with measured transmission loss data at different flow speeds and the agreement is excellent. The new complete model has also been compared with the 1-D model earlier suggested by the authors. It turns out that by using a wave number based on the Kirchhoff solution for plane waves in narrow pipes, the simple 1-D model works almost as well as the complete model. Another conclusion is that the effect of mean flow on the sound transmission through a filter is very small. Using the new model and existing models for standard pipe elements and the CC, the acoustic 2-port for a car ATD unit has been calculated and used to predict the transmission loss. The agreement between the predictions and the measured data for various flow speeds is good.

  • 14.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Hellwan University, Egypt .
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Experimental characterization of acoustic liners with extended reaction2008In: 14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference), 2008Conference paper (Refereed)
    Abstract [en]

    Suppressing of jet engine noise by inlet and exhaust duct liners and internal combustion engine (ICE) noise by intake and exhaust systems is an important part of developing environmentally acceptable vehicles. The acoustic liner is designed to provide an impedance boundary condition in the engine duct that reduces the propagation of engine noise through the duct. An accurate impedance boundary condition is necessary to optimally suppress the noise at different conditions. The goal of the research presented in this paper is to present a new technique to Educe and characterize the acoustic liner impedance for cases with extended reaction. This technique is depending on comparing both the measured and predicted 2-port transfer matrices. The measurement of the transfer matrix is performed using the two microphone technique, while the prediction of the transfer matrix is obtained assuming plane waves in the inner pipe and outer chamber coupled by a perforated wall impedance. By using a regression process the unknown wall impedance is then educed. The method is applied to investigate the effect of flow on the impedance of so called Micro-perforated panels (MPP). A MPP consists of a panel (here a plate made of Al or steel) with small perforations distributed over its surface. When these perforations are of sub-millimeter size they provide by themselves enough acoustic resistance and low acoustic mass reactance necessary for a wideband absorber.

  • 15.
    Allam, Sabry
    et al.
    Helwan University, Cairo, Egypt.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Fan Noise Control Using Microperforated Splitter Silencers2014In: Journal of Vibration and Acoustics-Transactions of the ASME, ISSN 1048-9002, E-ISSN 1528-8927, Vol. 136, no 3, p. 031017-Article in journal (Refereed)
    Abstract [en]

    Splitter or baffle silencers are commonly used, for example, in heating ventilation and air conditioning (HVAC) systems and as inlet/outlet silencers on gas turbines. Another application is to reduce noise from the cooling fan inlet for large IC-engines. A splitter silencer can be seen as a periodic arrangement of parallel rectangular absorbers, which can be placed in a rectangular duct. The noise reduction afforded by parallel splitters depends not only on the physical properties of the lining but also upon the angle of incidence of the impinging sound waves, and the splitter and duct dimensions. In this paper, the potential of using splitters made of microperforated plates (MPPs) is investigated, with a particular focus on cooling fan inlet/outlet applications.

  • 16. Allam, Sabry
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Investigation of damping and radiation using full plane wave decomposition in ducts2006In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 292, no 05-mar, p. 519-534Article in journal (Refereed)
    Abstract [en]

    A general plane wave decomposition procedure that determines both the wave amplitudes (or the reflection coefficient) and the wavenumbers is proposed for in-duct measurements. To improve the quality of the procedure, overdetermi nation and a nonlinear least-squares procedure is used. The procedure has been tested using a six microphone array, and used for accurate measurements of the radiation from an open unflanged pipe with flow. The experimental results for the reflection coefficient magnitude and the end correction have been compared with the theory of Munt. The agreement is very good if the maximum speed rather than the average is used to compare measurements and theory. This result is the first complete experimental validation of the theory of Munt [Acoustic transmission properties of a jet pipe with subsonic jet flow, 1: the cold jet reflection coefficient, Journal of Sound and Vibration 142(3) (1990) 413-436]. The damping of the plane wave (the imaginary part of the wavenumber) could also be obtained from the experimental data. It is found that the damping increases strongly, compared with the damping for a quiescent fluid.. when the acoustic boundary layer becomes thicker than the viscous sublayer. This finding is in agreement with a few earlier measurements and is also in agreement with a theoretical model proposed by Howe [The damping of sound by wall turbulent sheer layers, Journal of Acoustic Society, of America 98(3) (1995) 1723-17301. The results reported here are the first experimental verifications of Howe's model. It is found that the model works well typically up to a normalized acoustic boundary layer thickness delta(+)(A) of 30-40. For values of A a delta(+)(A) less than 10, corresponding to higher frequencies or lower flow speeds, the model proposed by Dokumaci [A note on A transmission of sound in a wide pipe with mean flow and viscothermal attenuation, Journal of Sound and Vibration 208(4) (1997) 653-655] is also in good agreement with the experimental data.

  • 17. Allam, Sabry
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Modeling and testing of after-treatment devices2006In: Journal of Vibration and Acoustics-Transactions of the ASME, ISSN 1048-9002, E-ISSN 1528-8927, Vol. 128, no 3, p. 347-356Article in journal (Refereed)
    Abstract [en]

    Driven by emission regulations in the US and the EU exhaust systems on new diesel engines are equipped with both a catalytic converter (CC) and a diesel particulate filter (DPF). The CC and DPF are normally placed after each other in an expansion chamber to create a complete after-treatment device (ATD) to reduce the exhaust pollutants. The ATD unit can also affect the acoustical performance of an exhaust system. in this paper an acoustic model of a complete ATD for a passenger car is presented. The model is made up of four basic elements: (i) straight pipes; (ii) conical inlet/outlet; (iii) CC unit, and (iv) DPF unit. For each of these elements, a two-port model is used and, with the exception of the DPF unit, known models from the literature are available. For the DPF unit, a new model suggested by the authors has been used. Using the models, the complete acoustic two-port model for the investigated ATD unit has been calculated and used to predict the sound transmission loss. The predictions have been compared to experimental data taken at cold conditions for various flow speeds and show a good agreement.

  • 18.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Noise control for cooling fans on heavy vehicles2012In: Noise Control Engineering Journal, ISSN 0736-2501, E-ISSN 2168-8710, Vol. 60, no 6, p. 707-715Article in journal (Refereed)
    Abstract [en]

    In this paper two different objects for fan passive noise control have been examined: heat exchangers and inlet/outlet parallel splitter silencers based on micro-perforated panels. The first object is theoretically and experimentally examined while the second is only examined experimentally. Throughout this paper two measurement methods were used. The ISO 15186-1:2000 to test the acoustic transmission for a diffuse field and plane wave testing in a duct of a sample cut from each heat exchanger type. Based on an anisotropic equivalent fluid model a theoretical model for the heat exchanger acoustic transmission is presented. A new type of splitter silencers based on micro-perforated plates, which can add damping up 10-20 dB in the frequency range of interest (<5 kHz), are also presented.

  • 19.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    On optimal design of mufflers using micro-perforated panels2010In: 17th International Congress on Sound and Vibration 2010, ICSV 2010: Volume 2, 2010, p. 1158-1165Conference paper (Refereed)
    Abstract [en]

    Passive mufflers are widely employed to reduce industrial and domestic ventilation noise as well as vehicle exhaust noise. Their basic geometry is formed by a simple expansion chamber and the performance is controlled by using complex geometries or by adding porous materials inside the chamber. However, when a clean absorbent system is desirable or when the muffler must support high air flux, it is not possible to add those fibrous materials and the use of micro perforated panels (MPP) as another alternative to improve the acoustic performance become important. The purpose of this work is not only to optimize the acoustic performance of low cost simple geometry mufflers using micro perforated panels (MPP) absorbers but also to find the best shape design under a limited space constraint. In this paper, on the basis of plane wave theory, the four-port system matrix for two wave guides coupled via a MPP tube is derived and used to compute the two-port transfer matrix for a expansion chamber muffler with a MPP tube. Moreover, a simulated annealing (SA) algorithm searching for the global optimum by imitating the softening process of metal has been adopted during the muffler's optimization. To assure SA's correctness, the sound transmission loss (TL) maximization of one-chamber perforated mufflers at a targeted frequency of 1500 Hz is tested first. The result of the optimized muffler is compared with the measured results at room temperature. Furthermore, a numerical case in dealing with a broadband noise emitted from an I.C. Engine by using one-chamber micro perforated mufflers has been introduced and is discussed.

  • 20. Allam, Sabry
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Sound propagation in an array of narrow porous channels with application to diesel particulate filters2006In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 291, no 05-mar, p. 882-901Article in journal (Refereed)
    Abstract [en]

    In an earlier work the authors have presented a 1-D acoustic model for diesel particulate filters (DPFs). One shortcoming of this first model is the approximate treatment of the viscous and thermal losses along the narrow channels. In the present paper this issue is analyzed in more detail, by solving the convective acoustic wave equations for two neighboring channels simplified in the manner of the Zwikker and Kosten theory. From the solution the acoustic two-port has been calculated to predict the sound transmission losses for an entire DPF unit. The theoretical results are compared with experimental data for clean filter units at room temperature and the agreement is very good and better, in particular for very small Mach numbers, than for the earlier presented 1-D model. A modified 1-D model using the classical (exact) Kirchhoff solution for a plane wave in a narrow tube is also presented. This modified 1-D model is in close agreement with the predictions of the new model. Furthermore, the earlier proposed 1-D model, which assumes isothermal sound propagation, works satisfactorily up to 800-1000Hz for a typical filter at operating (hot) conditions.

  • 21.
    Allam, Sabry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Whistling potential for duct components2013In: SAE Technical Papers: Volume 4, 2013, S A E Inc , 2013, Vol. 4, p. 2013-01-1889-Conference paper (Refereed)
    Abstract [en]

    Components in ducts systems that create flow separation can for certain conditions and frequencies amplify incident sound waves. This vortex-sound phenomena is the origin for whistling, i.e., the production of tonal sound at frequencies close to the resonances of a duct system. One way of predicting whistling potential is to compute the acoustic power balance, i.e., the difference between incident and scattered sound power. This can readily be obtained if the scattering matrix is known for the object. For the low frequency plane wave case this implies knowledge of the two-port data, which can be obtained by numerical and experimental methods. In this paper the procedure to experimentally determine whistling potential will be presented and some examples are given to show how this procedure can be used in some applications for automotive intake and exhaust system components.

  • 22.
    Bodén, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Allam, Sabry
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Holmberg, Andreas
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Experimental Techniques for Aeroacoustics in Low Mach Number Confined Flows: Keynote Paper2011In: Proceedings of the International Conference on Mechanical Engineering 2011(ICME2011) 18-20 December 2011, Dhaka, Bangladesh, ICME , 2011Conference paper (Other academic)
    Abstract [en]

    Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications such as in the development of mufflers for IC-engines. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. The wall mounted pressure transducers normally used will pick up unwanted flow noise mainly in the form of turbulent pressure fluctuations. The problem is then obtaining a signal-to- noise ratio high enough for quality measurements. Techniques to improve acoustic two-port determination have been developed in this paper, including test rig design, signal processing techniques and over-determination.

  • 23. Elnady, T.
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Allam, Sabry
    Modeling perforates in mufflers using two-ports2010In: Journal of Vibration and Acoustics-Transactions of the ASME, ISSN 1048-9002, E-ISSN 1528-8927, Vol. 132, no 6, p. 061010-Article in journal (Refereed)
    Abstract [en]

    One of the main sources of noise of a vehicle is the engine where its noise is usually damped by means of acoustic mufflers A very common problem in the modeling of automotive mufflers is that of two flow ducts coupled through a perforate A new segmentation approach is developed here based on two port analysis techniques in order to model perforated pipes using general two port codes which are widely available Examples are given for simple muffler configurations and the convergence of the technique is investigated based on the number of segments used The results are compared with closed form solutions form the literature Finally an analysts of a complicated multi chamber perforated muffler system is presented The two port simulation results show good agreement with both the measurements and the simulations using the classical four port elements [DOI 10 1115/1 4001510]

  • 24.
    Guo, Ying
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Allam, Sabry
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Micro-Perforated Plates for Vehicle Applications2008In: Proceedings of the 2008 Congress and exposition of noise control engineering, Inter-Noise 2008, 26-29 Oct, Shanghai, China, 2008Conference paper (Refereed)
  • 25.
    Guo, Ying
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Dolinar, Andreas
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Allam, Sabry
    Experimental Investigation of Acoustic Properties of Mufflers with Perforated Pipes2007In: 19th International Congress on Acoustics Madrid, Spain, September 2007, 2007Conference paper (Refereed)
  • 26.
    Kierkegaard, Axel
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Allam, Sabry
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Efraimsson, Gunilla
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Simulations of whistling and the whistling potentiality of an in-duct orifice with linear aeroacoustics2012In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 331, no 5, p. 1084-1096Article in journal (Refereed)
    Abstract [en]

    This paper demonstrates a linear aeroacoustic simulation methodology to predict the whistling of an orifice plate in a flow duct. The methodology is based on a linearized Navier-Stokes solver in the frequency domain with the mean flow field taken from a Reynolds-Averaged Navier-Stokes (RANS) solution. The whistling potentiality is investigated via an acoustic energy balance for the in-duct element and good agreement with experimental data is shown. A Nyquist stability criterion based on the simulation data was applied to predict whistling of the orifice when placed in a finite sized duct and experiments were carried out to validate the predictions. The results indicate that although whistling is a non-linear phenomena caused by an acoustic-flow instability feed-back loop, the linearized Navier-Stokes equations can be used to predict both whistling potentiality and a duct system’s ability to whistle or not.

  • 27. Veloso, R.
    et al.
    Elnemr, Y.
    Reich, F. M.
    Allam, Sabray
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Simulation of sound transmission through automotive turbochargers2012In: SAE Technical Paper 2012-01-1560, 2012, 2012Conference paper (Refereed)
    Abstract [en]

    The use of turbochargers for production cars has been increasing due to the current trend of engine downsizing. Acoustically, it acts as a damper of the pressure pulsations from the engine that propagate through the intake and exhaust system. This effect is referred to as the passive acoustic properties of a turbocharger. The aim of this paper is to investigate the passive acoustic properties of an automotive turbocharger compressor and turbine and to develop and verify an acoustical model of the turbocharger. To investigate the acoustic properties such as the transmission loss and the transfer function through these elements under different operating conditions, acoustic two-port measurements were performed on a turbocharger test rig at different flow conditions. A geometry-based acoustical model of the turbocharger was developed for both the compressor and turbine to provide both the transmission loss and the transfer function of these elements and verified by means of non-linear time-domain 1-D gas dynamics simulation. A good agreement between measurement and simulation of the upstream and downstream acoustic transmission loss of the turbocharger was achieved and it also illustrates an improvement of simulation quality using the new models.

  • 28.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Allam, Sabry
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Dissipative silencers based on micro-perforated plates2013In: SAE Technical Papers, 2013, Vol. 6Conference paper (Refereed)
    Abstract [en]

    Micro-perforated plates (MPP:s) can be defined as a perforated plate where the hole impedance is dominated by viscous losses. This will always be true for sufficiently low frequencies or small holes. In addition for a standard MPP the perforation ratio is chosen so that the normalized acoustic resistance is between 1-2, which yields optimum damping for incident plane waves. Historically MPP:s have been used as panel absorbers to reduce reflections in rooms and enclosures. More recently the potential for machinery and vehicle applications has come into focus, e.g., dissipative exhaust silencers. Some advantages offered by a MPP solution, when compared to traditional dissipative silencers, are that it can reduce the weight and the problem with fibre breakout. In this paper the work on cylindrical MPP dissipative silencers at KTH is summarized. One important question being how an optimum damping is achieved, for a certain frequency band and for a given volume (length & area ratio) of the silencer.

  • 29.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Allam, Sabry
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Industrial Education College, Cairo, Egypt .
    Boij, Susann
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Aero-acoustics of flow duct singularities at low mach numbers2006In: Collection of Technical Papers - 12th AIAA/CEAS Aeroacoustics Conference, 2006, p. 3708-3717Conference paper (Refereed)
    Abstract [en]

    This paper describes the application of an acoustic 2-port model to describe flow generated noise in ducts. An experimental procedure that enables determination of both the passive (the scattering matrix) as well as the active (source) 2-port data is described. The method is applied to investigate the aero-acoustics of an orifice plate in a duct. The passive data is compared with a simple quasi-stationary model and the active part is analyzed using a scaling law procedure, based on the assumption of a compact dipole source.

1 - 29 of 29
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