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  • 1.
    Alenius, Emma
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics. 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, MWL Flow acoustics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Large eddy simulations of acoustic-flow interaction at an orifice plate2015In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 345, p. 162-177Article in journal (Refereed)
    Abstract [en]

    The scattering of plane waves by an orifice plate with a strong bias flow, placed in a circular or square duct, is studied through large eddy simulations and dynamic mode decomposition. The acoustic-flow interaction is illustrated, showing that incoming sound waves at a Strouhal number of 0.43 trigger a strong axisymmetric flow structure in the orifice in the square duct, and interact with a self-sustained axisymmetric oscillation in the circular duct orifice. These structures then generate a strong sound, increasing the acoustic energy at the frequency of the incoming wave. The structure triggered in the square duct is weaker than that present in the circular duct, but stronger than structures triggered by waves at other frequencies. Comparing the scattering matrix with measurements, there is a good agreement. However, the results are found to be sensitive to the inflow, where the self-sustained oscillation in the circular duct simulation is an artefact of an axisymmetric, undisturbed inflow. This illustrates a problem with using an undisturbed inflow for studying vortex-sound effects, and can be of interest when considering musical instruments, where the aim is to get maximum amplification of specific tones. Further, it illustrates that at the frequency where an amplification of acoustic energy is found for the orifice plate, the flow has a natural instability, which is suppressed by non-axisymmetry and incoming disturbances.

  • 2.
    Alenius, Emma
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics. 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, MWL Flow acoustics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    LES of Acoustic-Flow Interaction at an Orifice Plate2012In: 18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference), 2012Conference paper (Other academic)
    Abstract [en]

    The scattering of plane waves by a thick orifice plate, placed in a circular or square duct with flow, is studied through Large Eddy Simulation. The scattering matrix is computed and compared to measurements, showing reasonably good agreement except around one frequency ($St \approx 0.4$). Here a stronger amplification of acoustic energy is observed in the circular duct simulations than in the measurements and the square duct simulations. In order to improve the understanding of the interaction between an incoming wave, the flow, and the plate, a few frequencies are studied in more detail. A Dynamic Mode Decomposition is performed to identify flow structures at significant frequencies. This shows that the amplification of acoustic energy occurs at the frequency where the jet in the circular duct has an axisymmetric instability. Furthermore, the incoming wave slightly amplifies this instability, and suppresses background flow fluctuations.

  • 3.
    Alenius, Emma
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics. 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, MWL Flow acoustics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Scattering of Plane Waves by a Constriction2011In: Proceedings of ASME Turbo Expo 2011, Vol 7, Parts A-C, American Society Of Mechanical Engineers , 2011, p. 1043-1052Conference paper (Refereed)
    Abstract [en]

    Liner scattering of low frequency waves by an orifice plate has been studied using Large Eddy Simulation and an acoustic two-port model. The results have been compared to measurements with good agreement for waves coming from the downstream side. For waves coming from the upstream side the reflection is over-predicted, indicating that not enough of the acoustic energy is converted to vorticity at the upstream edge of the plate. Furthermore, the sensitivity to the amplitude of the acoustic waves has been studied, showing difficulties to simultaneously keep the amplitude low enough for linearity and high enough to suppress flow noise with the relatively short times series available in LES.

  • 4.
    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.

  • 5. Allam, S.
    et al.
    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).
    Investigation of aerodynamic installation effects for an axial fan2013In: 19th AIAA/CEAS Aeroacoustics Conference, 2013Conference paper (Refereed)
    Abstract [en]

    The aim of the work reported in this paper is to investigate the aerodynamic installation effects for a generic cooling fan system. This is done by treating the ducted fan as a multi-port source and performing a source characterization, i.e., determining the source strength and source reflection matrix. From the source strength, which represents the sound radiated by the fan in a reflexion free (infinite) duct, the effect of various inflow distortions can be studied. The work is part of the EU-project ECOQUEST where the data will be used to validate the acoustic prediction tools.

  • 6. Allam, S.
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Noise reduction for automotive radiator cooling fans2015In: FAN 2015 - International Conference on Fan Noise, Technology and Numerical Methods, Institution of Mechanical Engineers , 2015Conference paper (Refereed)
    Abstract [en]

    Engine cooling fans have long been recognized as one of the major noise sources in a vehicle. As the engine and other vehicle components are made quieter, the need to reduce fan noise has become more and more urgent. To reduce fan noise in a cost-effective manner, it is necessary to incorporate the component of noise reduction into an early design stage. In this paper a detailed experimental study on an automotive vehicle cooling system is presented. The aim is to investigate the flow generated noise, characterize the heat exchanger damping properties and investigate the use of near-field noise control by micro-perforated (MPP) shrouds and tuned MPP dampers. For the tested standard automotive cooling fan system the MPP shroud gave a reduction in the range 1.5 to 4.5 dB(A) depending on the fan speed. Also the absorption on the back-side is significantly increased which can reduce the noise further. The near-field tuned MPP damper concept is also promising and gives a reduction around 3 dB(A) at the operating points. 

  • 7.
    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.

  • 8. 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.

  • 9.
    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.

  • 10.
    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

  • 11. 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.

  • 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.
    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.

  • 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.
    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.

  • 14.
    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.

  • 15.
    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.

  • 16.
    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.

  • 17. 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.

  • 18. 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.

  • 19.
    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.

  • 20.
    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.

  • 21. 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.

  • 22.
    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.

  • 23. Baro, S.
    et al.
    Corradi, R.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Tyre cavity noise: Porous materials as a countermeasure2016In: Proceedings of the INTER-NOISE 2016 - 45th International Congress and Exposition on Noise Control Engineering: Towards a Quieter Future, German Acoustical Society (DEGA) , 2016, p. 2313-2318Conference paper (Refereed)
    Abstract [en]

    The first resonance of the tyre air cavity can significantly affect vehicle interior noise at frequencies around 200 Hz. The insertion of a sound absorbing liner inside a tyre is known to be an efficient countermeasure to this problem and tyre manufacturers are already producing tyres implementing this solution. The present work proposes a methodology for predicting the damping performance of a lined tyre by means of numerical models. The geometry of the tyre cavity and the lining, as well as the properties of the sound absorbing material are taken into account and predictions are made concerning the effect of specific combinations of volume and properties of the liner. For fixed material characteristics, the volume of the lining treatment strongly influences the attenuation of the cavity resonance. Moreover, the simulations performed suggest that for fixed volume and material properties, larger attenuation of the cavity resonance peak can be obtained by adopting a discontinuous layout.

  • 24. Baumann, H. D.
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Valve-Induced Noise: Its Cause and Abatement2008In: Handbook of Noise and Vibration Control / [ed] Malcolm Crocker, John Wiley & Sons, 2008, p. 935-945Chapter in book (Refereed)
  • 25.
    Berglund, Per-Olof
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Feng, Leping
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    An application of the noise synthesis technology (NST) to a system with an axial fan2002In: Forum Acustica 2002, Seville, Spain, 2002Conference paper (Refereed)
  • 26.
    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.

  • 27.
    Boij, Susann
    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.
    Pieters, R.
    Eindhoven University of Technology, Department of Applied Physics, Netherlands.
    Reflection properties of a flow pipe with a small angle diffuser outlet2010In: 17th International Congress on Sound and Vibration 2010, ICSV 2010: Volume 1, 2010, p. 706-712Conference paper (Refereed)
    Abstract [en]

    The reflection of plane acoustic waves is studied for an open pipe termination where the outlet section about 10 cm long is a diffuser with a small angle. Diffuser angles up to 6o with a sharp outlet edge are considered both without and with a mean flow. The experiments are performed for Helmholtz numbers, He, based on the pipe diameter, up to 1.0 and mean flow Mach numbers, M, up to 0.25. A multi-microphone method is used for accurate measurements of the acoustic fields inside the pipe. With no mean flow, the reflection coefficient results are compared with the theories for thin walls by Levine and Schwinger [1] and for the end correction and thick walls by Ando [2], respectively. The data for the magnitude of the reflection coefficient for different pipe end geometries show that in the low frequency regime it is the outlet radius that determines the magnitude of the energy reflection coefficient. The same collapse in the data is not obtained for the end correction which is strongly affected by the pipe end geometry. Experimental results of the reflection coefficient in the presence of a mean flow show a similar behaviour as without flow. However, it is the Strouhal number of the outlet that governs the losses, i.e., radiation and flow losses. For a region of small Strouhal numbers, the reflection is larger than one, as predicted for straight pipes, [3, 4]. An increased diffuser angle and rounded edges both increase the reflection at the pipe termination in a critical range of Strouhal numbers, which indicates that the reflection coefficient is strongly dependent on the shape, half angle, and edge curvature of the pipe end in this region.

  • 28.
    Bolin, Karl
    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.
    Air-borne sound generated by sea waves2010In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 127, no 5, p. 2771-2779Article in journal (Refereed)
    Abstract [en]

    This paper describes a semi-empiric model and measurements of air-borne sound generated by breaking sea waves. Measurements have been performed at the Baltic Sea. Shores with different slopes and sediment types have been investigated. Results showed that the sound pressure level increased from 60 dB at 0.4 m wave height to 78 dB at 2.0 m wave height. The 1/3 octave spectrum was dependent on the surf type. A scaling model based on the dissipated wave power and a surf similarity parameter is proposed and compared to measurements. The predictions show satisfactory agreement to the measurements. (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3327815]

  • 29.
    Bournobuke, Peter
    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.
    Flow acoustic analysis at ciclope2010In: 17th International Congress on Sound and Vibration 2010, ICSV 2010, 2010, p. 1068-1080Conference paper (Refereed)
    Abstract [en]

    The new wind tunnel at CICLoPE - Center for International Cooperation in Long Pipe Experiments (www.ciclope.unibo.it) is an important research facility for the study of high Reynolds number turbulent flows. However, measuring turbulent fluctuations require low acoustic noise levels in the measurement section. A flow acoustic analysis of the wind tunnel has been completed at the maximum flow velocity case of 69 m/s in the measurement section with the purpose of concluding the final design of the up- and downstream mufflers. The acoustic analysis of the wind tunnel was carried out using a power based method (VDI 3733) along the two airborne sound propagation directions. However, it was concluded that a pure power based analysis was not an adequate approach in the low frequency region since where system resonance effects can be expected. For the low frequency range the wind tunnel was modeled using a one-dimensional sound propagation software for complex duct networks (SIDLAB).

  • 30.
    Du, Lin
    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). Beijing University of Aeronautics and Astronautics, China.
    Holmberg, Andreas
    Karlsson, Mikael
    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 Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sound amplification at a rectangular T-junction with merging mean flows2016In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 367, p. 69-83Article in journal (Refereed)
    Abstract [en]

    This paper reports a numerical study on the aeroacoustic response of a rectangular T-junction with merging mean flows. The primary motivation of the work is to explain the high sound amplification, recently seen experimentally, when introducing a small merging bias flow. The acoustic results are found solving the compressible Linearized Navier-Stokes Equations (LNSEs) in the frequency domain, where the base flow is first obtained using RANS with a k-epsilon turbulence model. The model predicts the measured scattering data well, including the amplitude and Strouhal number for the peak amplification, if the effect of eddy viscosity damping is included. It is found that the base flow changes significantly with the presence of a small bias flow. Compared to pure grazing flow a strong unstable shear layer is created in the downstream main duct starting from the T-junction trailing edge. This means that the main region of vortex-sound interaction is moved away from the junction to a downstream region much larger than the junction width. To analyze the sound amplification in this region Howe's energy corollary and the growth of acoustic density are used.

  • 31.
    Du, Lin
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Karlsson, Mikael
    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.
    Numerical study on the sound amplification of a T-junction with bias flow2016In: Springer Proceedings in Physics, 2016, p. 373-381Conference paper (Refereed)
    Abstract [en]

    This paper reports a numerical study on the aeroacoustic response of a rectangular T-junction with bias flow in the side-branch. The primary motivation of the present work is to study and explain the in recent experiments observed high sound amplification at small bias flows. The study is conducted by performing numerical simulation, which solves the 2D compressible linearized Navier-Stokes equations (LNSEs) in the frequency domain. The time averaged flow is first solved by using RANS along with a k-ε turbulence model. The overall agreement with the experimental acoustic 3-port scattering data is good. It is found that the base flow changes significantly with the presence of a small bias flow. Compared to the case with no bias flow, a strong shear layer is created along the downstream main duct by the mixed grazing-bias flow. For small bias flows (Mach-number < 0.02) this shear layers extends far downstream of the actual junction. This creates a region of vortex-sound interaction much larger than for the no bias flow case, which is the main explanation behind the large amplification. © Springer International Publishing Switzerland 2016.

  • 32.
    Du, Lin
    et al.
    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.
    Using micro-perforated plates to realize a silencer based on the Cremer impedance2014In: Proceedings of Forum Acusticum, European Acoustics Association (EAA), 2014Conference paper (Refereed)
    Abstract [en]

    Current trends for IC-engines are driving the development of more efficient engines with higher specific power. This is true for both light and heavy duty vehicles and has led to an increased use of charging. The charging can be both in the form of a single or multi-stage turbo-charger driven by exhaust gases or via a directly driven compressor. In both cases a possible noise problem can be a strong Blade Passing Frequency (BPF) typically in the kHz range and above the plane wave range. In this paper a novel type of compact dissipative silencer developed especially to handle this type of problem is described. The silencer is based on a combination of a micro-perforated tube backed by a locally reacting cavity. The combined impedance of micro-perforate and cavity is chosen to match the theoretical optimum known as the Cremer impedance at the mid-frequency in the frequency range of interest. Due to the high damping achieved at the Cremer optimum (hundreds of dB/m) it is easy to create a compact silencer with a significant damping (say >40 dB) in a range larger than an octave. Several principles are presented to determine the parameters of micro-perforate and cavity. The numerical results indicate that, following the principles, a silencer with broad-band damping can be achieved.

  • 33.
    Du, Lin
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Karlsson, Mikael
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Knutsson, M.
    Modelling of Acoustic Resonators Using the Linearized Navier Stokes Equations2016Conference paper (Refereed)
    Abstract [en]

    To tune the acoustics of intake systems resonators are often used. A problem with this solution is that the performance of these resonators can be affected a lot by flow. First, for low frequencies (Strouhal-numbers) the acoustic induced vorticity across a resonator inlet opening will create damping, which can reduce the efficiency. Secondly, the vorticity across the opening can also change the end-correction (added mass) for the resonator, which can modify the resonance frequency. However, the largest problem that can occur is whistling. This happens since the vortex-sound interaction across a resonator opening for certain Strouhal-numbers will amplify incoming sound waves. A whistling can then be created if this amplified sound forms a feedback loop, e.g., via reflections from system boundaries or the resonator. To analyse this kind of problem it is necessary to have a model that allows for both sound and vorticity and their interaction. This means using a convected wave equation type of model is not sufficient. A better approach is to apply the linearized Navier Stokes equations, which will give a full model of the vortex-sound effects. In this paper an effort to apply this approach on a set of generic resonators is described. Besides the numerical results comparisons with experiments are also presented.

  • 34.
    Ducret, Fabrice
    et al.
    Ørsted, DTU, Acoustic Technology, Technical University of Denmark.
    Jacobsen, F.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Development of micro-perforated acoustic elements for vehicle applications2005In: 12th International Congress on Sound and Vibration 2005: ICSV 2005, 2005, p. 2343-2350Conference paper (Refereed)
    Abstract [en]

    There is a growing interest in the automotive industry on micro-perforated acoustic elements for reducing noise radiation from engine systems. The traditional means of acoustic attenuation through porous materials has some serious drawbacks (poor efficiency at low frequencies, low capability to withstand flow and thermal conditions commonly encountered in engine systems). Novel elements consisting of holes/slits of sizes of about 10-1 mm are currently investigated. An indicator of paramount importance when designing such elements is the acoustic flow impedance (pressure drop across the sample/acoustic particle velocity). A detailed description of the impedance of these elements is necessary for evaluating the efficiency of reflection of acoustic waves. This is required for reducing the propagation of noise further downstream. A concise fundamental theoretical investigation for optimal design is not possible due to highly complex geometries attributed to micro-perforated patterns. Moreover, mean flow, thermal and non-linear effects come into play for vehicle applications. This paper describes preliminary results from a project investigating the above effects.

  • 35.
    Efraimsson, Gunilla
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aeroacoustics.
    Pieper, Timm
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aeroacoustics.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Boij, Susann
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Simulation of Wave Scattering at an Orifice by using a Navier-Stokes Solver2007In: 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference) , Rome, Italy, May 21-23, 2007, 2007Conference paper (Other academic)
  • 36. Elnady, T.
    et al.
    Elsaadany, S.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Flow and pressure drop calculation using two-ports2008In: Int. Congr. Sound Vib., ICSV, 2008, p. 743-750Conference paper (Refereed)
    Abstract [en]

    Exhaust systems should be carefully designed for different applications. The main objective of an exhaust system is to reduce the engine noise. Maximum noise reduction is usually desired to the limit of a certain back pressure which is set by the engine manufacturer in order not to deteriorate the engine efficiency. Therefore, a parallel calculation of the flow and pressure drop must be performed. The amount of flow flowing through each element will also affect its acoustic properties. Usually, acoustic and flow calculations are done separately on two different software. This paper describes a new technique that enables both calculations to be done using the same input data on the same platform. Acoustic calculations are usually performed in the frequency domain in the plane wave region using the two-port theory and then the acoustic pressure in the system is solved for using well-known algorithms to handle arbitrary connected two-ports. The stagnation pressure and volume flow can also be calculated using the same algorithm by deriving a flow two-port for each element using the stagnation pressure and the volume flow velocity as the state variables. The proposed theory is first discussed listing the flow matrices for common elements in exhaust elements, and then different systems are analyzed and compared with the measurements.

  • 37. Elnady, T.
    et al.
    Elsaadany, S.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Flow and Pressure Drop Calculation Using Two-Ports2011In: Journal of Vibration and Acoustics-Transactions of the ASME, ISSN 1048-9002, E-ISSN 1528-8927, Vol. 133, no 4, p. 041016-Article in journal (Refereed)
    Abstract [en]

    Exhaust systems should be carefully designed for different applications. The main objective of an exhaust system is to reduce the engine noise. Maximum noise reduction is usually desired to the limit of a certain backpressure, which is set by the engine manufacturer in order not to deteriorate the engine efficiency. Therefore, a parallel calculation of the flow and pressure drop must be performed. The amount of flow flowing through each element will also affect its acoustic properties. Usually, acoustic and flow calculations are done separately on two different software. This paper describes a new technique that enables both calculations to be done using the same input data on the same platform. Acoustic calculations are usually performed in the frequency domain in the plane wave region using the two-port theory and then the acoustic pressure in the system is solved for using well-known algorithms to handle arbitrary connected two-ports. The stagnation pressure and volume flow can also be calculated using the same algorithm by deriving a flow two-port for each element using the stagnation pressure and the volume flow velocity as the state variables. The proposed theory is first discussed listing the flow matrices for common elements in exhaust elements, and then different systems are analyzed and compared with the measurements.

  • 38. Elnady, T.
    et al.
    Elsahar, W.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Estimation of temperature drop in exhaust systems2012In: 41st International Congress and Exposition on Noise Control Engineering 2012, INTER-NOISE 2012: Volume 11, 2012, 2012, p. 9362-9375Conference paper (Refereed)
    Abstract [en]

    The analysis of the acoustic performance of exhaust systems is affected by the flow speed and temperature of the exhaust gases. The flow speed affects the convective wave speed and changes the acoustic properties of some acoustic elements such as perforates. The temperature of the exhaust gas affects its density and the speed of sound. It is important to model the flow and temperature distribution within the exhaust system in order to perform an accurate acoustic simulation. The acoustic propagation along an exhaust system can be modeled by dividing the exhaust system into a number of two-port elements. It has been previously shown that flow distribution and pressure drop calculation can be done using the same two-port elements but using a different set of transfer matrices. In this paper, we are proposing a new method to calculate the temperature drop using the same two-port elements but using another set of transfer matrices. This technique has the advantage to solve all physics by defining only one network. The heat is dissipated through the pipe wall via convection, conduction, and radiation. Several cases were designed to validate the new technique. They show that it works as expected.

  • 39. Elnady, T.
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    SIDLAB: New 1D sound propagation simulation software for complex duct networks2006In: 13th International Congress on Sound and Vibration 2006, 2006, p. 4262-4269Conference paper (Refereed)
    Abstract [en]

    Low frequency sound generation and propagation in duct and pipe systems is important in many applications. The examples range from ventilation systems to IC-engine exhaust and inlet systems. Plane wave two-port theory can be used to describe the sound transmission along the system. Based on a long experience of different applications, a new user-friendly software (SIDLAB) has been developed to meet today's engineering needs. The new software is MATLAB based and allows access to the source code. This gives the user a flexibility to do other calculations than those already defined and further post-process the data. SIDLAB includes typical two-port elements for many applications but also unique elements, e.g., for modelling devices for exhaust gas cleaning. It is also unique that the two-port elements are allowed to be active, i.e., contain sources. There is no limitation to the number of elements and they can be connected together in any arbitrary scheme. By introducing one-port elements, the network can handle several inlets and outlets. SIDLAB can simulate passive results, such as transmission loss and transfer matrices between arbitrary nodes (points) in the network. It can also simulate active results, such as pressure and sound power at each node in the network. In this paper an overview of the new software is presented together with an example of the analysis of a complex muffler.

  • 40. 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]

  • 41. Elnady, T.
    et al.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Yang, Y.
    Systematic Optimization of an Exhaust System to Meet Noise Radiation Criteria at Idle2014In: SAE International Journal of Passenger Cars - Mechanical Systems, ISSN 1946-3995, Vol. 7, no 3Article in journal (Refereed)
    Abstract [en]

    Exhaust noise is a major contributor to the radiated noise level of a vehicle, especially at idle. The radiated noise level has to meet a certain criteria based on regulation and consumer demand. In many cases, the problem appears after the vehicle is manufactured and the tailpipe noise measurement is performed indicating a high noise level that needs to be reduced. This paper describes one of those cases where the radiated noise level of a certain passenger car at idle was required to be reduced by 6 dB(A). The exhaust system consists of one main muffler and one auxiliary muffler. A 1D two-port model of the exhaust system including the two mufflers was built using commercial software. This model was validated against the measurement of the two-port matrix of both mufflers. The model was then used together with tailpipe noise measurements to estimate the characteristics of the source strength and impedance. Using a 1D model of the complete system, it was possible to propose several modifications for the vehicle manufacturer to choose from. A systematic optimization approach is presented in this paper describing several suggestions to reduce the radiated tailpipe noise.

  • 42. Elnady, Tamer
    et al.
    Elsaadany, Sara
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Investigation into modeling of multi-perforated mufflers2009Conference paper (Other academic)
    Abstract [en]

    There is a strong competition among automotive manufacturers to reduce the radiated noise levels. One main source is the engine where the main noise control strategy is by using efficient mufflers. Resistive mufflers are now widely used to attenuate IC-Engine noise due to its better performance over the reactive ones. Resistive damping can be achieved either by using absorbing material or perforates in the form of tubes or sheets. Perforates mufflers have an increased performance with flow when the acoustic impedance is increased by introducing flow through the perforate holes. On the other hand, perforates can deteriorate the engine performance, if badly designed, by increasing the flow back pressure. Modeling of perforated mufflers started in the seventies when simple geometries were used. There were two approaches to analyze two tubes connected with a perforate (i.e. four-port), segmentation and distributed. Both approaches were limited to a few specified geometries. Recently, the authors published a new technique based on the segmentation approach where four ports can be replaced by a number of two-ports so that it can be used in general two-port codes. This paper investigates the use of these techniques in modeling complex perforated muffler geometries. Fifteen different configurations were modeled and compared to measurements. There are some limitations to the use of these models in some configurations because of strong 3D effects that limits the validity of these models to almost half the plane wave region. These configurations are mainly the double plug flow muffler and the parallel tube mufflers.

  • 43.
    Elnady, Tamer
    et al.
    Sound and Vibration Lab., Faculty of Engineering, Ain Shams University.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Modelling perforates in mufflers using two-ports2005In: 12th International Congress on Sound and Vibration 2005: ICSV 2005, 2005, p. 2294-2301Conference paper (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 analysis 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.

  • 44.
    Elsaadany, Sara
    et al.
    Ain Shams Univeristy, Group for Advanced Resaerch in Dynamic Systems, Faculty of Engineering, Egypt.
    Elnady, Tamer
    Ain Shams Univeristy, Group for Advanced Resaerch in Dynamic Systems, Faculty of Engineering, Egypt.
    Boij, Susann
    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.
    Optimization of exhaust systems to meet the acoustic regulations and the enginespecifications2011Conference paper (Other academic)
    Abstract [en]

    Mufflers for internal combustion engines should be carefully designed. The main objective of a muffler is to reduce the engine noise while maintaining the back pressure below a certain limit. A specific target acoustic performance has to be met under space constraints and allowable engine back pressure limit. Usually, the insertion loss of the exhaust system is required to satisfy a certain target performance curve. The insertion loss is most appropriate to describe the exhaust system acoustic performance since it is dependent on the engine acoustic impedance, which varies with the engine loading and rotational speed. In this paper, a muffler optimization problem is formulated so that several shape parameters are optimized under some space constraints with flow. Any combination of linear space constraints can be imposed. The allowable engine back pressure is introduced as a non-linear constraint so that the optimum shape design will meet the engine back pressure specifications. The interior point optimization algorithm, which is available as a built-in MATLAB function "fmincon", is used in this paper. The formulated problem is applied to a real case study, where a truck exhau stsystem consists of a diesel engine, two mufflers, intermediate pipes, and a tailpipe. The first muffler is a typical EU-regulation compliant. The dimensions and location of the second muffler are to be optimized. A limit for the system back pressure is imposed by the engine manufacturer. An optimum design was investigated for different engine speeds and loadings. It was found that using the suggested formulation in this paper; one can obtain an applicable design of a muffler to meet both the acoustic regulations and the engine specifications.

  • 45.
    Elsaadany, Sara
    et al.
    Ain Shams University, Sound and Vibration Lab., Egypt.
    Elnady, Tamer
    Ain Shams University, Sound and Vibration Lab., Egypt.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Boij, Susann
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Insight into exhaust systems optimization techniques2010Conference paper (Other academic)
    Abstract [en]

    Exhaust system mufflers should be carefully designed for different applications. The main objective of an exhaust system is to reduce the engine noise. Maximum acoustic performance is usually desired under the limit of space constraints. Therefore obtaining the muffler optimum design is very crucial. In this paper, the muffler optimization problem is formulated allowing getting the optimum muffler design through calculating the acoustic properties conjugated with the optimization technique using a function "fmincon" from the MATLAB optimization tool-box that finds the minimum of a constrained nonlinear multivariable function. There are several possibilities to evaluate the acoustic performance of a muffler such as the sound transmission loss, the insertion loss, and the acoustic pressure measured by a receiver outside the exhaust system opening. By selecting one of these design targets, the optimum design of a specific muffler configuration in the frequency range of interest can be obtained. In this paper, a shape optimization approach is presented for different mufflers configurations, and the results of transmission loss, insertion loss, and the outside acoustic pressure are compared against optimum designs from the literature obtained using different optimization methods as well as design targets.

  • 46. Farooqui, Maaz
    et al.
    Elnady, Tamer
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Measurement of perforate impedance with grazing flow on both sides2016In: 22nd AIAA/CEAS Aeroacoustics Conference. American Institute of Aeronautics and Astronautics., American Institute of Aeronautics and Astronautics, 2016Conference paper (Refereed)
    Abstract [en]

    In this paper, the acoustic impedance of a microperforated plate (MPP) with two-sided grazing flow is educed by a modified semi-analytical inverse technique. The MPP sample is placed in the middle of a rectangular duct with grazing flow. The mode matching formulation is derived in terms of pressure for a MPP with two-sided grazing flow. The inputs to this mode matching technique are complex acoustic pressure measured at twelve positions at the wall of the duct, upstream and downstream of the MPP section. First the ability of the code to reproduce the pressure field for a given impedance is tested. Second the ability to educe the correct impedance for a given pressure distribution is tested. The results of the mode matching code are in good agreement with the simulations for no flow cases.

  • 47.
    Feng, Leping
    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.
    Orrenius, Ulf
    Engineering methods to predict noise levels at reference points with known source properties2015In: Applied Acoustics, ISSN 0003-682X, E-ISSN 1872-910X, Vol. 96, p. 68-74Article in journal (Refereed)
    Abstract [en]

    Two engineering methods are proposed to predict the sound pressure levels at a given point when the sound power level of a noise source is known and the transfer function between the source and the reference point can be obtained. The first method is applicable when the source is surrounded by many reflectors, or inside a box-like structure. A single monopole with average transfer function is suggested for this situation. For a source with a strong directivity placed in an essentially free space, the "box-source" method is recommended to take into account of the source directivity. The total sound power is in this case divided into five independent noise sources which are obtained via ordinary sound power measurement methods. Experimental verifications are made for several cases in laboratory. Satisfactory results are obtained for both methods.

  • 48.
    Glav, R
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Bodén, Hans
    KTH, Superseded Departments, Vehicle Engineering.
    Åbom, Mats
    KTH, Superseded Departments, Vehicle Engineering.
    An acoustic model for automobile catalytic converters1988Conference paper (Refereed)
  • 49.
    Glav, R
    et al.
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Åbom, M
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    A general formalism for analysing acoustic 2-port networks1997In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 202, no 5, p. 739-747Article in journal (Refereed)
  • 50. Glav, Ragnar
    et al.
    Costa, A.
    Åbom, Mats
    Application of the folded resonator2003In: Proceedings of the Tenth International Congress on Sound and Vibration, 2003, p. 3203-3210Conference paper (Refereed)
    Abstract [en]

    A folded side-branch resonator has been designed and tested on a heavy truck. The design procedure is based on numerical simulations of the complete exhaust line and cold flow measurements of resonator inlet resistance and end-correction. A reduction in exhaust noise of as much as 7dB relative to the standard exhaust system shows the potential of the suggested technique to handle low frequency problems without increased volume or backpressure.

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