A point collocation approach to modelling large dissipative silencers

A numerical matching technique known as point collocation is used to model mathematically large dissipative splitter silencers of a type commonly found in HVAC ducts. Transmission loss predictions obtained using point collocation are compared with exact analytic mode matching predictions in the absence of mean flow. Over the frequency range in which analytic mode matching predictions are available, excellent agreement with point collocation transmission loss predictions is observed for a range of large splitter silencers. The validity of using point collocation to tackle large dissipative silencers is established, as is the computational efficiency of the method and its suitability for tackling dissipative silencers of arbitrary, but axially uniform, cross sections

Transmission loss predictions for dissipative silencers of arbitrary cross section in the presence of mean flow

A numerical technique is developed for the analysis of dissipative silencers of arbitrary, but axially uniform, cross section. Mean gas flow is included in a central airway which is separated from a bulk reacting porous material by a concentric perforate screen. The analysis begins by employing the finite element method to extract the eigenvalues and associated eigenvectors for a silencer of infinite length. Point collocation is then used to match the expanded acoustic pressure and velocity fields in the silencer chamber to those in the inlet and outlet pipes. Transmission loss predictions are compared with experimental measurements taken for two automotive dissipative silencers with elliptical cross sections. Good agreement between prediction and experiment is observed both without mean flow and for a mean flow Mach number of 0.15. It is demonstrated also that the technique presented offers a considerable reduction in computational expenditure when compared to a three dimensional finite element analysis

Analysis of acoustic networks including cavities by means of a linear finite volume method

[EN] A procedure allowing for the analysis of complex acoustic networks, including three-dimensional cavities described in terms of zero-dimensional equivalent elements, is presented and validated. The procedure is based on the linearization of the finite volume method often used in gas-dynamics, which is translated into an acoustic network comprising multi-ports accounting for mass exchanges between the finite volumes, and equivalent 2-ports describing momentum exchange across the volume surfaces. The application of the concept to a one-dimensional case shows that it actually converges to the exact analytical solution when a sufficiently large number of volumes are considered. This has allowed the formulation of an objective criterion for the choice of a mesh providing results with a prefixed error up to a certain Helmholtz number, which has been generalized to three-dimensional cases. The procedure is then applied to simple but relevant three-dimensional geometries in the absence of a mean flow, showing good agreement with experimental and other computational results.This work has been partially supported by Ricardo Software, and by Ministerio de Ciencia e Innovacion through Grant DPI2009-14290. The authors thank Dr. F.D. Denia for his kind computational assistance.Torregrosa, AJ.; Broatch, A.; Gil, A.; Moreno Martínez, D. (2012). Analysis of acoustic networks including cavities by means of a linear finite volume method. Journal of Sound and Vibration. 331(20):4575-4586. https://doi.org/10.1016/j.jsv.2012.05.023S457545863312

The Flow Reversal Resonator

Copyright © 2007 SAE International The flow reversal chamber is a commonly used element in practical silencer design. To lower its fundamental eigenfrequency, it is suggested to acoustically short circuit the inlet and outlet duct. In the low frequency limit such a configuration will correspond to a Helmholtz resonator, but with a choked flow through the short circuit, the main flow will be forced through the expansion volume. For the proposed concept, the flow reversal resonator, a theoretical model is derived and presented together with transfer matrix simulations. The possible extension to a semi active device as well as the influence of mean flow on the system is investigated experimentally. Finally the concept is implemented on a truck silencer. The results indicate that the flow reversal resonator would prove an interesting complement to traditional side branch resonators. The attenuation bandwidth is broader and it can be packaged very efficiently. Mean flow effects are still an issue and should be studied further

Off-Peak Hours Deliveries: An Acoustic Perspective on the Stockholm Pilot Study

QC 20190918</p

Off-Peak Hours Deliveries: An Acoustic Perspective on the Stockholm Pilot Study

QC 20190918</p

Off-Peak Hours Deliveries: An Acoustic Perspective on the Stockholm Pilot Study

QC 20190918</p

On variation of absorption factor due to measurement method and correction factors for conversion between methods

Sound absorbing materials are used in many applications to reduce sound, and their soundabsorbing characteristics are most often determined experimentally since theoreticaldetermination is difficult. Sound absorption factors are used in material specifications aswell as input to numerical simulations.Several methods for experimental determination of the absorption factor exist, two of themstandardized and frequently used. It is commonly known that the absorption factorobtained by these two methods differs as different sound fields are prescribed by thestandards. However, the size of the differences has not been so well described. Due to thisdifference, the choice of method is critical in order to avoid errors in simulations andspecifications of material properties.Experimental determination of absorption factors for three commonly used absorbers wasperformed, resulting in significant differences between the two methods. Correction factorsto compensate the absorption factor determined at one acoustic state and used in anotherare given. Theory verifying the differences is also presented.References: Färm, A., Boij, S., Glav, R., On sound absorbing characteristics and suitable measurement methods (2012) Proceedings of the 7 International Styrian Noise, Vibration and Harshness Congress; (2003) Acoustics - Measurement of Sound Absorption in a Reverberation Room, , ISO 354; (1996) Acoustics - Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes - Part 1: Method Using Standing Wave Ratio, , International standard ISO 10534-1; Delany, M.E., Bazley, E.N., Acoustic properties of fibrous absorbent materials (1970) Applied Acoustics, 3, pp. 105-116; Corcos, G.M., The structure of the turbulent pressure field in boundary layer flows J. of Fluid Mechanics, 18, p. 1964; Biot, M.A., Generalized theory of acoustic propagation in porous dissipative media (1962) Journal of the Acoustical Society of America, 34 (9), pp. 1254-1264; Attenborough, K., Acoustical characterization of porous materials (1982) Physics Reports, 82 (3), pp. 179-227; Sastry, J.S., Munjal, M.L., A transfer matrix approach for evaluation of the response of a multi-layer infinite plate to a two-dimensional pressure excitation (1995) Journal of Sound and Vibration, 182 (1), pp. 109-128QC 20130913</p

Mihai Retegan,Război politic în blocul comunist. Relaţii româno-sovietice în anii şaizeci. Documente, I Bucureşti: ,2002 Mihai Retegan,Război politic în blocul comunist. Relaţii româno-sovietice în anii şaizeci. Documente, II Bucureşti: ,2004

QC 20210824</p