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Modal Filters for Mitigation of In-duct Sound
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.ORCID iD: 0000-0001-7898-8643
2016 (English)In: Proceedings of Meetings on Acoustics, American Institute of Physics (AIP), 2016, Vol. 29, no 040004, article id 040004Conference paper, Published paper (Refereed)
Abstract [en]

Investigations on noise mitigation strategies for duct systems with a low number of propagating modes, as found in middle size HVAC systems, are sparse. Here, the design of a silencer is presented based on cascades of modal filters and a Cremer silencer. The modal filter aims at mitigating spinning duct modes. The Cremer silencer is a silencer that realizes impedance for optimal attenuation for the plane wave at one frequency (Cremer impedance). The combination of both can create high damping both for planar and non-planar waves. The modal filter consists of micro-perforated plates parallel to the duct axis in mean flow direction. As the concept of modal filters is unprecedented, general investigations of its acoustic behaviour are carried out by solving the Helmholtz equation numerically to investigate the effect of different plate positions, the number of plates and the plate impedance on the acoustic absorption. Finally, the combination of the Cremer silencer and the modal filter is investigated by performing measurements on prototypes. It is shown, that even though the damping of the modal filter is low compared to the Cremer silencer, improved damping and dissipation of fields consisting of plane waves and spinning modes can be achieved by combining the two types.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016. Vol. 29, no 040004, article id 040004
Keywords [en]
multi-ports, silencer, acoustics, acoustic filter
National Category
Engineering and Technology
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-207472DOI: 10.1121/2.0000473Scopus ID: 2-s2.0-85044213978OAI: oai:DiVA.org:kth-207472DiVA, id: diva2:1096968
Conference
172nd Meeting of the Acoustical Society of America
Projects
IdealVent
Note

QC 20170522

Available from: 2017-05-20 Created: 2017-05-20 Last updated: 2018-05-17Bibliographically approved
In thesis
1. Experimental and Numerical Multi-port Eduction for Duct Acoustics
Open this publication in new window or tab >>Experimental and Numerical Multi-port Eduction for Duct Acoustics
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sound generation and propagation in circular ducts for frequencies beyond the cut-on frequencies of several higher order acoustic modes is investigated. To achieve this, experimental and numerical set-ups are designed and used to research aeroacoustic interactions between in-duct components and to conceive noise mitigation strategies.

Describing in-duct sound for frequencies with a moderate number of propagating modes is important, for example, for improving the noise emission from mid-size ventilation systems. Challenges that are largely unacknowledged in the literature involve efficient test rig design, quantification of limits in the methods, numerical modelling, and development of effective noise mitigation strategies for higher order modes.

In this thesis, in-duct sound is mapped on a set of propagating pressure eigenmodes to describe aeroacoustic components as multi-ports with sound scattering (passive properties) and a source strength (active properties). The presented analysis includes genetic algorithms and Monte Carlo Methods for test rig enhancement and evaluation, multi-port network predictions to identify model limitations, and scale resolving (IDDES) and Linearized Navier Stokes computations for numerical multi-port eduction and the silencer design.

It is first shown that test rig optimization improves the quality of multi-port data significantly. Subsequently, measurements on orifice plates are used to test the network prediction model. The model works with high accuracy for two components that are sufficiently separated. For small separations, strong coupling effects are observed for the source strength but not for the scattering of sound. The measurements are used for numerical validation, which gives reliable results for coupled and uncoupled systems. The total acoustic power of tandem orifices is predicted with less than 2 dB deviation and the passive properties for most frequencies with less than 5 % difference from the measurement. The numerical (FEM) models are also used to design a completely integrated silencer for spinning modes that is based on micro-perforated plates and gives broadband attenuation of 3-6 dB per duct diameter silencer length.

The multi-port method is a powerful tool when describing aerodynamically decoupled in-duct components in the low- to mid-frequency range. Due to a robust passive network prediction, multi-port methods are particular interesting for the design of silencer stages. Furthermore, the demonstrated applicability to numerical data opens novel application areas.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. p. 55
Series
TRITA-AVE, ISSN 1651-7660 ; 30
Keywords
acoustics, aeroacoustics, flowacoustics, multi-ports, orifice, silencer, experiment, numeric, IDDES, LNSE, optimization, test rig, Monte Carlo
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-207475 (URN)978-91-7729-402-3 (ISBN)
Public defence
2017-05-29, F3, Lindstedtsvägen 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Projects
IdealVent
Note

QC 20170522

Available from: 2017-05-22 Created: 2017-05-20 Last updated: 2017-05-22Bibliographically approved

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