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Experimental and Numerical Multi-port Eduction for Duct Acoustics
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
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 [en]
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: urn:nbn:se:kth:diva-207475ISBN: 978-91-7729-402-3 (print)OAI: oai:DiVA.org:kth-207475DiVA, id: diva2:1096973
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
List of papers
1. On Acoustic Multi-Port Characterisation Including Higher Order Modes
Open this publication in new window or tab >>On Acoustic Multi-Port Characterisation Including Higher Order Modes
2016 (English)In: Acta Acoustica united with Acustica, ISSN 1610-1928, E-ISSN 1861-9959, Vol. 192, no 5, p. 834-850Article in journal (Refereed) Published
Abstract [en]

Methods to design test-procedures for acoustic multi-ports in ducts with a focus on pressure sampling positions for accurate modal decomposition are demonstrated. Acoustic fields up- and downstream of an in-duct acoustic element are excited by external sources and decomposed into transmitted and reflected aeroacoustic modal pres- sure amplitudes in order to first determine the acoustic scattering of the element. Secondly, the determination of the element source strength requires tests with no external sources, but with known terminations and scattering data. Unfavourable source and sensor positions lead to mode coupling and to ill-conditioned or even singular decomposition matrices, which results in high amplifications of uncertainties within the wave decomposition. An unoptimised but over-determined assembly is compared with a setup containing a minimum of sensors but with optimised positions. Lower uncertainty amplification, despite the usage of fewer sensors, is achieved for most frequencies, especially after the cut-on of t he higher order acoustic modes. A genetic algorithm (GA) is used to achieve this optimised setup by minimising the condition number of the decomposition matrix, which is a multi-dimensional optimisation problem with numerous local minima. To estimate the stability of the optimised configuration, a Monte-Carlo Method (MCM) is deployed to introduce normal distributed complex pressure un- certainties into the decomposition. In order to estimate the wave number, different approaches are compared - namely the classical non-dissipative wav e number estimate, an extended Kirchhoff method for viscous-thermal damping and an eigenvalue solution of the Linearised Navier Stokes Equations by Dokumaci. The presented de- composition method is not only applicable to measurement data but is equally useful to post-process results from numerical computation.

Keywords
multi-port, acoustic, acoustic duct modes, aeroacoustic, duct-acoustic, optimization, Monte Carlo, Genetic Algorithm, acoustic Measurements, orifice flow
National Category
Mechanical Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-192482 (URN)10.3813/AAA.918998 (DOI)2-s2.0-84987847639 (Scopus ID)
Projects
IDEAL VENT
Note

QC 20160916

Available from: 2016-09-13 Created: 2016-09-13 Last updated: 2017-11-21Bibliographically approved
2. Investigation of orifice aeroacoustics by means of multi-port methods
Open this publication in new window or tab >>Investigation of orifice aeroacoustics by means of multi-port methods
2017 (English)In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 407, p. 32-45Article in journal (Refereed) Published
Abstract [en]

Comprehensive methods to cascade active multi-ports, e.g., for acoustic network prediction, have until now only been available for plane waves. This paper presents procedures to combine multi-ports with an arbitrary number of considered duct modes. A multi-port method is used to extract complex mode amplitudes from experimental data of single and tandem in-duct orifice plates for Helmholtz numbers up to around 4 and, hence, beyond the cut-on of several higher order modes. The theory of connecting single multi-ports to linear cascades is derived for the passive properties (the scattering of the system) and the active properties (the source cross-spectrum matrix of the system). One scope of this paper is to investigate the influence of the hydrodynamic near field on the accuracy of both the passive and the active predictions in multi-port cascades. The scattering and the source cross-spectrum matrix of tandem orifice configurations is measured for three cases, namely, with a distance between the plates of 10 duct diameter, for which the downstream orifice is outside the jet of the upstream orifice, 4 duct diameter, and 2 duct diameter (both inside the jet). The results are compared with predictions from single orifice measurements. It is shown that the scattering is only sensitive to disturbed inflow in certain frequency ranges where coupling between the flow and sound field exists, whereas the source cross-spectrum matrix is very sensitive to disturbed inflow for all frequencies. An important part of the analysis is based on an eigenvalue analysis of the scattering matrix and the source cross-spectrum matrix to evaluate the potential of sound amplification and dominant source mechanisms.

Place, publisher, year, edition, pages
Academic Press, 2017
Keywords
multi-ports, network predictions, orifice, acoustics, aeroacoustics
National Category
Fluid Mechanics and Acoustics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-207473 (URN)10.1016/j.jsv.2017.06.026 (DOI)2-s2.0-85025098210 (Scopus ID)
Projects
IdealVent
Funder
EU, European Research Council, 314066
Note

QC 20170522

Available from: 2017-05-20 Created: 2017-05-20 Last updated: 2017-08-21Bibliographically approved
3. Numerical Eduction of Active Multi-port Data for In-duct Obstructions
Open this publication in new window or tab >>Numerical Eduction of Active Multi-port Data for In-duct Obstructions
2016 (English)In: Article in journal (Refereed) Submitted
Keywords
multi-port, numerics, orifice, IDDES, LNSE
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-207474 (URN)
Projects
IdealVent
Note

QC 20170522

Available from: 2017-05-20 Created: 2017-05-20 Last updated: 2017-11-07Bibliographically approved
4. Modal Filters for Mitigation of In-duct Sound
Open this publication in new window or tab >>Modal Filters for Mitigation of In-duct Sound
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
Keywords
multi-ports, silencer, acoustics, acoustic filter
National Category
Engineering and Technology
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-207472 (URN)10.1121/2.0000473 (DOI)2-s2.0-85044213978 (Scopus ID)
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

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