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On Acoustic Multi-Port Characterisation Including Higher Order Modes
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.
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.ORCID iD: 0000-0001-7898-8643
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aerodynamics.ORCID iD: 0000-0002-9061-4174
2016 (English)In: Acta Acoustica united with Acustica, ISSN 1610-1928, E-ISSN 1861-9959, Vol. 192, no 5, 834-850 p.Article 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.

Place, publisher, year, edition, pages
2016. Vol. 192, no 5, 834-850 p.
Keyword [en]
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: urn:nbn:se:kth:diva-192482DOI: 10.3813/AAA.918998Scopus ID: 2-s2.0-84987847639OAI: oai:DiVA.org:kth-192482DiVA: diva2:968884
Projects
IDEAL VENT
Note

QC 20160916

Available from: 2016-09-13 Created: 2016-09-13 Last updated: 2017-11-21Bibliographically 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. 55 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 30
Keyword
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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Sack, StefanÅbom, MatsEfraimsson, Gunilla
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