Modal Decomposition on Sound Propagation in Ducts with and without Flow Aerospace Engineering

108 Pages·2014·2.22 MB·English

Modal Decomposition on Sound Propagation
in Ducts with and without Flow
João Luís Aguiar Oliveira Rosa
Thesis to obtain the Master of Science Degree in
Aerospace Engineering
Supervisors
Prof. Fernando José Parracho Lau
Prof. Christophe Schram
Examination Committee
Chairperson: Prof. Filipe Szolnoky Ramos Pinto Cunha
Supervisor: Prof. Fernando José Parracho Lau
Member of the Committee: Prof. João Manuel Gonçalves de Sousa Oliveira
September 2014
ii
“Whenever you are asked if you can do a job, tell ’em, ’Certainly I can!’
Then get busy and find out how to do it.”
- Theodore Roosevelt
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Acknowledgements
Throughout the time I spent in Belgium at VKI many people contributed to the accomplishment which
this thesis represents. Some of them directly involved in IDEALVENT and its test campaigns or theoretical
background, others simply with their company and friendship.
To these I pay my respects, taking also the opportunity to apologize to those mistakenly left out.
Christophe Schram and Korcan Kucukcoskun, for their help and kind support. Korcan’s exigence, always
tempered with patience, contributed to the best in this thesis.
To Fernando Lau, my supervisor in Técnico Lisboa in Lisbon, Portugal, I must thank for his readiness in
accepting a project even if at such a distance.
Julien Christophe, for his excellent data recording system, without which IDEALVENT measurements would
have taken probably ten times longer to finish; Ugur Karban, who made possible that all data processing was
done on schedule; and Stefan Sack, for his always valuable opinions regarding the best mathematical approach
to follow.
To my fellow interns and friends at VKI, Jose Torres, Gaylord Durand, Alberto Serrano, Valeria Andreoli,
Jorge Ramos, Anna Bru and Artur Carvalho. Their friendly and joyful spirit made all my working time better.
And also the Master’s students who kept me the most enjoyable of companies, Jorge Saavedra, Pablo Solano,
Andreu Molina, Pedro Carrascal, and my good ol’ friend David Cuadrado, an example in character and hard
work.
To Aude Lahalle, who although I only met for such a short time earned a place on this page.
And finally my parents and sister, for all the help and support to their absent son and brother.
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Abstract [English]
With the increasing number of airline passengers every year the reduction of noise generated by aircraft
Environmenal Control Systems (ECS) has become an important issue to tackle with more research being
devoted to it, mainly in what concerns duct acoustics and fan-generated noise.
Acoustic tests were carried out to describe the scattering matrix of two different kinds of duct terminations:
an experimental anechoic termination and a horn-shaped flow inlet.
The methodology previously outlined for the study of two-port acoustic sources allowed the characterization
of the modal scattering matrix for different obstacles, an ECS fan and a diaphragm, with the determination of
transmission and reflection coefficients for each side with and without in-duct flow.
It was concluded that flow alters reflection mechanisms at duct terminations, decreasing direct reflections
and increasing convertive reflections both upstream and downstream. For two-port sources flow increased
transmission factors on the upstream side at the same time it decreased reflections downstream.
Activepartmeasurementsallowedtoidentifyhowadiaphragminthepresenceofflowgeneratesflow-induced
noise.
The passive part of the ECS fan showed that the spinning induced flow favored upstream transmission
factors for modes with the same spinning direction of the fan and suppressed its downstream counterparts.
Fan-generated noise proved to be dominant over transmitted and reflected noise.
This work constituted the first time such a complex modal decomposition was carried out at VKI.
Keywords: Duct Acoustics, Modal Decomposition, Two Microphones Method, Two-Port Acoustic Sources
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Resumo [Português]
Com o crescente número de passageiros em viagens aéreas a redução do ruído produzido pelo Sistema de
Controlo de Climatização (ECS) de cada aeronave tornou-se um aspecto importante sobre o qual inside cada
vez mais investigação, nomeadamente realcionada com acústica de condutas.
Ensaios acústicos foram levados a cabo a fim de descrever a matriz de disperção modal de dois tipos
diferentes de extremidades de condutas: uma terminação anecóica experimental e um inlet de secção variável
em corneta.
Metodologias anteriormente descritas para o estudo de fontes acústicas de dois terminais permitiram ainda
caracterizar a matrix de dispersão modal para diferentes obstáculos: uma ventoinha original de um ECS e um
diafragma, determinando os coeficientes de transmissão e reflexão para cada secção com e sem escoamento.
Concluiu-se que o escoamento altera os mecanismos de reflexão nas extremidade de conductas, diminuindo
reflexões directas e favorecendo reflecões conversivas tanto a jusante como a montante. Para fontes de dois
terminais fez aumentar os factores de transmissão a montante ao mesmo tempo que o diminuiu os coeficientes
de reflexão a jusante.
Medições da componente activa permitiram identificar como o diafragma na presença de escoamento gera
ruído por este induzido.
A componente passiva da ventoínha mostrou que o escoamento induzido, por possuir velocidade rotacional,
tendeafavorecerosfactoresdetransmissãoamontanteparamodosrodandonamesmadirecçãodaventoínhae
minimizar os seus equivalentes a jusante. O ruído directamente proveniente da ventoínha provou ser dominante
sobre as suas reflexões e restantes transmissões.
Este trabalho contituiu a primeira vez que uma decomposição modal desta complexidade foi levada a cabo
no VKI.
Palavras-Chave: AcústicadeCondutas,DecomposiçãoModal,Métododedoismicrofones,Fontesacústicas
de dois terminais
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Table of Contents
Acknowledgements v
Abstract vii
Table of Contents xi
List of Figures xiv
List of Tables xv
List of Symbols xvii
1 Introduction 1
1.1 Cabin Environment and Comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Motivation - Environmental Control Systems (ECS) . . . . . . . . . . . . . . . . . . . . 3
1.1.3 IDEALVENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Duct Acoustics and The Two-Microphone Method - A Literature Review . . . . . . . . . . . . 6
1.3 Aim of the Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2 Theoretical Background 9
2.1 Description of the Sound Field. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.1 General Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.2 Modal Decomposition in Cylindrical ducts. . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.3 Modes and Cut On Frequencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2 Propagation Modes in Cylindrical Ducts with Axial Mean Flow . . . . . . . . . . . . . . . . . . 16
3 Experimental Methods 19
3.1 The Two Microphones Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1.1 The Two Microphones Method - One-Port Analysis (Terminations) . . . . . . . . . . . 20
3.1.2 The Two Microphones Method - Two-Port Analysis . . . . . . . . . . . . . . . . . . . . 22
3.2 Expanding The Two Microphones Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.1 Expanded analysis on terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
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x Table of Contents
3.2.2 Expanded analysis on two-port sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 Experimental Over-Determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3.1 Experimental Over-Determination for one-port sources . . . . . . . . . . . . . . . . . . 34
3.3.2 Experimental Over-Determination for two-port sources . . . . . . . . . . . . . . . . . . 35
3.4 Including Radial Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.4.1 Radial Modes for terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.4.2 Radial Modes for two-port sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.5 Transfer Functions and Flow Noise Suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.5.1 Transfer Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.5.2 Transfer Functions’ effect on Flow Noise Suppression . . . . . . . . . . . . . . . . . . . 39
3.6 Coherence and Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.6.1 Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.6.2 Coherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4 Facilities and Installation 43
4.1 Installation at VKI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.2 Microphone Placements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5 Reflection Matrices for Terminations: Results 47
5.1 No Flow situation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.1.1 Anechoic Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.1.2 Inlet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.1.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5.2 Flow Speed=10 m/s (M=0.0294) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.2.1 Anechoic Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.2.2 Inlet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.2.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.3 Flowspeed=20 m/s (M=0.0588) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.3.1 Anechoic Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3.2 Inlet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
6 Two-Port Analysis: Results 69
6.1 Validation case: empty duct with no flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.2 Diaphragm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.2.1 Without Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.2.2 With Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.2.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.3 Operating ECS Fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6.3.1 Passive part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77