Modelling and Characterization of Perforates in Lined Ducts ...kth.diva- 9676/ Lined Ducts

download Modelling and Characterization of Perforates in Lined Ducts ...kth.diva- 9676/  Lined Ducts

of 40

  • date post

    11-Jul-2021
  • Category

    Documents

  • view

    0
  • download

    0

Embed Size (px)

Transcript of Modelling and Characterization of Perforates in Lined Ducts ...kth.diva- 9676/ Lined Ducts

Microsoft Word - Introduction.docModelling and Characterization of Perforates in Lined Ducts and Mufflers
Tamer Elnady
Stockholm 2004
Doctoral Thesis The Marcus Wallenberg Laboratory for Sound and Vibration Research
Department of Aeronautical and Vehicle Engineering The Royal Institute of Technology (KTH)
Akademiska avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framläggs till offentlig granskning för avläggande av teknologie doktorexamen den 14 september 2004, kl 10:00 i Kollegiesalen, Valhallavägen 79, KTH, Stockholm. Fakultetsopponent är Dr. Yves Aurégan, Laboratoire d’Acoustique de l’Université du Maine, Le Mans, France. Avhandlingen försvaras på engelska.
2
3
Abstract Increased national and international travel over the last decades has caused an increase in the global number of passengers using different means of transportation. Great effort is being directed to improving the noisy environment in the residential community. This is to face the growing strict noise requirements which are implemented by international noise regulatory authorities, governments, and local airports. There is also a strong competition between different manufacturers to make their products quieter. The propulsion system in an aircraft is the major source of noise during relevant flight conditions. The engine noise in a vehicle dominates the total radiated noise at low speeds especially inside cities. Many recent studies on noise reduction involve the use of perforated plates in the air and gas flow ducting connected to the engine. This thesis deals with the modelling of perforates as an absorbent.
There are many difficulties in using liners in these applications. The most important is that there is no large surface area to which the linings may be applied. Equally, the environment in which linings have to survive is hostile. Therefore, liners have to be carefully tailored in order to achieve the most efficient attenuation. The full-scale simulation testing, which is usually necessary to define the noise attenuation produced by a liner installation, is both time-consuming and expensive. Therefore, a need for accurate models is a must. This thesis fills some gaps in the impedance modelling of perforated liners. It also concentrates on those complicated situations of sound propagation in ducts that were solved earlier using Finite Element Methods. Alternate analytical solutions to these problems are developed here, which gives more physical insight into the results.
The key design parameter of perforates is the acoustic impedance. The impedance is what determines their efficiency to absorb sound waves. A semi empirical impedance model was developed to be capable of accurately predicting the liner impedance as a function of its physical properties and the surrounding conditions. It was compared to all previous models in the literature. Nothing in the literature has been reported on the effect of temperature on the perforate impedance, therefore a complete study was performed. A new inverse analytical impedance measurement technique was proposed. It is based on educing the impedance value based on the measurement of the attenuation across a lined duct section. Two applications were further considered: The effect of hard strips in lined ducts on there attenuation properties; and the modelling of perforations in a complicated automotive muffler system.
Keywords: Perforates – Liners – Acoustic impedance – Hot stream liners – Hard splices – Mufflers – Lined ducts – Collocation – Flow duct.
4
This thesis consists of the following papers:
Paper I Application of the Point Matching Method to Model Hard Strips in Lined Ducts.
Paper II Investigation of Mode Scattering by Hard Strips in Lined Ducts Using Collocation.
Paper III On the Modelling of the Acoustic Impedance of Perforates with Flow.
Paper IV An Inverse Analytical Method for Extracting Liner Impedance from Pressure Measurements.
Paper V Impedance of SDOF Perforated Liners at High Temperatures.
Paper VI On Acoustic Network Models for Perforated Tube Mufflers and the Effect of Different Coupling Conditions.
Division of work between authors:
The theoretical and experimental work presented in this thesis was done by Tamer Elnady under the supervision of Hans Bodén. Paper VI was supervised by Mats Åbom. In Paper V, Timo Kontio, an M.Sc. student at the department, participated in the measurements with the help of Tamer Elnady and supervision of Hans Bodén.
5
The content of this thesis has been presented in the following conferences:
1. 7th AIAA/CEAS Aeroacoustics Conference, Maastricht, The Netherlands, 28-30 May, 2001. AIAA 2001-2202: “Application of the Point Matching Method to Model Circumferentially Segmented Non-Locally Reacting Liners”.
2. 8th AIAA/CEAS Aeroacoustics Conference, Breckenridge, CO, USA, 17-19 May, 2002. AIAA 2002-2444: “Hard Strips in Lined Ducts”.
3. 9th AIAA/CEAS Aeroacoustics Conference, Hilton Head, SC, USA, 12-14 May, 2003. AIAA 2003-3304: “On Semi-Empirical Liner Impedance Modelling with Grazing Flow”.
4. 10th AIAA/CEAS Aeroacoustics Conference, Manchester, UK, 10-12 May, 2004. AIAA 2004-2836: “An Inverse Analytical Method for Extracting Liner Impedance from Pressure Measurements”. And AIAA 2004-2842: “Impedance of SDOF Perforated Liners at High Temperatures”.
5. Sixth International Conference on Production Engineering and Design for Development (PEDD-6), Cairo, Egypt, 12-14 Feb., 2002. “Impedance Modelling of Acoustic Liners”.
6. 10th International Congress on Sound and Vibration (ICSV10), Stockholm, Sweden, 7-10 July, 2003. “Mode Scattering by Hard Strips in Lined ducts”.
The work in this thesis was performed within the following Projects:
1. DUCAT: funded by EC (European Commission), contract number BRPR-CT-97- 0479. (Paper I)
2. Aeroacoustics: funded by NFFP (National Flight Research Program), project number P460. (Papers II to V)
3. ARTEMIS: funded by EC (European Commission), contract number G3RD-CT- 2001-0511. (Paper IV)
6
Acknowledgements If I want to write a detailed acknowledgement, I will have a long list to thank. But, the top of this list is always booked to the two persons who were the reason that I am writing these lines now, and those to whom I relate any success I achieve, my Parents. They say that I behave like my father and I hope that I can achieve half of what he did. My mother is always there for me as a friend more than a mother. Now, two other people have jumped to the top of my priorities, my wife Eman and my son Yusuf. Eman has smoothly joined my life giving it strength and encouragement to always be the best. Yusuf added joy and happiness to the life of all the family with his charming smile. I dedicate this thesis and all my future work to both of them.
In historical order, I must thank all my teachers at school and at Ain Shams University in Cairo. On the other hand, it is a must to mention Dr. Ahmed Hussein and Dr. Mansour Elbardisi. The first contributed a lot to my engineering sense and the second introduced me to the field of Acoustics.
The person who invited me to Sweden and welcomed me at the airport is my supervisor, Hans Bodén. Since then, he has been helping me in almost everything. He is always there for me whenever I need him and it usually takes no more than ten minutes of discussion with him until I find the answer to my question or the solution to my problem. For me, he is the kind of supervisor that anyone would like to work with. I would also like to thank Mats Åbom, the head of MWL, for helping me throughout the thesis especially with Paper VI.
I really enjoyed working in the friendly atmosphere of the Department of Vehicle Engineering and I wish to thank all its members for that, especially Anders Nilsson. I learned from him many things I will need when I become a Head of a Department. I wish I could name all of them, but I just say:
Tack, Thanks, Merci, Gracias, Grazie, Danke, Kiitus,
Financial support from NFFP as well as European funded projects DUCAT and ARTEMIS is gratefully acknowledged. Last but not least, I must thank my Egyptian friends in Stockholm, Hussein, Magdy, and Sabry who reduced the feeling of homesickness.
7
A. Paper I: Application of the Point Matching Method to Model Hard Strips in Lined Ducts..........................................................................................................................23
B. Paper II: Investigation of Mode Scattering by Hard Strips in Lined Ducts using Collocation ................................................................................................................24
C. Paper III: On the Modelling of the Acoustic Impedance of Perforates with Flow ....26 D. Paper IV: An Inverse Analytical Method for Extracting Liner Impedance from
Pressure Measurements .............................................................................................29 E. Paper V: Impedance of SDOF Perforated liners at High Temperatures ....................31 F. Paper VI: On acoustic network models for perforated tube mufflers and the effect of
different coupling conditions.....................................................................................33 VII. Recommendations for Future Research ......................................................................35 References ..........................................................................................................................35
8
9
I. Introduction The attenuation of sound propagating in fluids contained in ducts can be due to a variety of causes. The least significant of these for gases, normally, is the “volume absorption” due to irreversible conversion of acoustic energy into heat in the fluid itself, through viscous and thermal diffusion processes. More prominent is the absorption at the wall. This can be due either to motion of the wall and the resulting transmission of sound through it, or to viscous and thermal diffusion processes at the wall. For relatively rigid walls of fairly high thermal conductivity relative to that of the fluid, these processes are more effective at the wall, in producing acoustic attenuation, than in the fluid itself. In general, the attenuation is dependent on frequency and on the degree of complexity of the individual amplitude patterns of the duct modes by which the sound is being transmitted. Presence of a mean flow modifies the attenuation both by convection effects and by shear layer effects. It can also change the acoustic properties of the wall.
A sound absorbing material is placed along a flow passage to attenuate noise generated by some upstream device, i.e. a method for passive noise control. It is usually attached to a hard surface to attenuate noise by converting sound energy into heat through viscous and thermal diffusion processes at the walls.
Liner effectiveness is a function of several parameters; some have to do with the configuration of the liner itself and others with the environment in which it has to operate. The full-scale simulation testing, which is usually necessary to define the noise attenuation produced by a liner installation, is both expensive and time consuming. Therefore, it is desirable to produce mathematical models which are capable of predicting liner performance inside the system. The key design parameter is the acoustic impedance of the treatment panel. The impedance is used as input for propagation models to predict overall attenuation inside the engine duct.
A typical liner structure consists of a perforated face-sheet covering a honeycomb or porous material, and having a solid back-plate (Fig. 1). Acoustic liners are of two types namely locally and non-locally reacting. These are defined below.
Locally reacting liners do not allow sound propagation inside the liner, parallel to the liner surface. The face-sheet is backed by a regular partitioned single-layer cellular structure (such as metal honeycomb (Fig. 2)) with solid walls perpendicular to the face-sheet plate. Such a design is called a single degree-of-freedom liner (SDOF). The locally reacting liner can be considered as an array of Helmholtz resonators, whose theory of operation is analogous to that of the mechanical mass-spring system. The mass of the gas oscillating in the aperture corresponds to a mechanical mass sliding over a resistive surface and the compressibility of the gas in the resonator cavity acts as the spring in the mechanical system. Non-locally reacting liners allow the sound waves to propagate within the liner material (Fig. 1). They are often called bulk absorbers. They usually have a single-layer construction in which fibrous material fills the panel between the porous face-sheet and the back-plate. A typical example of this material is the glass wool that is used, for instance, inside anechoic rooms.
The SDOF liner is effective over the narrowest range of frequencies (one octave) and must be tuned to the frequency band containing the single tone of greatest concern. A multiple- degree-of-freedom (MDOF) liner has a wider bandwidth that can cover the main tone and its next two harmonics (about two octaves), using various internal depths and several
10
layers with different resistance, but it is more difficult to manufacture. Bulk absorbers have the widest bandwidth, extending over three octaves if the panel is made sufficiently deep to be effective at the lowest frequency.
Fig. 1 Types of acoustic liners.
Fig. 2 Honeycomb structure of SDOF locally reacting liner
Perforates are used to attenuate sound in ducts in many applications. The applications considered in this thesis are related to the reduction of transportation noise, either due to aircraft or vehicles. Increased national and international travel over the last decades has caused an increase in the global number of passengers using different means of transportation. Great effort is being directed to improving the noisy environment in the residential community. There has been a significant progress during the last forty years concerning the overall vehicle and aircraft noise reduction; but the noise level and annoyance for people living in residential areas and in the vicinity of airports is constantly increasing. The appearance of low-cost airlines helped increasing air traffic in small airports that compete with low airport taxes and more relaxed regulations. Residents around airports and close to highways blame aircraft and vehicle noise for the appearance of sleeping disorders and other noise related diseases such as hypertension, fatigue, and loss of concentration.
Stricter noise requirements are being implemented by international noise regulatory authorities and governments. In order to achieve long-term improvements, a need for quieter means of transportation has evolved. The propulsion system in an aircraft is the major source of noise during relevant flight conditions. The engine noise in a vehicle dominates the total radiated noise at low speeds especially inside cities. Many recent studies on noise reduction involve the use of perforated plates in the air and gas flow ducting connected to the engine.
One of the main applications of using liners is inside aircraft jet engines. Forty years ago, the engines were of the turbojet type where jet noise dominated. Recently, turbofans, having large bypass ratios, are commonly used in modern aircrafts. In a turbofan engine, a large-diameter fan spins at a high speed and fan noise dominates all other noise sources in
Perforated face-sheet Partitions (e.g. honeycomb)
Porous material Solid back-plate
11
the engine. Fig. 3 shows the sources of noise and the share of each source in the total radiated noise. The engine is wrapped in a nacelle and fan noise has two paths to propagate; the inlet and bypass ducts. The ducts have acoustic treatment to damp and absorb noise generated by the fan. Sound, which is not absorbed, propagates out the front and back of the nacelle and can be heard at the ground. In addition, liners are used inside the exhaust duct to minimize turbine and combustor noise.
(a) Turbojet (low bypass ratio engine). (b) Turbofan (high bypass ratio engine).
Fig. 3 The percentage of each noise source in different types of engines 1.
The liners in service inside aircraft engines are of the locally reacting type. Bulk absorber treatment has not been used in aircraft engines in commercial service because of structural design difficulties. A liner with the fibrous form will be faced with a high-porosity screen to hold the fibres in place; however, because of its high porosity, it allows the material it contains to absorb and retain the fluid. Bulk absorbers cannot be used either in the cold ducts (where freeze/thaw action on retained water is the problem) or in the hot ducts (where combustible fluid retention is a fire hazard). Aircraft manufacturers are interested to use porous ceramic materials or metallic foams in liners to obtain better performance. This will be the main trend for future liner research.
Fig. 4 The fan of an aircraft engine (Courtesy of Airbus).
12
Other applications of perforates are inside car mufflers to reduce the engine noise, inside combustion chambers of rockets or gas turbines to control combustion instabilities, and inside ventilation ducts to attenuate flow induced noise.
II. Acoustic Propagation in Ducts Although much success has been achieved in understanding and optimizing the attenuation of sound waves in acoustically lined ducts, there still remain various areas that require further investigation. In particular the effects of a circumferential variation in wall admittance on the propagation and radiation of sound from ducts has received relatively little attention. Although the lined wall may have been designed to be uniform, a variation in admittance may result from the mounting techniques, which can make the existence of hard strips inevitable, to hold the liners in place. On the other hand, it may be possible that by a deliberate choice of a non-uniform liner, an increase in attenuation can be achieved.
Fig. 5 The inlet nacelle being mounted in front of the fan (Courtesy of Boeing).
The segmented liner problem (circumferential or axial variation of liner impedance) was first addressed in the seventies. A mathematical model was developed for rectangular ducts with uniform mean flow 2. The aim was to define methods for obtaining improved multi- segment lining performance by taking advantage of relative placement of each lining segment. Properly phased liner segments reflected and spatially redistributed the incident acoustic energy and thus provided additional attenuation. Optimal configurations were presented to improve the attenuation rate inside infinite ducts.
Watson 3 developed Finite Element Methods to analyze sound attenuation in ducts with a peripherally variable liner. For a finite duct with no flow, he showed that the attenuation rate near the frequency of maximum attenuation drops significantly if a small portion of a peripheral liner is removed.
From the in-flight circumferential modal spectra of the Rolls-Royce Tay 650 engine mounted on the Fokker 100 aircraft, Sarin and Rademaker 4 found that the sound field propagating upstream in the inlet is strongly modulated by hard-walled strips in the lined area, the non-cylindrical geometry of the duct and the non-axisymmetric flow velocity distribution. In order to study the effect of the modulation of the acoustic field by the hard- walled strips separately, an experimental program in the NLR spinning mode synthesizer
13
was carried out 5. The m-mode spectra of the transmitted field clearly showed modulation effects caused by the hard-walled strips and the modulation increased with increasing mode number. Nevertheless, it was found that the total in-duct transmitted acoustic energy flux is not much influenced by the scattering of the incident modes.
It is believed that this modal modulation is caused by scattering which occurs at the hard- wall strips. As an incident mode passes over these hard-walled strips, its acoustic energy tends to be scattered into different lower-order modes, thus altering the modal character of the radiated field. The major concern is that scattering into lower-order modes has an adverse effect on the liner performance, since energy is transferred into modes which are less attenuated by the liner. In addition, it is possible that cut-off modes may scatter into cut-on modes with a resulting increase in radiated energy.
Regan & Eaton developed a FE model 6, 7 and used it to analyze a finite duct lined with locally reacting liner with different number of hard strips of different widths. They demonstrated that the transmitted modal spectrum can be significantly modulated by the presence of hard strips, but for the frequency range considered (ka < 10), the overall transmitted power is not significantly affected.
McAlpine et al. 8 presented an assessment of the effect of scattering by the liner hard strips for both finite element and boundary element calculations. The results demonstrated that the scattering depends on the fan speed and the number and widths of strips. They showed that the level of the scattered tones can be significantly reduced by having thinner splices. But at high fan speeds, the…