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Accurate experimental two-port analysis of flow generated 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
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.ORCID iD: 0000-0002-8474-8563
2011 (English)In: Sound & vibration, ISSN 1541-0161, Vol. 330, no 26, 6336-6354 p.Article in journal (Other academic) Published
Abstract [en]

In the study of in-duct aero-acoustic phenomena two-port analysis based upon in-duct measurements has become an important method for the plane wave region. However already at moderate Mach numbers (0.2 - 0.3) the errors in the results can be hard to suppress. Ways of dealing with this includes the use of over-determination methods and methods of obtaining more accurate wave numbers. In this paper different methods for the determination of the source data and the scattering matrix are evaluated. For the source data in the form of a cross spectrum matrix an over-determination method is introduced. In addition, a method of obtaining the mean Mach number from experimentally determined wave numbers is described.

In order to evaluate the methods measurements are conducted at a mean Mach number of 0.2 for two test cases – an empty duct and a mixer plate inside the duct. The main improvements in the scattering matrix results are achieved by discarding measurements from an abundant set, based upon the measured coherence between excitation and fluctuating pressures. For the source part, it is shown that the error in the magnitude of the source cross spectrum matrix can be significantly suppressed by having additional (> 2) reference microphones on each side of the two-port.

The mean Mach number obtained from an experimentally determined wave number is observed to yield more accurate scattering matrix results in both phase and magnitude, than that based upon flow velocity measurements at one point and an assumed flow profile.

Place, publisher, year, edition, pages
2011. Vol. 330, no 26, 6336-6354 p.
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-26137DOI: 10.1016/j.jsv.2011.07.041ISI: 000296403600004Scopus ID: 2-s2.0-80053254335OAI: oai:DiVA.org:kth-26137DiVA: diva2:370442
Funder
Swedish Research Council
Note
QC 20101118. Uppdaterad från Manuskript till Artikel (20111121) Available from: 2010-11-16 Created: 2010-11-16 Last updated: 2012-11-23Bibliographically approved
In thesis
1. Experimental Determination of Aeracoustic Sources in Low Mach Number Internal Flows
Open this publication in new window or tab >>Experimental Determination of Aeracoustic Sources in Low Mach Number Internal Flows
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, the in-duct experimental methods for determining aeroacoustic N-ports of in-duct elements are discussed and improved. The scattering matrix determination methods and the related wave decomposition methods are evaluated from measurements in an empty duct carrying a mean flow. The improvements of a new over-determination method for the source part of the N-port is studied using simulations and measurements; in quiescent air as well as measurements of the flow associated noise of a mixer plate, here a triangular plate inserted at an angle in a duct. The new method is shown to improve suppression of random errors while no improvement is achieved for bias errors.

 

Further, the methods are applied in the study of two different aeroacoustic phenomena; one is the effect on the flow associated noise of the triangular plate achieved by varying the bending stiffness. For the most resilient plate tested, it is observed that when the Strouhal number of the flow noise coalesce with the Helmholtz number of a specific eigen-mode of the plate, the noise is drastically dampened. There is also a weaker broad band effect.

 

The other phenomena studied is the amplification and attenuation obtained for sound waves propagating in a T-junction of rectangular ducts. It is found that by adding only 10% of inflow in the side branch relative to that in the main branch, the amplification is heavily increased. By adding another 10% the amplification is again similar to that of no side branch flow. Adding further flow lessens the effects still.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010
Series
Trita-AVE, ISSN 1651-7660 ; 2010:69
Keyword
Aeracoustics, ducts, N-ports, scattering matrix, wave decomposition, source determination, over-determination, flow noise, fluid-acoustic interaction, fluid-structure-acoustic interaction, wave number, T-junction, power balance, grazing-bias flow
National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-26133 (URN)
Presentation
2010-12-09, MWL 74, Teknikringen 8, Stockholm, KTH, 13:00 (English)
Opponent
Supervisors
Projects
Experimental characterization of aero-acoustic sources
Funder
Swedish Research Council
Note
QC 20101118Available from: 2010-11-18 Created: 2010-11-16 Last updated: 2010-11-18Bibliographically approved
2. Aeroacoustic Characterization using Multiport Methods
Open this publication in new window or tab >>Aeroacoustic Characterization using Multiport Methods
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Noise is a major environmental pollutant, which can inict physical and psychologicalinjury. An important noise contribution stems from aeroacousticsources, which are found in e.g., ventilation ducts, engine exhaust systems andairplane engines.In this thesis, research methods for low Mach number aeroacoustic sourcesin ducts are developed. The basis of the methods is the ability to describe theintrinsic linear properties of the source as an N-port (multiport), where theoutput sound eld is related to the input sound eld and the generated soundeld, all consisting of plane waves. The methods presented are both numericaland experimental. The numerical method treats the passive properties, i.e.,scattering, attenuation and amplication of incident sound, while the experimentalmethod treats the active part (intrinsic sound generation) as well. Themethods are applied in the study of noise generation by a vortex mixer plate,placed in an airow of Mach 0.2, and in the study of acoustic-hydrodynamicinteraction in a T-junction of rectangular ducts.It is found that the accuracy of the experimental methods is signicantly increasedwhen the equations are over-determined, which is achieved by addingadditional microphones to the test rig. In the frequency range studied, themixer plate is found to generate less sound when made exible, without disturbingthe mixing quality.For the numerical method { based on the linearized Navier-Stokes equations,a model of the oscillation of the Reynolds stress (\turbulent damping")due to the acoustic eld is introduced. By comparing with experimental results,it is found that not using this model results in an over-prediction of theamplication at higher frequencies with several factors in magnitude, whileimplementing the model results in a much better agreement.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. vi, 66 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2012:76
Keyword
Aeroacoustics, Multiport, Experiments, Computational Aeroacoustics, Mixer Plate, T-junction.
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-105396 (URN)978-91-7501-563-7 (ISBN)
Public defence
2012-12-10, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilTrenOp, Transport Research Environment with Novel Perspectives
Note

QC 20121123

Available from: 2012-11-23 Created: 2012-11-20 Last updated: 2013-04-11Bibliographically approved

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