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Experimental Determination of Aeracoustic Sources in Low Mach Number Internal Flows
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
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 [en]
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: urn:nbn:se:kth:diva-26133OAI: oai:DiVA.org:kth-26133DiVA: diva2:370447
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
List of papers
1. The Error Suppression of an Experimental Over-Determination of In-Duct Flow Noise Sources
Open this publication in new window or tab >>The Error Suppression of an Experimental Over-Determination of In-Duct Flow Noise Sources
2010 (English)In: 16th AIAA/CEAS Aeroacoustics Conference, Stockholm, June 2010, 2010, 1-19 p.Conference paper, Published paper (Refereed)
Abstract [en]

The full aeroacoustic two-port measurement procedure in flow ducts involves the determination of a scattering matrix and a source term. In this paper, the improved error suppression of a recently developed over-determination technique for the source is investigated. By inducing disturbances in the reflection coefficients it is shown that by applying the over-determination the accuracy is improved. For errors introduced via the scattering matrix the results are identical regardless of method applied. From measurements it is shown that random errors in the microphone data are better suppressed using the over-determination method. While this is true for bias errors due to external sound sources as well, the accuracy is unsatisfying when the external source generated sound is more than about 3 dB louder than that of the two-port source.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-26140 (URN)2-s2.0-78649590915 (Scopus ID)
Conference
16th AIAA/CEAS Aeroacoustics Conference
Funder
Swedish Research Council
Note

QC 20101118

Available from: 2010-11-16 Created: 2010-11-16 Last updated: 2016-06-08Bibliographically approved
2. Accurate experimental two-port analysis of flow generated sound
Open this publication in new window or tab >>Accurate experimental two-port analysis of flow generated sound
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.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-26137 (URN)10.1016/j.jsv.2011.07.041 (DOI)000296403600004 ()2-s2.0-80053254335 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20101118. Uppdaterad från Manuskript till Artikel (20111121) Available from: 2010-11-16 Created: 2010-11-16 Last updated: 2017-12-12Bibliographically approved
3. The effect of grazing-bias flow on the self sustained oscillations in a side branch
Open this publication in new window or tab >>The effect of grazing-bias flow on the self sustained oscillations in a side branch
2010 (English)In: 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, 2010, 176-181 p.Conference paper, Published paper (Refereed)
Abstract [en]

The flow-acoustic interaction in a T-junction can result in both amplification and attenuation of incoming waves. The frequency ranges of amplification are mainly governed by the time scales of convection of hydrodynamic instabilities over the side branch opening. With bias flow, i.e. flow through the side branch the amplification can increase or completely vanish. In this paper this effect is studied for a T-junction of rectangular ducts, with a grazing Mach number of 0.1 and a varying bias inflow. When the bias inflow Mach number changes from 0 to 0.01 the amplification is increased severely. Further increasing the bias inflow has the effect of lower the amplification again towards zero at a bias Mach number of 0.05.

Keyword
Flow-acoustic interaction, Flowthrough, Frequency ranges, Hydrodynamic instabilities, Inflow Mach number, Rectangular ducts, Self-sustained oscillations, Side branches, T junctions, Time-scales
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-26141 (URN)2-s2.0-84869137211 (Scopus ID)978-161782745-7 (ISBN)
Conference
20th International Congress on Acoustics 2010, ICA 2010 - Incorporating the 2010 Annual Conference of the Australian Acoustical Society; Sydney, NSW; Australia; 23 August 2010 through 27 August 2010
Funder
Swedish Research Council
Note

QC 20101118

Available from: 2010-11-16 Created: 2010-11-16 Last updated: 2014-08-15Bibliographically approved
4. Experimental determination of the aero-acoustic properties of an in-duct flexible plate
Open this publication in new window or tab >>Experimental determination of the aero-acoustic properties of an in-duct flexible plate
Show others...
2008 (English)In: 14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference), 2008Conference paper, Published paper (Refereed)
Abstract [en]

The potential reduction of the aeroacoustic noise generated by an in duct plate, byallowing it to be flexible, is studied experimentally. The test object is a triangular plateinserted at an angle in a circular flow duct. Results are given for the active and passiveacoustic properties. In addition the flow field and the vibrations of the plate arecharacterized. It is found that an appropriately yielding plate reduces the flow generatednoise while keeping the mean flow field unaffected.

Keyword
Aeroacoustic noise, Circular flow, Experimental determination, Flexible plates, Flow-generated noise, Mean flow fields, Potential reduction, Test object, Triangular plates
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-26135 (URN)2-s2.0-78249257017 (Scopus ID)9781563479397 (ISBN)
Conference
14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference); Vancouver, BC; Canada; 5 May 2008 through 7 May 2008
Note

QC 20101118

Available from: 2010-11-16 Created: 2010-11-16 Last updated: 2014-10-13Bibliographically approved

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