Relevant bibliographies by topics / Air bubbles; Acoustic impedance

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  1. Journal articles
  2. Dissertations / Theses
  3. Books
  4. Book chapters
  5. Conference papers

Academic literature on the topic 'Air bubbles; Acoustic impedance'

Author: Grafiati
Published: 4 June 2021
Last updated: 15 February 2022

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Journal articles on the topic "Air bubbles; Acoustic impedance"

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Lynnworth, Lawrence C. "Air transducers with high acoustic impedance." Journal of the Acoustical Society of America 103, no. 5 (May 1998): 2833. http://dx.doi.org/10.1121/1.421383.

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Hsiao, P. Y., M. Devaud, and J. C. Bacri. "Acoustic coupling between two air bubbles in water." European Physical Journal E 4, no. 1 (January 2001): 5–10. http://dx.doi.org/10.1007/s101890170136.

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Ye, Zhen. "Acoustic scattering by arrays of air bubbles in water." Journal of the Acoustical Society of America 108, no. 5 (November 2000): 2639. http://dx.doi.org/10.1121/1.4743829.

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Deane, Grant B., and M. Dale Stokes. "The acoustic excitation of air bubbles fragmenting in sheared flow." Journal of the Acoustical Society of America 124, no. 6 (December 2008): 3450–63. http://dx.doi.org/10.1121/1.3003076.

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Gubaidullin, Damir, and Anatolii Nikiforov. "Interaction acoustic waves with a layered structure containing layer of bubbly liquid." MATEC Web of Conferences 148 (2018): 15006. http://dx.doi.org/10.1051/matecconf/201814815006.

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The results of a theoretical study of the effect of a bubble layer on the propagation of acoustic waves through a thin three-layered barrier at various angles of incidence are presented. The barrier consists of a layer of gel with polydisperse air bubbles bounded by layers of polycarbonate. It is shown that the presence of polydisperse air bubbles in the gel layer significantly changes the transmission and reflection of the acoustic signal when it interacts with such an obstacle for frequencies close to the resonant frequency of natural oscillations of the bubbles. The frequency range is identified where the angle of incidence has little effect on the reflection and transmission coefficients of acoustic waves.
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Gomez Alvarez-Arenas, T. E. "Acoustic impedance matching of piezoelectric transducers to the air." IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 51, no. 5 (May 2004): 624–33. http://dx.doi.org/10.1109/tuffc.2004.1302770.

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Gomez Alvarez-Arenas, T. E. "Acoustic Impedance Matching of Piezoelectric Transducers to the Air." IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 51, no. 5 (May 2004): 624–33. http://dx.doi.org/10.1109/tuffc.2004.1308697.

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Alvarez-Arenas, T. E. G. "Acoustic impedance matching of piezoelectric transducers to the air." IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 51, no. 5 (May 2004): 624–33. http://dx.doi.org/10.1109/tuffc.2004.1320834.

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Ye, Zhen. "Resonant scattering of acoustic waves by ellipsoid air bubbles in liquids." Journal of the Acoustical Society of America 101, no. 2 (February 1997): 681–85. http://dx.doi.org/10.1121/1.418279.

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Bin, Liang, Zhu Zhe-Min, and Cheng Jian-Chun. "Acoustic Localization in Weakly Compressible Elastic Media Permeated with Air Bubbles." Chinese Physics Letters 23, no. 4 (March 30, 2006): 871–74. http://dx.doi.org/10.1088/0256-307x/23/4/031.

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Dissertations / Theses on the topic "Air bubbles; Acoustic impedance"

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Ramble, David Gary. "Characterisation of bubbles in liquids using acoustic techniques." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.390369.

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McIntyre, Trevor A. "Ultrasonic acoustic characteristics of air bubbles in the surf zone." Thesis, Monterey, California. Naval Postgraduate School, 1995. http://hdl.handle.net/10945/26821.

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Understanding the movement of sediment in the nearshore region due to wave motion and longshore currents is important in beach erosion studies, and has tactical significance in beach front mine warfare. In the surf zone, an bubbles and sediment are both suspended within the water column. At the Naval Postgraduate School in Monterey, California, a sediment flux probe has been developed to study small scale processes. Using ultrasonic acoustic backscatter, the Coherent Acoustic Sediment Flux Probe (CASP) is capable of tracking the movement of scatterers within the surf zone. As it is important that the CASP system is capable of distinguishing between sediment and entrained air bubbles, laboratory experiments were run to determine the ultrasonic acoustic backscatter characteristics of surf zone bubbles. Bulk void fraction and optical sizing methods were explored to develop a means of measuring bubble populations produced in the laboratory for calibration of the backscattered energy received by the CASP system in the presence of bubbles
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Michaud, Alexander Page. "Experimental Investigation of Reflection of Airborne Noise at Duct Terminations." Thesis, Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/16209.

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Noise between 25-500 Hz is a common problem in Heating, Ventilating, and Air Conditioning (HVAC) systems. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Handbook lists values of end reflection loss (ERL), a frequency dependent parameter describing energy reflected back up a duct at a termination impedance, to help engineers design and account for noise. The ASHRAE Handbook does not account for common termination variations and only lists ERL values using octave bands down to 63 Hz. This thesis experimentally determined the ERL of a variety of rectangular duct configurations and termination conditions between 25-500 Hz. This research also compared experimental ERL results with analytic predictions and ASHRAE Handbook values. Seven duct sizes were tested, from 6X6 to 18X54 inches. Duct termination baffle hardness was varied between acoustically hard (plywood) and soft (ceiling tiles) for the 6X6, 6X10, and 6X18 ducts. Five duct termination distances above the termination baffle were tested, between flush and 1D for the 6X10 and 6X18 ducts and between flush and 5D for the 6X6 duct, where D equals the duct s effective diameter. Diffusers and flex duct configurations were installed at the end of the rigid duct to test their effect on ERL on the 6X6, 6X10, and 6X18 ducts. ERL was determined using an adaptation of the ASTM E1050 Standard, an application of the two-microphone impedance tube method. Experimental results closely conformed to analytic predictions and are an improvement over ASHRAE Handbook ERL values. The results indicate that baffle hardness has a negligible impact on ERL, which contradicts the ASHRAE assumption that diffusers that terminate in a suspended lay-in acoustic ceiling can be treated as terminating in free space. Termination distance above the baffle has a negligible impact on ERL at distances less than six inches for the 6X6 duct. Termination distances above the baffle greater than six inches exhibit limited free space ERL behavior for the 6X6 duct. The use of flex duct greatly reduces low frequency ERL and this is not accounted for by the ASHRAE Handbook. The impact from flex duct usage also negates any influence from downstream termination variations.
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Du, Liangfen. "Characterisation of air-borne sound sources using surface coupling techniques." Thesis, Lyon, 2016. http://www.theses.fr/2016LYSEI028/document.

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La thèse se base sur la recherche des possibilités de caractérisation du son aérien de sources sonores arbitraires. A cette fin, une approche particulière est étudiée à l’endroit où la caractérisation de la source est faite via une surface d’interface qui enveloppe totalement ou partiellement la source physique. Deux descripteurs qui dépendent de la fréquence sont definis au travers d’une telle surface: la pression sonore bloquée et l’impédance de la source. Le précédent représente la pression sonore créée par le système d’exploitation source qui agit sur la surface enveloppante quand elle est rendue immobile. Cette dernière représente le rapport des amplitudes de réponse de pression et les amplitudes de vitesse d’excitation normales au travers de la surface. La surface enveloppante définit un volume d’air qui contient la source physique appelée l’espace source. Les deux descripteurs définis sur l’espace source, la pression bloquée et l’impédance de la source sont montrés comme étant intrinsèques à la source, c’est-à-dire indépendants de l’espace acoustique environnant. Une fois définis, ces descripteurs permettent de trouver la pression sonore et la vitesse particulaire normale à la surface de l’interface quand l’espace source est couplé à un espace récepteur arbitraire, c’est-à-dire une pièce. Cela permet alors la prédiction du son dans l’espace récepteur. Les conditions de couplage nécessitent que l’espace récepteur soit caractérisé en utilisant la même surface enveloppante telle que l’espace source. En acceptant de garder à l’esprit la simplicité de la mesure, la surface enveloppante a été conçue vu qu’elle comporte une ou plusieurs surfaces rectangulaires planes. Le défi de la recherche était alors d’obtenir une impédance significative de la surface au travers de la surface plane rectangulaire (continue) ainsi que celle de la pression bloquée compatible avec la formulation de l’impédance. Cela a conduit à une décomposition dans l’espace de la pression sonore et de la vitesse des particules au sein du nombre fini des composants, chacun défini par une amplitude complexe et une distribution dans l’espace particulière. De cette façon, la pression bloquée se réduit à un vecteur d’amplitude de pression complexe, tandis que l’impédance devient une matrice de pression et des rapports d’amplitudes complexes de la vitesse de défauts de de décompositions ont été recherchés dans le détail: la méthode harmonique de surface et la méthode du patch. Le premier se rapproche de la pression de surface et de la vitesse normale par des combinaisons de fonctions de surface trigonométriques en 2D tandis que ce dernier partage la surface en petites parcelles et intervient sur chaque parcelle de façon discrète en utilisant les valeurs moyennes du patch
The thesis investigates possibilities of air-borne sound characterisation of arbitrary sound sources. To this end a particular approach is studied where the source characterisation is done via an interface surface which fully or partially envelopes the physical source. Two frequency dependent descriptors are defined across such a surface: the blocked sound pressure and the source impedance. The former represents the sound pressure created by the operating source which acts on the enveloping surface when this is made immobile. The latter represents the ratio of pressure response amplitudes and normal velocity excitation amplitudes across the surface. The enveloping surface defines an air volume containing the physical source, called the source space. The two source descriptors defined on the source space, the blocked pressure and the source impedance, are shown to be intrinsic to the source, i.e. independent of the surrounding acoustical space. Once defined, these descriptors allow one to find the sound pressure and normal particle velocity at the interface surface when the source space is coupled to an arbitrary receiver space, i.e. a room. This in turn allows for sound prediction in the receiver space. The coupling conditions require that the receiver space is characterised using the same enveloping surface as the source space. Bearing the measurement simplicity in mind, the enveloping surface has been conceived as consisting of one or several rectangular plane surfaces. The research challenge was then to obtain meaningful surface impedance across a (continuous) rectangular plane surface as well as the blocked pressure compatible with impedance formulation. This has led to a spatial decomposition of sound pressure and particle velocity into finite number of components, each defined by a complex amplitude and a particular spatial distribution. In this way the blocked pressure reduces to a vector of complex pressure amplitudes while the impedance becomes a matrix of pressure and velocity complex amplitude ratios. Two decomposition methods have been investigated in detail: the surface harmonic method and the patch method. The former approximates the surface pressure and normal velocity by combinations of 2D trigonometric surface functions while the latter splits the surface into small patches and treats each patch in a discrete way, using patch-averaged values
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Zander, Anthony Charles. "Influence of error sensor and control source configuration and type upon the performance of active noise control systems / Anthony C. Zander." 1994. http://hdl.handle.net/2440/21488.

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Bibliography : leaves 237-251.
x, 251 leaves : ill. ; 30 cm.
Title page, contents and abstract only. The complete thesis in print form is available from the University Library.
Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1994
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Books on the topic "Air bubbles; Acoustic impedance"

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McIntyre, Trevor A. Ultrasonic acoustic characteristics of air bubbles in the surf zone. Monterey, Calif: Naval Postgraduate School, 1995.

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Book chapters on the topic "Air bubbles; Acoustic impedance"

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Chimenti, Dale, Stanislav Rokhlin, and Peter Nagy. "Air-Coupled Ultrasonics." In Physical Ultrasonics of Composites. Oxford University Press, 2011. http://dx.doi.org/10.1093/oso/9780195079609.003.0013.

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Ultrasonic material characterization or inspection for defects is conventionally performed using either liquid coupling (water, usually) or some type of gel or oil in contact-mode coupling. Mechanical waves can be transmitted only through some sound-supporting medium from their source (a transducer) to the object under study, and back again. Using distilled, degassed water to couple ultrasound to an object under test works quite well and has many technical advantages, including relatively low signal loss over laboratory or shop dimensions at typical frequencies, almost zero toxicity, and low cost. For many applications, the use of water is acceptable and preferred. There are, however, certain testing applications for which water can be a disadvantage. These situations include materials that are sensitive to contact with water, such as uncured graphite-epoxy composites or certain electronics. Large objects, whose total immersion is impractical, or objects for which rapid scanning is required might also be unsuitable for water coupling. Recent technological developments are beginning to permit the judicious replacement of water by a far more ubiquitous sound coupling medium—air. Ultrasonic testing in air has been investigated for more than 30 years, but recently there has been an upsurge in interest and application because of the availability of much more efficient sound-generating devices designed specifically for operation in air. In water- or direct-coupled ultrasonics, one typically employs piezoelectric transducers to generate sound waves because they are well suited to the generation of sound in water or in solids because of their high acoustic impedance. In air, however, we need just the opposite. Air is very compliant, so waves from a high-impedance source couple poorly into air. Much effort has been invested in finding suitable impedance matching materials that will render the familiar piezoelectric probe efficient in air-coupled (A-C) ultrasound. The problem, however, is nearly insurmountable because of the large acoustic impedance difference between air and quartz, for example. Quartz has an acoustic impedance of about 15 MRayl, while air’s impedance is about 425 Rayl, a ratio of about 35,000. The challenge is to find a material with an acoustic impedance that nearly equals the geometric average of these two widely disparate values.
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Newnham, Robert E. "Acoustic waves II." In Properties of Materials. Oxford University Press, 2004. http://dx.doi.org/10.1093/oso/9780198520757.003.0026.

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Acoustic impedance, acoustic losses, acoustic waves in piezoelectric solids, and surface waves are discussed in this chapter, along with a number of nonlinear acoustic phenomena. The reflection and transmission of acoustic waves across a boundary is governed by acoustic impedance. One of the most important boundary value problems in acoustics concerns a plane wave incident upon a planar surface, dividing one medium from another. In the general case of an anisotropic medium, the incident beam consists of three waves (one quasilongitudinal, two quasitransverse), each traveling at a different velocity. Each of the three incident waves will be refracted and reflected at the boundary. If the second medium is also anisotropic, each incident wave will generate three reflected waves and three refracted waves, a total of 27 waves in all. Wave propagation in a polycrystalline solid where there are many grain boundaries becomes very complicated. The simpler case of a pure longitudinally-polarized wave at normal incidence to the boundary provides insight into the more general problem. In this case the reflection and transmission coefficients are governed by the relatively simple acoustic impedance parameter (ρc)1/2 = ρv, where ρ is the density, c the stiffness coefficient, and v the phase velocity. The reflection coefficient R at the interface between medium I and medium II is The MKS unit for acoustic impedance is the Rayl (=kg/m2 s). Atypical value for a solid is about 107 rayls. In many acoustic applications it is desirable to reduce reflection by matching the acoustic impedance of the two media. Lithium tantalate transducers are well-matched to iron, for example. Sound transmission from the transducer to the medium can be enhanced with composite materials or with graded coupling layers. Backing materials are often selected to promote reflection. In this case acoustic impedances are mismatched. Tungsten and air are two commonly used backing materials. In an isotropic material the acoustic impedance is (ρc11)1/2 for longitudinal waves and (ρc44)1/2 for shear waves. For anisotropic materials the wave velocities and acoustic impedance change with direction as indicated earlier.
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Conference papers on the topic "Air bubbles; Acoustic impedance"

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Ueno, Ichiro, Keishi Matsumoto, Atsumi Machida, and Tsuyoshi Hanyu. "Shape Oscillation of Bubble(s) in Acoustic Field." In 2010 14th International Heat Transfer Conference. ASMEDC, 2010. http://dx.doi.org/10.1115/ihtc14-22636.

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We focus on dynamics of multiple air bubbles exposed to acoustic pressure field while ascending in water. The bubbles are injected into the pool filled with water from a vertical capillary tube, and then the acoustic wave of designated frequency is applied toward the bubbles. The frequency of the acoustic wave is varied from 0.5 to 20 kHz. Volume and shape oscillations of the bubbles are captured by a high-speed camera at frame rates up to 40000 fps with a back-lighting system. Through this system, we succeed in capturing the dynamics of the axisymmetric shape oscillation with a distinct mode number; the bubble exhibits the volume oscillation first with a fundamental frequency f0, and then the gradual transition to the shape oscillation with a fundamental frequency fnm takes place. We evaluate the correlation through the careful observations between the f0 and fnm as f0 ∼ 2.1fnm, which brings almost perfectly confirmation of the prediction through the preceding theoretical works. We also indicate the criterion of the excitation of the shape oscillation by varying the frequencies of the adding pressure field.
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Schuller, T., N. Tran, N. Noiray, D. Durox, S. Ducruix, and S. Candel. "The Role of Nonlinear Acoustic Boundary Conditions in Combustion/Acoustic Coupled Instabilities." In ASME Turbo Expo 2009: Power for Land, Sea, and Air. ASMEDC, 2009. http://dx.doi.org/10.1115/gt2009-59390.

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Triggering, frequency shifting, mode switching and hysteresis are commonly encountered during self-sustained oscillations in combustors. These mechanisms cannot be anticipated from classical linear stability analysis and the nonlinear flame response to incident flow perturbations is often invoked to interpret these features. However, the flame may not be solely responsible for nonlinearities. Recent studies indicate that interactions with boundaries can be influenced by the perturbation level and that this needs to be considered. The nonlinear response of acoustic boundary conditions to flow perturbations is here exemplified in two configurations which typify practical applications. The first corresponds to a perforated plate backed by a cavity conveying a bias flow and the second corresponds to a set of flames stabilized at a burner outlet. These systems are submitted to acoustic perturbations of increasing amplitudes as can be encountered during unstable operation. It shown that these terminations can be characterized by an impedance featuring an amplitude dependent response. The classical linear impedance Z(ω) is then replaced by its nonlinear counterpart an Impedance Describing Function (IDF), which depends on the perturbation level input Z(ω, |p′| or |u′|). Using this concept, it is shown that the passive perforated plate optimized to damp instabilities of small amplitudes may eventually loose its properties when submitted to large sound pressure levels and that the flame response shifts when the amplitude of incoming flow perturbations is amplified. The influence of these nonlinear elements on the stability of a generic burner is then examined using a methodology which extends a previous analysis based on the Flame Describing Function (FDF) to systems with complex flow interactions at the boundaries.
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Bourgoin, M., C. Baudet, A. Cartellier, P. Gervais, and Y. Gagne. "3D Acoustic Lagrangian Velocimetry." In ASME 2006 2nd Joint U.S.-European Fluids Engineering Summer Meeting Collocated With the 14th International Conference on Nuclear Engineering. ASMEDC, 2006. http://dx.doi.org/10.1115/fedsm2006-98210.

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We report Lagrangian measurements obtained with an acoustic Doppler velocimetry technique. From the Doppler frequency shift of acoustic waves scattered by tracer particles in a turbulent flow, we are able to measure the full three-component velocity of the particles. As a first application, we have studied velocity statistics of Lagrangian tracers in a turbulent air jet at Rλ∼320 and at various distances from the nozzle. The choice of an air jet is motivated by the fact that jets produce a well characterized high level tubulence and open air flows are well suited to simultaneaously achieve classical hot wire Eulerian measurements. Therefore, we are also able to explicitly address the question of the differences between Eulerian and Lagrangian statistics. As Lagrangian tracers we use soap bubbles inflated with Helium which are neutrally buoyant in air and can be assimilated to fluid particles. Velocity statistics are analysed. We show that the Lagrangian autocorrelation decays faster in time than its Eulerian counterpart. Finally we present Lagrangian time velocity increments statistics which, as already reported by previous work, exhibits stronger intermittency than Eulerian velocity increments.
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Bothien, Mirko R., Jonas P. Moeck, and Christian Oliver Paschereit. "Impedance Tuning of a Premixed Combustor Using Active Control." In ASME Turbo Expo 2007: Power for Land, Sea, and Air. ASMEDC, 2007. http://dx.doi.org/10.1115/gt2007-27796.

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In early design phases new burner concepts are mostly tested in single or multi burner test rigs. These test rigs generally exhibit a different acoustic behavior than the full scale engine. The acoustic behavior, however, is crucial to predict whether thermoacoustic instabilities are likely to occur. Tuning the test rig’s acoustic boundary conditions to that of the engine could overcome this issue. Through this, an effective assessment of new burners is possible even in early design phases. In this work a method is proposed, which uses an active control scheme to manipulate the acoustic boundary conditions. It is applied to an atmospheric combustor test rig with a swirl-stabilized burner. In a first step it is shown that the acoustic boundary conditions can be controlled in the cold flow case. Almost arbitrary frequency dependent impedances can be prescribed ranging from fully reflecting (both pressure and velocity node) to anechoic. In particular, an additional virtual length can be added to the combustor outlet by manipulation of the reflection coefficient’s phase. This introduces resonance frequencies different from those of the uncontrolled case. In a second step the impedance tuning concept is applied to the reacting flow. It is demonstrated that the concept is feasible despite the harsh environmental conditions in a combustion chamber. The effect of different levels of reflection at the combustion chamber outlet on the combustion process is investigated. In addition to that, a study of the influence of the simulated combustor length on the system’s resonance frequencies is conducted.
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Albors, Gabriel O., Aaron M. Kyle, George R. Wodicka, and Eduardo J. Juan. "Computer Simulation Tool for Predicting Sound Propagation in Air-Filled Tubes with Acoustic Impedance Discontinuities." In 2007 29th Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE, 2007. http://dx.doi.org/10.1109/iembs.2007.4352761.

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Huber, Andreas, Philipp Romann, and Wolfgang Polifke. "Filter-Based Time-Domain Impedance Boundary Conditions for CFD Applications." In ASME Turbo Expo 2008: Power for Land, Sea, and Air. ASMEDC, 2008. http://dx.doi.org/10.1115/gt2008-51195.

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For flow simulations, proper boundary conditions are essential for realizing a well-posed, physically meaningful and numerically stable problem formulation. This is particularly difficult for compressible flow, where in general boundary conditions have to be imposed both for mean flow and acoustic quantities. For the acoustic variables, boundary conditions can be formulated in terms of the acoustic impedance or alternatively the reflection coefficient, which are general a complex-valued, frequency dependent quantity. The present work presents a novel, efficient and flexible approach to impose time-domain impedance boundary conditions (TDIBC) for computational fluid dynamics (CFD): The acoustic boundary conditions are represented as a discrete filter model with appropriately optimized filter coefficients. Using the z-transformation the filter model is transferred to a time-domain formulation and applied to the CFD environment in form of advanced filter realizations. Validation studies using various acoustic boundary conditions have been carried out with the new formulation. The results demonstrate that the method works in an accurate and robust manner.
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Krebs, Werner, Günther Walz, Patrick Flohr, and Stefan Hoffmann. "Modal Analysis of Annular Combustors: Effect of Burner Impedance." In ASME Turbo Expo 2001: Power for Land, Sea, and Air. American Society of Mechanical Engineers, 2001. http://dx.doi.org/10.1115/2001-gt-0042.

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For the development of modern Low-NOx gas turbine combustors featuring high power densities due to their compact design a detailed knowledge about thermoacoustically induced combustion oscillations is required. In order to design passive and active means to suppress thermoacoustic oscillations and to extend the stable operation range of the gas turbine an investigation of the acoustic eigenmodes of the combustor already in the design phase is necessary. In a combined experimental and computational project, tools to determine the mode shapes of a gas turbine combustor have been developed. The mode shapes of an annular combustor of the 3A series have been identified under operating conditions in the test bed of Berlin by cross correlating pressure signals mounted on twelve different azimuthal locations. These data have been used in order to validate the new numerical steady state response method. It has been found that taking into account appropriate acoustic boundary conditions at the burner outlet the numerical predictions are in good agreement with the measurements.
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Dong, Qian, Xiaolei Song, Subhrodeep Ray, and Haijun Liu. "Acoustic Metamaterial With Air-Backed Diaphragm for Broadband Absorption: A Preliminary Study." In ASME 2020 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2020. http://dx.doi.org/10.1115/imece2020-23928.

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Abstract Membrane-based acoustic metamaterials have been reported to achieve 100% absorption, the acoustic analogue of photonic black-hole. However, the bandwidth is usually very narrow around some local resonance frequency, which limits its practical use. To address this limitation and achieve a broadband absorption, this paper first establishes a theoretical framework for unit cells of air-backed diaphragms, modeled as an equivalent mass-spring-dashpot system. Based on the impedance match principle, three different approaches are numerically investigated by tuning the cavity length, the static pressure in the cavity, and the effective damping of perforated plates. A prototype with polyimide diaphragm and 3D printed substrate is then fabricated and characterized using an acoustic impedance tube. Preliminary experiments show the feasibility to achieve an absorption bandwidth of ∼200 Hz at center frequency of 1.45 kHz. This work pays the way for developing a sub-wavelength light weight broadband acoustic absorber for a variety of applications in noise control.
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Widenhorn, Axel, Berthold Noll, and Manfred Aigner. "Impedance Boundary Conditions for the Numerical Simulation of Gas Turbine Combustion Systems." In ASME Turbo Expo 2008: Power for Land, Sea, and Air. ASMEDC, 2008. http://dx.doi.org/10.1115/gt2008-50445.

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The design process of modern gas turbine combustion systems relies more and more on CFD methods. To capture unsteady combustion phenomena like combustion instabilities or direct combustion noise both the reacting flow field and the acoustic field have to be modeled precisely. To take into account the accurate simulation of the acoustic wave reflection at a boundary condition time-domain impedance formulations have to be used. These conditions allow specifying the frequency depending impedance quantities for example of the fuel line, air supply system, combustion chamber outlet and walls in the time-domain. In the present paper the theory and the practical implementation of the time-discrete impedance formulation are discussed. Here, the link between the time and frequency domain is established by utilizing both the z-transform and Fourier transform. By means of simple test cases the physical effects of acoustically treated boundary conditions on the flow and acoustic field are worked out. Furthermore, the accuracy is analyzed and the need for such boundary conditions in the framework of gas turbine combustion system research and development is discussed.
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Richards, Geo A., and Edward H. Robey. "Effect of Fuel System Impedance Mismatch on Combustion Dynamics." In ASME Turbo Expo 2005: Power for Land, Sea, and Air. ASMEDC, 2005. http://dx.doi.org/10.1115/gt2005-68386.

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Combustion dynamics are a challenging problem in the design and operation of premixed gas turbine combustors. In premixed combustors, pressure oscillations created by the flame dynamic response can lead to damaging pressure oscillations. These dynamics are typically controlled by designing the combustor to achieve stable operation for planned conditions, but dynamics may still occur with minor changes in ambient operating conditions, or fuel composition. In these situations, pilot flames, or adjustment to fuel flow splits can be used to stabilize the combustor, but often with a compromise in emissions performance. As an alternative to purely passive design changes, prior studies have demonstrated that adjustment to the fuel system impedance can be used to stabilize combustion. Prior studies have considered just the response of individual fuel injector and combustor. However, in practical combustion systems, multiple fuel injectors are used. In this situation, individual injector impedance can be modified to produce a different dynamic response from individual flames. The resulting impedance mismatch prevents all injectors from strongly coupling to the same acoustic mode. In principle, this mismatch should reduce the amplitude of dynamics, and may expand the operating margin for stable combustion conditions. In this paper, a 30 kW laboratory combustor with two premixed fuel injectors is used to study the effect of impedance mismatch on combustion stability. The two fuel injectors are equipped with variable geometry resonators that allow a survey of dynamic stability while changing the impedance of the individual fuel systems. Results demonstrate that a wide variation in dynamic response can be achieved by combining different impedence fuel injectors. A baseline 7% RMS pressure oscillation was reduced to less than 3% by mismatching the fuel impedance.
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