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Ray Kirby - One of the best experts on this subject based on the ideXlab platform.
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A three dimensional investigation into the acoustic performance of dissipative splitter Silencers
The Journal of the Acoustical Society of America, 2014Co-Authors: Ray Kirby, Paul T. Williams, James HillAbstract:Splitter Silencers are found in ventilation and gas turbine systems and consist of parallel baffles of porous material placed within a duct so that they split the mean gas flow. Theoretical investigations into dissipative splitter Silencers have generally been limited to two dimensions and this limits the analysis to finding the Silencer eigenmodes or, for a finite length Silencer, to rectangular baffles only. In this article a numerical point collocation approach is used to extend theoretical predictions to three dimensions. This facilitates the analysis of more complex Silencer designs such as “bar” Silencers and theoretical predictions are validated by comparison with experimental measurements. The insertion loss of different Silencer designs is evaluated and the performance of a bar Silencer is compared to traditional designs for rectangular and circular ducts. It is shown that a bar Silencer with a volume of material identical to an equivalent parallel baffle design delivers a significant improvement in insertion loss at higher frequencies, although this is at the expense of a small reduction in performance at low frequencies. It is also shown that under most circumstances it is possible to get good agreement between prediction and experiment even for relatively large Helmholtz numbers.
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A comparison between the performance of different Silencer designs for gas turbine exhaust systems
2012Co-Authors: Ray Kirby, Paul Williams, James HillAbstract:The dissipative Silencers used to attenuate noise emanating from air moving devices such as fans are normally of a simple splitter design, with parallel baffles of absorbent material arranged over the width of a duct. However in more specialist applications, such as the exhaust systems of gas turbines, different Silencer geometries are often used. One such geometry is a so-called bar Silencer, in which rectangular bars, or bricks, of absorbing material are placed in a lattice arrangement over the duct cross section. The acoustic performance of these bar Silencers is investigated here using a finite element based numerical mode matching scheme. The insertion loss of the bar Silencers is then calculated and compared against traditional splitter designs in order to investigate the relative efficiency of each design.
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Predicting the acoustic performance of HVAC splitter Silencers
Noise & Vibration Worldwide, 2007Co-Authors: Ray KirbyAbstract:Mathematically modelling sound propagation in splitter Silencers has the potential to provide a fast and effective method for optimising Silencer design, and for alleviating the need to undertake repetitive and expensive experimental measurements. Moreover, understanding the physics behind sound attenuation in Silencers will ultimately help to improve in situ Silencer performance and to avoid current problems whereby commissioned Silencers do not provide the attenuation expected from laboratory measurements. Accordingly, a mathematical model is discussed here that is capable of predicting Silencer insertion loss and predictions are compared with experimental measurements. Results demonstrate the potential of this model for predicting Silencer performance under laboratory conditions, but also illustrate that multi-modal sound fields significantly affect Silencer performance so that laboratory tests are unlikely to represent accurately in situ Silencer performance.
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Multi-mode sound propagation in pod Silencers
2006Co-Authors: Ray KirbyAbstract:Pod Silencers are dissipative Silencers used in HVAC applications in which the absorptive material is used to line the outer wall of the duct and a central cylindrical pod is inserted to form a Silencer with an annular flow passage. Pod Silencers are used in a wide range of ductwork, from the relatively small to very large applications with duct diameters of the order of several metres, and so in general Silencer performance is likely to be influenced by higher order mode propagation. Accordingly, a numerical mode matching technique is used here to model higher order mode propagation in a pod Silencer. A comparison of predictions with those previously found assuming plane wave propagation is presented and it is demonstrated that, even for relatively small Silencers, higher order modes play an important role in the performance of pod Silencers. INTRODUCTION The cross-sectional shape of HVAC ductwork is typically rectangular, circular or flat oval. Dissipative Silencers are used within this HVAC ductwork to attenuate broadband noise emanating from a fan, although the Silencers themselves tend to be restricted to rectangular or circular geometries. Rectangular dissipative Silencers are normally fabricated using a series of parallel baffles, at least one of which is placed centrally in the duct to act as a “splitter”. These central splitters are known to increase sound attenuation over and above that which may be expected simply by lining the walls of the duct. For a circular duct the same principle is adopted, only here the splitter is formed from a central cylindrical “pod” so that the Silencer creates an annular flow passage. Thus, the design of pod Silencers shares many similarities with the design of rectangular splitter Silencers and one should account for sound
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Modelling dissipative Silencers using point collocation
2003Co-Authors: Ray KirbyAbstract:Dissipative Silencers are widely used in ducts to attenuate broad band noise. Silencers may assume many different shapes and sizes, ranging from a relatively compact automotive exhaust Silencer, to a much larger HVAC splitter Silencer. To accommodate such a wide range of Silencer geometries it is convenient to use numerical methods such as the finite element method, although for many Silencers the use of a fully three dimensional approach may computationally be too expensive. A convenient alternative is to use a method known as point collocation which seeks to mesh only the cross-section of a Silencer (assuming a uniform cross-section in the axial direction) and then numerically matches continuity conditions at discrete transverse locations over the inlet and outlet planes of the Silencer. This paper examines the suitability of using point collocation to compute the transmission loss for a wide range of Silencer configurations. The robustness and flexibility of the method is reviewed with a view to adopting this technique as a single, efficient, design tool for studying dissipative Silencers of any cross-sectional shape and size.
Ya-hui Chiu - One of the best experts on this subject based on the ideXlab platform.
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Position Effect on the Directionality of Silencer Function in Saccharomyces cerevisiae
Genetics, 2006Co-Authors: Yanfei Zou, Ya-hui ChiuAbstract:In Saccharomyces cerevisiae, Silencers flanking the HML and HMR loci initiate the establishment of transcriptional silencing. We demonstrate that the activity of a Silencer pertaining to its potency and directionality is dependent on its genomic position. The context of the HML-E Silencer is more permissive to Silencer function than that of HML-I or HMR-E, despite that HML-E and HML-I are only 3.3 kb apart. The apparent strength and directionality of a Silencer in a particular location is affected by other silencing elements (Silencers and protoSilencers) present in its context. We show that at the HML locus, at least four silencing elements engage in multiple functional interactions that contribute to the activities of the Silencers. Notably, these dispersed silencing elements can synergize to silence genes located not only inside, but also outside the HML sequence that harbors them. Moreover, the relative positions and orientations of these elements are important for silencing, indicating that they belong to an intricate silencing network.
Jasper Rine - One of the best experts on this subject based on the ideXlab platform.
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the establishment inheritance and function of silenced chromatin in saccharomyces cerevisiae
Annual Review of Biochemistry, 2003Co-Authors: Laura N Rusche, Ann L Kirchmaier, Jasper RineAbstract:▪ Abstract Genomes are organized into active regions known as euchromatin and inactive regions known as heterochromatin, or silenced chromatin. This review describes contemporary knowledge and models for how silenced chromatin in Saccharomyces cerevisiae forms, functions, and is inherited. In S. cerevisiae, Sir proteins are the key structural components of silenced chromatin. Sir proteins interact first with Silencers, which dictate which regions are silenced, and then with histone tails in nucleosomes as the Sir proteins spread from Silencers along chromosomes. Importantly, the spreading of silenced chromatin requires the histone deacetylase activity of Sir2p. This requirement leads to a general model for the spreading and inheritance of silenced chromatin or other special chromatin states. Such chromatin domains are marked by modifications of the nucleosomes or DNA, and this mark is able to recruit an enzyme that makes further marks. Thus, among different organisms, multiple forms of repressive chromati...
D. Alexandre - One of the best experts on this subject based on the ideXlab platform.
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Gamma-Delta Lineage-Specific Transcription of Human T-Cell Receptor-Gamma Genes by a Combination of a Non-Lineage-Specific Enhancer and Silencers
European Journal of Immunology, 1995Co-Authors: M. P. Lefranc, D. AlexandreAbstract:The expression of the T cell receptor (TcR) gamma genes is restricted to TcR gamma delta(+) T lymphocytes. Transgenic and somatic cell hybrid experiments had suggested that the expression of a functionally rearranged TcR gamma gene was extinguished in TcR alpha beta(+) T cells, possibly by putative cis-acting transcriptional Silencers. We have identified such negative cis-acting sequences in the 3' non-coding region of the human TcR gamma (TRG) locus, upstream of an enhancer located at 6.5 kb of the TcR C gamma 2 gene (TRGC2). These Silencers were capable of repressing the transcription from a minimal heterologous promoter in a position- and orientation-independent fashion. When analyzed individually, the Silencers and the enhancer were equally active in the TcR alpha beta(+) and TcR gamma delta(+) T cell lines studied. In contrast, the association of the enhancer with either Silencer was shown to restrict transcription to the TcR gamma delta(+) T cell lines.
Rohinton T. Kamakaka - One of the best experts on this subject based on the ideXlab platform.
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Long-range communication between the Silencers of HMR
Molecular and Cellular Biology, 2008Co-Authors: Lourdes Valenzuela, Namrita Dhillon, Rudra N. Dubey, Marc R. Gartenberg, Rohinton T. KamakakaAbstract:Gene regulation involves long-range communication between Silencers, enhancers, and promoters. In Saccharomyces cerevisiae, Silencers flank transcriptionally repressed genes to mediate regional silencing. Silencers recruit the Sir proteins, which then spread along chromatin to encompass the entire silenced domain. In this report we have employed a boundary trap assay, an enhancer activity assay, chromatin immunoprecipitations, and chromosome conformation capture analyses to demonstrate that the two HMR Silencer elements are in close proximity and functionally communicate with one another in vivo. We further show that silencing is necessary for these long-range interactions, and we present models for Sir-mediated silencing based upon these results.
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Chromatin insulators.
Annual review of genetics, 2006Co-Authors: Lourdes Valenzuela, Rohinton T. KamakakaAbstract:Active and silenced chromatin domains are often in close juxtaposition to one another, and enhancer and Silencer elements operate over large distances to regulate the genes in these domains. The lack of promiscuity in the function of these elements suggests that active mechanisms exist to restrict their activity. Insulators are DNA elements that restrict the effects of long-range regulatory elements. Studies on different insulators from different organisms have identified common themes in their mode of action. Numerous insulators map to promoters of genes or have binding sites for transcription factors and like active chromatin hubs and silenced loci, insulators also cluster in the nucleus. These results bring into focus potential conserved mechanisms by which these elements might function in the nucleus.
