3,470 research outputs found
On acoustic propagation in three-dimensional rectangular ducts with flexible walls and porous linings
This is the post-print version of the Article. The official published version can be accessed from the links below - Copyright @ 2012 Acoustical Society of AmericaThe focus of this article is toward the development of hybrid analytic-numerical mode-matching methods for model problems involving three-dimensional ducts of rectangular cross-section and with flexible walls. Such methods require first closed form analytic expressions for the natural fluid-structure coupled waveforms that propagate in each duct section and second the corresponding orthogonality relations. It is demonstrated how recent theory [Lawrie, Proc. R. Soc. London, Ser. A 465, 2347–2367 (2009)] may be extended to a wide class of three-dimensional ducts, for example, those with a flexible wall and a porous lining (modeled as an equivalent fluid) or those with a flexible internal structure, such as a membrane (the “drum-like” silencer). Two equivalent expressions for the eigenmodes of a given duct can be formulated. For the ducts considered herein, the first ansatz is dependent on the eigenvalues/eigenfunctions appropriate for wave propagation in the corresponding two-dimensional flexible-walled duct, whereas the second takes the form of a Fourier series. The latter offers two advantages: no “root-finding” is involved and the method is appropriate for ducts in which the flexible wall is orthotropic. The first ansatz, however, provides important information about the orthogonality properties of the three-dimensional eigenmodes
Comments on a class of orthogonality relations relevant to fluid-structure interaction
Copyright @ 2011 Springer Science+Business Media B.V
Friction in inflaton equations of motion
The possibility of a friction term in the equation of motion for a scalar
field is investigated in non-equilibrium field theory. The results obtained
differ greatly from existing estimates based on linear response theory, and
suggest that dissipation is not well represented by a term of the form
.Comment: 4 pages, 2 figures, RevTex4. An obscurity in the original version has
been clarifie
On tuning a reactive silencer by varying the position of an internal membrane
A mode-matching method is used to investigate the performance of a two-dimensional, modified reactive silencer. The modification takes the form of a membrane which is attached to the internal walls of the expansion chamber parallel to the axis of the inlet/outlet ducts. The height of the membrane above the level of the inlet/outlet ducts can be varied and, by this means, the device is tuned. It is shown that the stopband produced by the silencer can be broadened and/or shifted depending upon the height to which the membrane is raised. Attention is focused on the efficiency of the device at low-frequencies - the regime where dissipative silencers are usually least effective. The potential use of the device as a component in a hybrid silencer for heating ventilation and air-conditioning (HVAC) ducting systems is discussed
Perturbative nonequilibrium dynamics of phase transitions in an expanding universe
A complete set of Feynman rules is derived, which permits a perturbative
description of the nonequilibrium dynamics of a symmetry-breaking phase
transition in theory in an expanding universe. In contrast to a
naive expansion in powers of the coupling constant, this approximation scheme
provides for (a) a description of the nonequilibrium state in terms of its own
finite-width quasiparticle excitations, thus correctly incorporating
dissipative effects in low-order calculations, and (b) the emergence from a
symmetric initial state of a final state exhibiting the properties of
spontaneous symmetry breaking, while maintaining the constraint . Earlier work on dissipative perturbation theory and spontaneous symmetry
breaking in Minkowski spacetime is reviewed. The central problem addressed is
the construction of a perturbative approximation scheme which treats the
initial symmetric state in terms of the field , while the state that
emerges at later times is treated in terms of a field , linearly related
to . The connection between early and late times involves an infinite
sequence of composite propagators. Explicit one-loop calculations are given of
the gap equations that determine quasiparticle masses and of the equation of
motion for and the renormalization of these equations is
described. The perturbation series needed to describe the symmetric and
broken-symmetry states are not equivalent, and this leads to ambiguities
intrinsic to any perturbative approach. These ambiguities are discussed in
detail and a systematic procedure for matching the two approximations is
described.Comment: 22 pages, using RevTeX. 6 figures. Submitted to Physical Review
Witnessing Macroscopic Entanglement in a Staggered Magnetic Field
We investigate macroscopic entanglement in an infinite XX spin-1/2 chain with
staggered magnetic field, B_l=B+e^{-i\pi l}b. Using single-site entropy and by
constructing an entanglement witness, we search for the existence of
entanglement when the system is at absolute zero, as well as in thermal
equilibrium. Although the role of the alternating magnetic field b is, in
general, to suppress entanglement as do B and T, we find that when T=0,
introducing b allows the existence of entanglement even when the uniform
magnetic field B is arbitrarily large. We find that the region and the amount
of entanglement in the spin chain can be enhanced by a staggered magnetic
field.Comment: Accepted for publication in Physical Review A, minor changes from
previous version. 5 pages, 3 figure
Synthetic Approaches to Complex Natural Coumarins
The first example of a successful para-Claisen rearrangement of a 1,1-dimethylallyl aryl ether has been realised. The rearrangement product, a natural coumarin, on methylation gave another natural coumarin, furopinnarin
Increasing pathology utilisation lies behind increasing pathology costs
Recent increases in pathology costs per scheme member are a concern to medical schemes and pathologists alike. To better understand the observed increasing costs, the National Pathology Group commissioned Prognosys to analyse the trends affecting these increases. We found that these increases are driven by inflation, increases in utilisation, and redistribution of the burden of cost. The identification of utilisation as a cost driver for pathology services is noteworthy as almost all pathology services are by referral from another doctor
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