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Modeling the Effect of Propagating Acoustic Noise over a Smooth-random Surface by Using the Equivalent Source Method and the Finite Element Method
Başlık:
Modeling the Effect of Propagating Acoustic Noise over a Smooth-random Surface by Using the Equivalent Source Method and the Finite Element Method
Yazar:
Ocansey, Daniel Teye, author.
ISBN:
9780355984064
Yazar Ek Girişi:
Fiziksel Tanımlama:
1 electronic resource (122 pages)
Genel Not:
Source: Dissertation Abstracts International, Volume: 79-10(E), Section: B.
Advisors: Marwan Bikdash Committee members: Kenneth Flurchick; Dukka KC; Janis Oldham; Guoqing Tang.
Özet:
Noise generated by large explosions at military bases causes discomfort to residents living in the vicinity, for up to 20km away. This noise explosion has strong low-frequency content and can travel long distances. A lot of theoretical and experimental work has been done to reduce this type of noise, but there are still many modeling and simulation difficulties due to the semi-indefinite domain. In this dissertation, we focus on three significant aspects of evaluating the outdoor propagation acoustic noise. First, an algorithm is proposed to locate the equivalent sources for a general smooth random or undulating surface. The standard method in solving such a problem is the Finite Element Method (FEM) which is implemented in COMSOL in this research. However, its memory allocation grows the mesh density cubically and its computational time grows super-cubically despite sparsity. We advocate the use of the Equivalent Source Method (ESM) for semi-definite domain which is also generalized to the case of a hard and soft randomly undulating boundary surface representing a reasonable approximation of many landscapes. The least-squares problem, derived from the boundary conditions, is used to determine the complex amplitudes of the equivalent sources. The acoustic field, caused by a source above an impedance boundary, is computed by using a superposition of equivalent point sources located below the surface properties adjusting and generalizing Ochmann's approach.
The proposed method is applied to and tested on various landscapes (namely: one-hump, two-hump, undulating, and smooth random surfaces). We compare the accuracy, memory requirement and computational time of the ESM and the FEM methods. The QR decomposition was used to solve the least-squares problem. We implemented the FEM in COMSOL.
To account for some of the environmental factors that affect the outdoor propagation noise, we examined the frequency-dependent attenuation of the atmosphere. Three models of frequency dependence of the atmospheric attenuation were examined (frequency-proportional, proportional to the absolute value of the frequency, and proportional to the square of frequency). In each case, we compute the effect of the attenuation on the Green's function, the propagation over a hard flat surface, both in the frequency and in the time domains. We also show that for sound frequencies and distances and in a uniform atmosphere, the method for calculation of the absorption of sound is best approximated by an attenuation model that is proportional to frequency squared. In all cases, the impact on the shape of the pulse is characterized but is found in general to be minimal. Curiously, a constant attenuation atmosphere is the most effective, while an attenuation coefficient that is proportional to the squared frequency is the least effective because this latter case favors high frequencies and ignores the low frequencies, which are the more important frequencies for long-distance atmospheric propagation.
Extensive simulations and comparison have shown that the ESM, augmented by Ochmann's term and the arbitrary surface interface that we develop is generally quite capable and accurate and is superior to the FEM method for a low-frequency semi-definite domain bounded by a reasonably smooth impedance surface.
Notlar:
School code: 1544
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Yer Numarası | Demirbaş Numarası | Shelf Location | Lokasyon / Statüsü / İade Tarihi |
---|---|---|---|
XX(679252.1) | 679252-1001 | Proquest E-Tez Koleksiyonu | Arıyor... |
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