• Journal of Korean Society of Coastal and Ocean Engineers
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• v.11 no.3
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• pp.135-140
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• 1999

To simulate the wave transformation by an energy dissipation region, a numerical model is suggested by discretizing the elliptic mild-slope equation. Generalized conjugate gradient method is used as solution algorithm to apply parabolic approximation to open boundary condition. To demonstrate the applicabil-ity of the numerical procedure suggested, the wave scattering by a circular damping region is examined. The feature of reflection in front of the damping region is captured clearly by the numerical solution. The effect of the size of dissipation coefficient is examined for a rectangular damping region. The recovery of wave height by diffraction occurs very slowly with distance behind the damping region.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.4 no.1
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• pp.34-44
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• 1992

A numerical model using Hybrid Element Method(HEM) is presented for the prediction of wave agitations in a harbor which are induced by the intrusion and transformation of incident short-period waves. A linear mild-slope equation including bottom friction is used as the governing equation and a partial absorbing boundary condition is used on solid boundaries. Functional derived in the present paper is based on the Chen and Mei(1974)'s concept which uses finite element net in the inner region and analytical solution of Helmholtz equation in the outer region. Final simultaneous equations are solved using the Gaussian Elimination Method. The model appears to be reasonably good from the comparison of numerical calculation with hydraulic experimental results of short-wave diffraction through a breakwater gap(Pos and Kilner, 1987). The problem of requring large computational memory could be overcome using 8-noded isoparametric elements.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.05a
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• pp.1062-1073
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• 2007

Acoustic Target Strength (TS) is a major parameter of the active sonar equation, which indicates the ratio of the radiated intensity from the source to the re-radiated intensity by a target. In developing a TS equation, it is assumed that the radiated pressure is known and the re-radiated intensity is unknown. This research provides a TS equation for polygonal plates, which is applicable to near field acoustics. In this research, Helmholtz-Kirchhoff formula is used as the primary equation for solving the re-radiated pressure field; the primary equation contains a surface (double) integral representation. The double integral representation can be reduced to a closed form, which involves only a line (single) integral representation of the boundary of the surface area by applying Stoke's theorem. Use of such line integral representations can reduce the cost of numerical calculation. Also Kirchhoff approximation is used to solve the surface values such as pressure and particle velocity. Finally, a generalized definition of Sonar Cross Section (SCS) that is applicable to near field is suggested. The TS equation for polygonal plates in near field is developed using the three prescribed statements; the redection to line integral representation, Kirchhoff approximation and a generalized definition of SCS. The equation developed in this research is applicable to near field, and therefore, no approximations are allowed except the Kirchhoff approximation. However, examinations with various types of models for reliability show that the equation has good performance in its applications. To analyze a general shape of model, a submarine type model was selected and successfully analyzed.

• Journal of KSNVE
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• v.8 no.1
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• pp.146-156
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• 1998

To mitigate excessive noise from highways, and high speed rail road, it is often necessary to construct a noise barrier. Absorptive barroer attenuation solution is obtained for the problem of diffration of a plane wave sound source by a semi-infinite plane. A finite region in the vicinity of the edge has an highly absorbing boundary condition ; the remaining portion of the half plane is rigid. The problem which is solved is a mathematical model for a hard barrier with an absorbing edge. If the wavelength of the sound is much smaller than the length scale associated with the barrier, the diffraction process is governed to all intents and purpose by the solution to a standard problem of diffraction by a semi-infinite hard plane with an absorbent edge. It is concluded that the absorbing material that comprises the edge need only be of the order of a wavelength long to have approximately the same effect, on the sound attenuation in the shadow side of the barrier. Traffic noise is composed of thousands of sources with varying frequency content. To simplify noise predictions when barriers are present, an effective frequency of 550Hz may be used to represent all vehicles. The wavelength of sound at f=550Hz for traffic noise is about 2 feet. According to the above conclusion, an absorptive highway noise barrier is only needed to cover to cover approximately a 2 foot length of absorbing material. It would be more economical to cover only the region in the immediate vicinity of the edge with highly sound obsorbent material.