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구조물 진동.소음의 수치해석시 최적 요소크기는 .lambda./4이다.
김정태,강준수,Kim, Jeung-Tae,Kang, Jun-Soo 대한기계학회 1997 大韓機械學會論文集A Vol.21 No.11
An engineering goal in vibration and noise professionals is to develope quiet machines at the preliminary design stage, and various numerical techniques such as FEM, SEA or BEM are one of the schemes toward the goal. In this paper, the research has been focused on the sensitivity effect of mesh sizes for FEM application so that the optimum size of the mesh that leads to engineering solution within acceptable computing time could be generated. In order to evaluate the mesh size effect, three important parameters have been examined : natural frequencies, number of modes and driving point mobility. First, several lower modes including the fundamental frequency of a 2-D plate structure have been calculated as mesh size changes. Since theoretical values of natural frequencies for a simple structure are known, the deviation between the numerical and theoretical values is obtained as a function of mesh size. The result shows that the error is no longer decreased if the mesh size becomes a quarter wavelength or smaller than that. Second, the mesh size effect is also investigated for the number of modes. For the frequency band up to 1.4 kHz, the structure should have 38 modes in total. As the mesh size reaches to the quarter wavelength, the total count in modes approaches to the same values. Third, a mobility function at the driving point is compared between SEA and FEM result. In SEA application, the mobility function is determined by the modal density and the mass of the structure. It is independent of excitation frequencies. When the mobility function is calculated from a wavelength to one-tenth of it, the mobility becomes constant if the mesh becomes a quarter wavelength or smaller. We can conclude that dynamic parameters, such as eigenvalues, mode count, and mobility function, can be correctly estimated, while saving the computing burden, if a quarter wavelength (.lambda./4) mesh is used. Therefore, (.lambda./4) mesh is recommended in structural vibration analysis.
김정태,조성호,Kim, Jeung-Tae,Cho, Sung-Ho 한국철도학회 1999 한국철도학회논문집 Vol.2 No.1
Many residential areas are situated near to railroad tracks so that a railroad noise has been one of the major environmental issues. In this paper two important aspects have been investigated in order to properly evaluate the railroad vehicle noise : sound power levels for different types and sound propagation characteristics of the railroad vehicles. For noise source characteristics of railroad vehicles, sound power values for various types of trains that are in active service have been measured. In this paper, domestic railroad vehicles are measured and compared with high speed train(TGV). Based on sound power information of railway vehicles, prediction on the sound pressure level and equivalent noise level near to railway areas have been evaluated.
김정태(Kim Jeung-Tae),전형옥(Jeon Hyueng-Wook) 한국철도학회 1998 한국철도학회 학술발표대회논문집 Vol.- No.-
A Systematic approach to estimate an interior noise level for a railway passenger car has been proposed. The prediction is based on the sound power values obtained from measured sound pressure lever, when a high speed train runs at 300 ㎞/hr. Then, the exterior sound pressure value is calculated by using the BEM code. After that, an interior sound lever is estimated, considering the transmission loss of body structures and absorption effect inside of the train. In this application, the estimated noise level is between 66 ㏈A and 74 ㏈A. The proposed approach could be useful for rough estimation of a noise level inside a passenger car at the design stage, although the method has some limitation to be implement for a general situation.