• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2913-2917
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• 2007

Turbocharger has been widely used in many passenger cars in application with diesel engines because of high power and fuel efficiency. However, flow-induced noise (whoosh or hissing noise) which is generated within the compressor during its operation at marginal surge line can deteriorate noise characteristics. Hissing noise excitation was associated with the generation of turbulence within the turbocharger compressor and radiated through the transmission path in turbocharger system. In this study, a sharp-edged reactive-type muffler was devised and installed in the transmission path to reduce the hissing noise. Acoustic and fluid dynamic characteristics for the muffler were investigated which is related to the unsteadiness of turbulence and pressure in turbocharger system. A transfer matrix method was used to analyze the transmission loss of the muffler. Simple expansion muffler with extended tube of the reactive type is proposed for the reduction of high frequency component noise. Turbulence computation was carried out by a standard ${\kappa}-{\varepsilon}$ model. An optimal design condition of the muffler was obtained by extensive acoustic and fluid dynamic analysis on the engine dynamometer with anechoic chamber. A significant reduction of the hissing noise was achieved at the optimal design of the muffler as compared with the conventional turbocharger system.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.2
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• pp.79-84
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• 2009

Turbocharger has been widely used in many passenger cars in application with diesel engines because of high power and fuel efficiency. However, flow-induced noise (whoosh or hissing noise) which is generated within a compressor during its operation at marginal surge line can deteriorate noise characteristics. Hissing noise excitation is associated with the generation of turbulence within the turbocharger compressor and radiated through the transmission path in a turbocharger system. In this study, a expansion muffler with lids is devised and installed in the transmission path to reduce the hissing noise. Acoustic and fluid dynamic characteristics for the muffler are investigated which are related to the unsteadiness of turbulence and pressure in the turbocharger system. A transfer matrix method is used to analyze the transmission loss of the muffler. A simple expansion muffler with lids is proposed for the reduction of high frequency component noise. Turbulence simulation is carried out by a standard k - ${\varepsilon}$ model. An optimal design condition of the muffler is obtained by extensive acoustic and fluid dynamic analysis on the engine dynamometer with anechoic chamber. A significant reduction of the hissing noise is achieved at the optimal design of the muffler as compared with the conventional muffler.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1994.10a
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• pp.323-330
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• 1994

본 연구에서는 다공질 흡음재료 중 closed cell 구조를 갖는 발포수지재료(Foamed Material)에 음(sound)이 입사할 때 발생하는 흡음현상을 보다 정확히 예측하기 위해서 다공질 흡음재료에 대한 Biot 이론에 근거한 Allard의 모델링기법[4]을 이용하여 해석 프로그램을 개발하였고, 이를 이용하여 다공질 흡음재료가 단층(single layer)일 때 이 재료의 Surface Impedance와 흡음률(Absorption Coefficient)을 예측하고, 물성치(parameters)변화에 따른 다공질 흡음재료의 흡음특성을 분석하였으며, 이 재료가 자동차 제조시 사용되는 압연강판(rolled steel piate)에 부착되었을 때의 투과손실(Transmission Loss)을 예측하였고, 또한 다공질 흡음재료는 중고주파 대역의 음에 대한 흡음특성은 좋지만 저주파 대역의 음에 대한 흡음특성은 좋지 않으므로 흡음 재료의 저주파 대역의 흡음특성을 향상시키기 위해서 2층(two layers)으로 하였을 때의 흡음특성을 분석하였다. 본 논문의 연구결과는 자동차 제조시 사용되어지는 다공질 흡음재료는 물론 산업용기계나 건축용등 여러 분야에서 사용되어지는 다공질 흡음재료의 흡음특성 분석에 응용될 수 있으리라 기대된다.

• Journal of KSNVE
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• v.9 no.4
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• pp.687-692
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• 1999

Digital holographic reconstruction method [W. A. Veronesi, et al, 1989, JASA, Vol. 85, pp. 588∼598] reconstructs a whole sound field by using measured sound pressure. This paper reports numerical errors of the method that occur at specific frequencies. The errors occur due to the truncation errors included in the calculation of transfer matrixes. The frequencies of the errors depend on the size of boundary element surfaces. Moreover, a modified calculation technique is proposed in the paper. The technique prevents the truncation errors by employing an indirect calculation procedure.

• Advances in aircraft and spacecraft science
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• v.6 no.6
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• pp.463-478
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• 2019

The present work investigates the effect on the flow-induced vibrations of the lay-up sequence of composite laminated axisymmetric structures, using an hybrid approach based on a wave finite element and a transfer matrix method. The structural vibrations, under deterministic distributed pressure loads, diffuse acoustic field and turbulent boundary layer excitations, are analysed and compared. A multi-scale approach is used for the dynamic analysis of finite structures, using an elementary periodic subsystem. Different flow regimes and shell curvatures are analysed and the computational efficiency is also discussed.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2007.04a
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• pp.309-314
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• 2007

Optimal layer sequencing of a multi-layered acoustical foam is solved to maximize its sound transmission loss. A foam consisting of air and poroelastic layers can be optimized when a limited amount of a poroelastic material is allowed. By formulating the sound transmission loss maximization problem as a one dimensional topology optimization problem, optimal layer sequencing and thickness were systematically found for several frequencies. For optimization, the transmission losses of air and poroelastic layers were calculated by the transfer matrix derived from Biot's theory. By interpolating five intrinsic parameters among several poroelastic material parameters, dear air-poroelastic layer distributions were obtained; no filtering or post-processing was necessary. The optimized foam layouts by the proposed method were shown to differ depending on the frequency of interest.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.11a
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• pp.731-736
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• 2000

The recently developed BEM-based NAH(nearfield acoustical holography) is a useful technique for identifying the sound source of vibrating objects. The acoustic parameters of a sound source can be reconstructed by using the vibro-acoustic transfer matrix, which is determined by means of BEM, and the sound pressure measured in the nearfield. Theoretically, one can come up with a very nice reconstructed result as the field plane gets near to the source surface. However, when a microphone is placed in the very close nearfield of the source surface, the scattering, reflection, or resonance in the gap between the source and the microphone can distort the acoustic field, and therefore, the measured field pressure would differ from the actual one in the absence of the microphone. In order to analyze this problem, the interference effect of the microphone is numerically calculated by using the nonsingular BEM that yields very small error in the nearfield. From this analysis, it is found that the prediction error of the field pressure decreases firstly and then increases as the microphone approaches the vibrating surface from the farfield to the close nearfield. It is noted that the microphone should be separated from the source surface by at least a diameter of the microphone for an error ratio less than 2% in the low frequency range less than about 2.7kHz. This means that if one wants to put a microphone in the very close nearfield. a microphone with small diameter should be used.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.10a
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• pp.821-832
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• 2011

Recently, the noise of vehicle is the one of the key factors for customers to purchase a vehicle and the most important part which is related to the noise is the exhaust system. Thus, car makers have their own ways to assess this exhaust noise not only to decrease the level of noise but to enhance the feeling of it. Typically, to do these things in the early stage of development, the tuning code of the exhaust system has to be made by CAE tool, which is very reliable but expensive, and the prototype parts of this code would be made for the validation test. Then this process can be iterated to meet the target of the performance. In this study, a new algorithm which adapts the '3 dimensional convective sound wave theory 'and 'Doppler effect' has been developed. With this new algorithm, a brand new system for the calculation of tail pipe noise has been developed and validated by acoustic wind tunnel test. As a result of this study, various comparisons and have been carried out, for example, the comparison with other CAE tool has been performed for the validity and the improvement of the new calculation code could be achieved.

• The Journal of the Acoustical Society of Korea
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• v.20 no.1
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• pp.13-20
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• 2001

Adaptive digital filters with long impulse response such as acoustic echo canceller and active noise controller suffer from slow convergence and computational burden. Subband techniques and multirate signal processing have been recently developed to improve the problem of computational complexity and slow convergence in conventional adaptive filter. Any FIR transfer function can be realized as a serial connection of interpolators followed by subfilters with a sparse impulse response. In this case, each interpolator which is related to the column vector of Hadamard matrix has band-pass magnitude response characteristics shifted uniformly. Subband technique using Hadamard transform and decimation of subband signal to reduce sampling rate are adapted to system modeling and acoustic noise cancellation In this paper, delayless subband structure with nonuniform bandwidth has been proposed to improve the performance of the convergence speed without aliasing due to decimation, where input signal is split into subband one using tree-structured filter bank, and the subband signal is decimated by a decimator to reduce the sampling rate in each channel, then subfilter with sparse impulse response is transformed to full band adaptive filter coefficient using Hadamard transform. It is shown by computer simulations that the proposed method can be adapted to general adaptive filtering.

• Proceedings of the KIPE Conference
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• 1998.10a
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• pp.961-965
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• 1998

The rapid development of small-scale (1-10 MJ) Superconducting Magnetic Energy Storage Systems (SMES) can be explained by real perspective of practical implementation of these devices in electro power nets. However the serious problem of all high mechanically stressed superconducting coils-problem of training and degradation (decreasing) of operating current still exists. Moreover for SMES systems this problems is more dangerous because of pulsed origin of mechanical stresses-one of the major sources of local heat disturbances in superconducting coils. We investigated acoustic emission (AE) phenomenon on model and 0.5 MJ SMES coils taking into account close correlation of AE and local heat disturbances. Two-coils 0.5 MJ SMES system was developed, manufactured and tested at Russian Research Center in the frames of cooperation with Korean Electrical Engineering Company (KEPCO) [1]. The two-coil SMES operates with the stored energy transmitted between coils in the course of a single cycle with 2 seconds energy transfer time. Maximum operating current 1.55 kA corresponds to 0.5 MF in each coil. The Nb-Ti-based conductor was designed and used for SMES manufacturing. It represents transposed cable made of Nb-Ti strands in copper matrix, several cooper strands and several stainless steel strands. The coils are wound onto fiberglass cylindrical bobbins. To make AE event information more useful a real time instrumentation system was used. Two main measured and computer processed AE parameters were considered: the energy of AE events (E) and the accumulated energy of AE events (E ). Influence of current value in 0.5 MJ coils on E and E was studied. The sensors were installed onto the bobbin and the external surface of magnets. Three levels of initial current were examined: 600A, 1000A, 2450 A. An extraordinary strong dependence of the current level on E and E was observed. The specific features of AE from model coils, operated in sinusoidal vibration current changing mode were investigated. Three current frequency modes were examined: 0.012 Hz, 0.03 Hz and 0.12 Hz. In all modes maximum amplitude 1200 A was realized.