• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.05a
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• pp.107-110
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• 2009

Combustion oscillations are caused by a coupling between acoustic waves and unsteady heat release. They can be eliminated using passive controller such as a helmholtz resonator. But, helmholtz resonator is normally only effective over a narrow frequency range. In this work, helmholtz resonator is applied for reducing the combustion oscillations and we vary the helmholtz resonator volume using piston in oder to tune in the wide range of operating conditions. As the result, it is found that the dominant combustion oscillations can be reduced by optimizing the size of resonator volume. Also, from these results, we investigate feasibility of actively tuned passive control

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1990.10a
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• pp.47-50
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• 1990

종래에는 공조덕트의 소음 감소를 위하여 주로 흡음재료를 사용하여 왔다. 그러나 이러한 흡음재료를 이용한 소음감소대책은 500HZ 이상의 고주파 영 역에서는 효과가 있으나 500HZ 이하의 저주파 영역에서는 효과가 적다. 이 러한 500HZ이하의 저주파 소음은 각종 구조물의 진동을 야기시켜 정밀작업 이 요구되는 작업환경을 파괴하고, 심지어는 구조물에 손상을 입힐수도 있 다. 또한 이러한 저주파 소음에 노출된 사람의 심리를 불안하게 하여 일의 효율을 떨어뜨릴 뿐만 아니라 청력장애를 일으키기도 한다. 또한 청정환경이 요구되는 곳에서의 소음감소를 위하여 흡음재료를 사용할 경우 흡음재료로 부터 떨어져 나오는 미세분에 의해 청정환경의 보존이 어려워진다. 이러한 흡음재료를 이용한 수동소음제어 대책의 단점을 해결하기 위하여 소음을 그 소음과 동일한 진폭을 갖으며 위상차가 180도인 인위적인 음파로 상쇠시키 는 능동소음제어대책의 개발이 요구되어 왔다. 본 논문은 덕트 또는 이와 유 사한 음향환경내에서의 적응필터기법(adaptive filtering technique)을 이용한 능동소음제어방법의 개발, 실제구현 및 성능평가에 관한 것으로서, 특히 다 수의 에러 마이크로폰과 다수의 소음제어용 스피커를 사용하여 제어계통의 가관측성(Observability)및 가제어성(Controllability)을 향상시켜 평면파로 전 달되는 소음의 감소를 증대시킬수 있을 뿐만 아니라 고차모드(Higher-order mode)로 전달되는 소음의 제어도 가능케하는 능동소음제어 방법에 관하여 논하였다.

• Korean Journal of Optics and Photonics
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• v.16 no.6
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• pp.490-493
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• 2005

We have experimentally studied supercontinuum generation in a photonic crystal fiber as a function of pump wavelength and intensity with 100 fs pulsewidth. A supercontinuum over 750 nm spectral width with amplitude variation less than 10 dB has been achieved. It was generated by coupling femtosecond pulses from a mode-locked Ti:Sapphire laser into a 2 m long photonic crystal fiber. Adjusting the parameters of the pump source, it was also possible to control different spectral features of the supercontinuum radiation.

• Korean Journal of Optics and Photonics
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• v.14 no.6
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• pp.600-605
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• 2003

We have experimentally demonstrated a Direct Optical Switching (DOS) CDMA for future wide-band mobile communication systems at the 40 ㎓ band by using orthogonal (crosscorrelation $\leq$2) unipolar type codes with code length of 16 and chip rate of 2.5 Gcps for radio-over-fiber (RoF) systems. Pulse-amplitude-equalized 40 ㎓ laser pulses were provided by rational-harmonically mode-locking a 10 ㎓ fiber ring laser.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.21 no.6
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• pp.573-582
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• 2010

In this paper, X-band transceiver for high resolution wideband SAR systems is designed and fabricated. Also as a technique for enhancing the performance, error compensation algorithm is presented. The transceiver for SAR system is composed of transmitter, receiver, switch matrix and frequency generator. The receiver especially has 2 channel mono-pulse structure for ground moving target indication. The transceiver is able to provide the deramping signal for high resolution mode and select the receive bandwidth for receiving according to the operation mode. The transceiver had over 300 MHz bandwidth in X-band and 13.3 dBm output power which is appropriate to drive the T/R module. The receiver gain and noise figure was 39 dB and 3.96 dB respectively. The receive dynamic range was 30 dB and amplitude imbalance and phase imbalance of I/Q channel was ${\pm}$0.38 dBm and ${\pm}$3.47 degree respectively. The transceiver meets the required electrical performances through the individual tests. This paper shows the pulse error term depending on SAR performance was analyzed and range IRF was enhanced by applying the compensation technique.

• Geophysics and Geophysical Exploration
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• v.8 no.1
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• pp.41-49
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• 2005

We have developed a random heterogeneous velocity model with bimodal distribution in methane hydrate-bearing Bones. The P-wave well-log data have a von Karman type autocorrelation function and non-Gaussian distribution. The velocity histogram has two peaks separated by several hundred metres per second. A random heterogeneous medium with bimodal distribution is generated by mapping of a medium with a Gaussian probability distribution, yielded by the normal spectral-based generation method. By using an ellipsoidal autocorrelation function, the random medium also incorporates anisotropy of autocorrelation lengths. A simulated P-wave velocity log reproduces well the features of the field data. This model is applied to two simulations of elastic wane propagation. Synthetic reflection sections with source signals in two different frequency bands imply that the velocity fluctuation of the random model with bimodal distribution causes the frequency dependence of the Bottom Simulating Reflector (BSR) by affecting wave field scattering. A synthetic cross-well section suggests that the strong attenuation observed in field data might be caused by the extrinsic attenuation in scattering. We conclude that random heterogeneity with bimodal distribution is a key issue in modelling hydrate-bearing Bones, and that it can explain the frequency dependence and scattering observed in seismic sections in such areas.

• Journal of the Korea institute for structural maintenance and inspection
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• v.26 no.4
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• pp.30-39
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• 2022

Maintaining structural integrity of major apparatuses in a nuclear power plant, including piping system, is recognized as a critical safety issue. The integrity of piping system is also a critical matter related to the safety of a nuclear power plant. The actual failure mode of a piping system due to a seismic load is the leakage due to a fatigue crack, and the structural damage mechanism is the low-cycle fatigue due to large relative displacement that may cause plastic deformation. In this study, in-plane cyclic loading tests were conducted under various constant amplitudes using specimens composed of steel straight pipes and a steel pipe tee in the piping system of a nuclear power plant. The loading amplitude was increased to consider the relative displacement generated in the piping system under seismic loads, and the test was conducted until leakage, which is the limit state of the steel pipe tee, occurred due to fatigue cracks. The limit state of the steel pipe tee was expressed using a damage model based on the damage index that used the force-displacement relationship. As a result, it was confirmed that the limit state of the steel pipe tee can be quantitatively expressed using the damage index.

• Journal of Korean Society of Steel Construction
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• v.29 no.2
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• pp.123-134
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• 2017

Ultimate axial strength of longitudinally stiffened cylindrical steel shells for wind turbine tower was investigated by applying the geometrically and materially nonlinear finite element method. The effects of radius to thickness ratio of shell, shape and amplitude of initial imperfections, area ratio between effective shell and stiffener, and stiffener spacing on the ultimate axial strength of cylindrical shells were analyzed. The ultimate axial strengths of stiffened cylindrical shells by FEA were compared with design buckling strengths specified in DNV-RP-C202. The shell buckling modes obtained from a linear elastic bifurcation FE analysis as well as the weld depression during fabrication specified in Eurocode 3 were introduced in the nonlinear FE analysis as initial geometric imperfections. The radius to thickness ratio of cylindrical shell models was selected to be in the range of 50 to 200. The longitudinal stiffeners were designed according to DNV-RP-C202 to prevent the lateral torsional buckling and local buckling of stiffeners.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.275-278
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• 2006

Systematic experiments in $CH_4/Air$ counterflow diffusion flames diluted with He have been undertaken to study the oscillatory instability in which lateral heat loss could be remarkable at low global strain rate. The oscillatory instability arises for Lewis numbers greater than unity and occurs near extinction condition. The dynamic behaviors of extinction in this configuration can be classified into three modes; growing, harmonic and decaying oscillation mode near extinction. As the global strain rate decreases, the amplitude of the oscillation becomes larger. This is caused by the increase of lateral heat loss which ran be confirmed by the reduction of lateral flame size. Oscillatory edge flame instabilities at low global strain rate are shown to be closely associated with not only Lewis number but also heat loss (radiation and lateral heat loss).

• Journal of Navigation and Port Research
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• v.27 no.5
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• pp.519-526
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• 2003

For preliminary structural analysis of superstructures on very large floating structures(VLFS), superstructures are analyzed considering elastic deformations of barge type lower-structures subjected to wave loads. In this case, to consider the effect of wave loads on the superstructure, initial displacements at the support points of superstructures are evaluated as input data for the analysis. However, the evaluation and application of displacement loads are tedious and very time-consuming processes. Therefore, this paper proposes a simplified static analysis method to analyze the structural behaviors of superstructures on very large floating structures subjected to wave loads. In this study, the member forces due to the variation of beam span and the amplitude and period of wave load are analyzed by using an example 4 span -3 story structure and the amplification factors for beam moments are represented by the specific regression equation.