• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
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• 2003.08a
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• pp.53-57
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• 2003

불규칙파를 사용하여 설계 자료로 이용하기 위해서는 설계해역에서 불규칙파의 파랑변형을 예측할 수 있는 수치모형의 개발이 선행되어야 한다. 비선형 불규칙파의 거동을 해석할 수 있는 Boussinesq 방정식은 상대파고인 $\alpha$/h($\alpha$는 수면의 진폭, h는 수심임)를 비선형의 매개변수로 하고 상대수심인 kh(k는 파수임)를 분산성의 매개변수로 하여 섭동법을 사용하여 유도된다. Boussinesq 식은 수심이 일정한 경우에 Boussinesq(1872)가 비선형 항을 O($\alpha$/h,(kh)$^2$)까지 포함하여 처음으로 개발하였고 수심의 변화가 완만한 경우에 Peregrine(1967)이 개발하였다. (중략)

• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
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• 2003.08a
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• pp.240-244
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• 2003

파랑의 변형 가운데 천수, 굴절, 회절, 반사를 예측하는 수학적 모형은 크게 두 가지 유형으로 나눌 수 있는데, 첫 번째로 파형경사인 ha(k：파수. $\alpha$：진폭)를 비선형의 매개변수로 하는 Stokes 파랑식이 있고, 두 번째로 상대파고인 $\alpha$/h를 비선형의 매개변수로 하고 상대수심인 kh를 분산성의 매개변수로 하는 천수방정식(Shallow water equation)이 있다. 파랑의 변형 가운데 천수, 굴절만을 예측하고 회절, 반사를 예측하지 못하는 수학적 모형으로는 에너지 이송방정식이 있다. (중략)

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.11 no.4
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• pp.201-215
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• 1999

Field measurements were made for long- and short-period waves and current velocities at the harbor mouth using pressure-type wave gauges and a current meter, respectively, at the Gamcheon Harbor which has a rectangular shape with a narrow entrance. The measured pressure data were subjected to spectral analyses after removing tidal effects by applying trend removal and high-pass filtering. For the band averaging of the raw spectra, in order to obtain good resolution over the entire frequency, instead of a constant band width, variable band widths were used, which gradually increase as marching from the lowest frequency towards higher frequencies. The Helmholtz resonance mode at the Gamcheon Harbor shows the relative amplification ratio of 9.2 at the wave period of 31.7 minutes, which is quite large compared with those at the harbors located on the east coast of Korea. The second and the third resonance period was 10.3 and 5.4 minute, respectively. On the other hand, the analysis of every 24 hours data shows that during storms the spectral densities are very large compared to those during calm seas and also the second and third resonances are predominant.

• The Journal of the Acoustical Society of Korea
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• v.18 no.3
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• pp.55-61
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• 1999

In this study, we have constructed the acoustic microscope using quadrature technique and analyzed the relative variation of image intensity and the quality of image by reconstructing the amplitude and phase image for surface defects with tiny hight variation. In this experiment, we have constructed the scanning acoustic microscope using the focused transducer with 3㎒ center frequency and the quadrature detector. And we have fabricated aluminum samples with round defects whose depth is different and reconstructed the amplitude and phase images for the samples. One sample has round defects with 2㎜ diameter and 100㎛ depth and the other has round defects with 4㎜ diameter and 5㎜ depth. In the result of line scanning for the sample with 100㎛ round defects, it has been shown that the variation rate of amplitude image intensity is 7% and the variation rate of phase image intensity is 89%. The phase image has better contrast than amplitude image for the sample. In contrast to this, the amplitude image has better contrast than phase image for the sample with 5㎜ depth's defects. Accordingly there is big difference between amplitude image and phase image for depth variation of defects whose boundary is 1 wavelength. Consequently the acoustic microscope using quadrature detector can be evaluated efficiently more than using envelope detector, for detecting defects which have height variation less than 1 wavelength. And also the phase image and the amplitude image can be used for detecting defects of tiny height variation with complimentary relation.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.1
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• pp.131-136
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• 1988

This study is a research on the dynamic characteristics of non-linear dynamic vibration absorber in which harmonic motion is applied to the foundation of the main system. The amplitude ratio of the system with non-linear dynamic vibration absorber was obtained by harmonic balance methods and the unstable region was determined by stability analysis. As a result of study, the amplitude ratio decreases as mass ratio increases.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.13 no.3
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• pp.209-229
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• 2001

Simultaneous field measurements of short-period and long-period waves were made at five stations inside or outside Okkye Harbor, which is located in the east coast of Korea. Based on the measured data, spacial and temporal variations of the long-period wave energy were examined. Three smoothing methods were examined for the spectral estimates: fixed interval averaging method, incremental interval averaging method, and moving averaging method. It was shown that a proper smoothing method should be chosen depending on the period of first resonant mode and the length of data being used. By comparing the results obtained using the long-term data with those obtained using two-day data, we showed that it is necessary to analyze the data of calm seas and storm seas separately. The Helmholtz resonant period in Okkye Harbor was found to be about 9.6 minutes with its relative amplification ratio of 9 to 10, and local amplifications were apparent at the periods of 1.2 to 1.3 minutes and 0.7 minute. During calm seas, both at the harbor entrance and inside the harbor the energy of the waves of 9 minutes or longer period was larger than the infra-gravity wave energy by more than 100 times. However, during storm seas the energy level was very high all over the period band, and local amplification was larger than that during calm seas by more than 100 times, especially inside the harbor, Finally it was shown that the energies of the Helmholtz resonant mode and the infra-gravity waves of 1 to 2 minutes are proportional to the storm wave height.

• The KIPS Transactions:PartC
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• v.12C no.1 s.97
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• pp.63-68
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• 2005

In this paper, performance of a maximum-likelihood modulation classification for quadrature amplitude modulation (QAM) is studied. Unlike previous works, the relative classification performance with respect to the available modulations and performance limit with single-sample observation are presented. For those purposes, all constellations are set to have the same minimum Euclidean distance between symbols so that a smaller constellation is a subset of the larger ones. And only one sample of received waveform is used for multiple hypothesis test. As a result, classification performance is improved with increase in signal-to-noise ratio in all the experiments. Especially, when the true modulation format used in the transmitter is 4 QAM, almost perfect classification can be achieved without any additional information or observation samples. Though the possibility of false classification due to the symbols shared by subset constellations always exists, correct classification ratio of $80{\%}$ can be obtained with the single-sample observation when the true modulation formats are 16 and 64 QAM.

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.6
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• pp.490-500
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• 2000

A new technique for the source location of acoustic emission (AE) in plates whose thichness are close to or thinner than the wavelength has been studied by introducing wavelet transform de-noising technique. The detected AE signals were pre-processed using wavelet transform to be decomposed into the low-frequency, high-amplitude flexural components and the high-frequency, low-amplitude extensional components. If the wavelet transform de-noising was employed, we could successfully filter out the extensional wave component, one of the critical errors of source location in plates by arrival time difference method. The accuracy of source location appeared to be significantly improved and independent of the setting of gain and threshold, plate thickness, sensor-to-sensor distance, and the relative position of source to sensors. Since the method utilizes the flexural component of relatively high amplitude, it could be applied to very large, thin-walled structures in practice.

• Geotechnical Engineering
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• v.11 no.4
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• pp.37-48
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• 1995

The dynamic behavior of dry sand under different vibration conditions is studied through laboratory experiments. Sinusoidal and random vibration experiments of sand are carried out in vertical direction under various surcharge loads. Five different sand samples are selected for the azperiment. They are composed of four different -size samples of particles and one sample which is simulated the field condition. In case of sinusoidal vibration, the change in relative density is measured with acceleration levels. To produce an acceleration, the vibration amplitude is maintained within the range of 0.4mm~0.6mm and the vibration frequency is changed within the range of 3Hz~40Hz. In case of random vibration, the combined sinusoidal acceleration is produced by a random vibration generator and the change in relative density is measured by an accelerometer. Based on the experimental results, it is found that the sandy soil is compacted to 94%~99% of relative density by vertical acceleration and the peak acceleration producing the maximum relative density is proportional to the difference between maximum and minimum void ratios. It is also found that the effect of surcharge loading : the greater the surcharge loading, the larger the change in relative density and the greater the acceleration required to change the relative density.

• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.2
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• pp.162-170
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• 1984

대부분의 동적댐퍼들은 주구조물의 진동진폭을 정해진 주파수 변위내에서 최대로 줄이는 것을 목 표로 한다. 그러나 공작기계의 안정성은 시편과 공구사이의 상대변위와 절삭력에 의해 결정되는 전달함수의 최대크기에 의해서보다는 실수부분의 최소치에 의해 결정된다는 것이 잘 알려져 있 다. 본 논문에서는 이 사실에 착안하여 공작기계에서 발생하는 채터를 흡수하기 위한 최적의 댐 퍼를 설계하는 절차를 제시하고 1 자유도로 대표될 수 있는 구조물의 경우에 대하여 구체적인 방 법을 예시하였다. 종래의 최적 댐퍼의 성질을 구하는 방법에 비해 수학적인 절차가 약간 복잡해 지기는 하나 전산기를 이용하여 큰 어려움이 없이 최적의 설계자료를 얻을 수 있다. 댐퍼 질량이 정해졌을 때 감쇠율과 스프링 계수를 변수로 하는 목표함수가 하나의 식으로 유도될 수 없기 때 문 에 간단한 최적화 방법으로 이변수 황금분할법을 사용하였다. 수치적인 예를 통하여 종래의 다른 방법에 의한 결과와 비교하고 제안된 방법론의 타당성을 입증하였다.