• The Journal of the Acoustical Society of Korea
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• v.20 no.4
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• pp.94-100
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• 2001

In October 1997, low-frequency reverberation was measured at an experimental site off the west coast of the Jeju island using the explosive charge, Signals Underwater Sound (SUS). Received signals were separated into the noise, the reflection, and the scattering region, and then were analyzed for the spectral and statistical characteristics of each region. In the analysis of the spectrum we verified that each region had a unique frequency band and statistical characteristics as well. The results of this analysis showed that the real and imaginary portions were shown to be both normal distributions in each frequency bin. The reverberation envelope had a Rayleigh distribution and the phase had a uniform distribution.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.23 no.7
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• pp.822-830
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• 2012

Clutters are all unwanted radar returns to affect on detection of targets. Radar clutter is characterized by amplitude distributions, spectrum, etc. Clutter is modelled with considering these kinds of characteristics. In this paper, a Weibull distribution function approximated by uniform distribution function is suggested. Weibull distribution function is used to model the various clutters. This paper shows that the data generated by the approximated solution of Weibull distribution function satisfy the Weibull probability density function. This paper shows that the data generation time of approximated Weibull distribution function solution is reduced by 20 % compared with the generation time of original Weibull probability density function.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2001.10a
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• pp.180-184
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• 2001

본 연구에서는 반잠수식 Rig의 형상에 따른 최적 형상을 검토하기 위하여 최근 건조되고 있는 4-Column과 2-Lowhull 을 가진 원통형 Column과 4각 Column형을 공시모형으로 택하여 규칙파중 두 구조물에 작용하는 유체력과 유체력에 의한 운동 응답을 계산, 형상에 따른 유체 역학적 특성과 동요 진폭의 관계를 검토하였다. 수치계산은 3차원 특이점 분포법을 이용하여 정상상태에 대하여 실행하였다. 구조물의 형상과 역학적인 간섭의 영향을 이용, 보다 성능이 우수한 형상을 얻을 수 있음을 확인하였다.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2003.05a
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• pp.128-131
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• 2003

본 논문에서는 채널의 진폭 특성만을 고려한 실내 무선 페이딩 환경에서 PPM 변조된 UWB 통신 시스템의 채널 성능을 분석하였다. 페이딩 채널은 기존에 발표되었던 UWB 전파 실험을 통한 데이터를 바탕으로 Nakagami-m분포 모델을 이용하여 페이딩 지수 m에 따른 다양한 채널 환경을 고려하였다. 또한, 채널 성능을 평가하기 위해 신호대 잡음비, 페이딩 지수 m, 그리고 시스템 성능 파라미터인 펄스반복개수(Ns)를 이용하여 다양한 페이딩 환경에서의 UWB 시스템 성능을 분석하였다.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.4
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• pp.376-381
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• 2013

위상잠금 열화상에서는 일반적으로 변조된 주파수의 광램프를 쓰고 있다. 하지만 램프의 광도 분포는 입력신호가 평단(flat)특성임에 불구하고 심지어 불균일하여 검사 시편내에 측면 열유동을 만들어낸다. 이러한 열유동은 원치 않는 효과로서 측면의 분해능을 감소시키는 등과 같이 관심 결함구조의 영상에 부정적인 영향을 미친다. 본 고에서 검토되는 방식은 열원으로서 LCD 프로젝터와 같은 것을 이용하여 각 가진픽셀에 개별적으로 진동진폭, 광도 오프셋, 위상지연 등을 할당하는 방법에 대한 접근기술이다. 이러한 반복적인 자체학습 과정에 의한 조명 패턴을 통하여 측면 열유동이 제거되고 분해능이 향상되도록 제공하는 것이다.

• Geophysics and Geophysical Exploration
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• v.17 no.4
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• pp.216-230
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• 2014

Based on the interpretation of 3D seismic profiles acquired in the northwestern area of the Ulleung Basin, East Sea, the shallow sediments consist of five seismic units separated by regional reflectors. An anticline is present in the study area that documents activity of many faults. Bottom simulating reflectors are characterized by high RMS amplitude. Acoustic blanking with low RMS amplitude is distinctively recognized in the gas hydrate stability zone. Seismic attribute analysis shows that if gas hydrates are underlain by free gas, the high reflection strength and the low instantaneous frequency are displayed below the boundary between them. Whereas, if not, the reflection strength is low and instantaneous frequency is high continuously below the gas hydrate zone. Based on the spectral decomposition of the bottom simulating reflector, the high envelope at the specific high frequency range indicates the generation of the tuning effect due to the lower free gas content. Four models for the occurrence of the gas hydrate are suggested considering the slope of sedimentary layers as well as the presence of gas hydrate or free gas.

• Composites Research
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• v.14 no.4
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• pp.15-26
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• 2001

Interfacial properties and microfailure degradation mechanisms of the bioabsorbable composites fur implant materials were investigated using micromechanical technique and nondestructive acoustic emission (AE). As hydrolysis time increased, the tensile strength, the modulus and the elongation of poly(ester-amide) (PEA) and bioactive glass fibers decreased, whereas these of chitosan fiber almost did not change. Interfacial shear strength (IFSS) between bioactive glass fiber and poly-L-lactide (PLLA) was much higher than PEA or chitosan fiber/PLLA systems using dual matrix composite (DMC) specimen. The decreasing rate of IFSS was the fastest in bioactive glass fiber/PLLA composites whereas that of chitosan fiber/PLLA composites was the slowest. AE amplitude and AE energy of PEA fiber decreased gradually, and their distributions became narrower than those in the initial state with hydrolysis time. In case of bioactive glass fiber, AE amplitude and AE energy in tensile failure were much higher than in compression. In addition, AE parameters at the initial state were much higher than those after degradation under both tensile and compressive tests. In this work, interfacial properties and microfailure degradation mechanisms can be important factors to control bioabsorbable composite performance.

• Journal of Korean Ophthalmic Optics Society
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• v.5 no.1
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• pp.13-18
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• 2000

In this paper, we use the C light which is a daylight and consider the incident beam having the Gaussian amplitude. We investigated the illuminance distributions and the variation in chromaticity of optical system having the truncated Gaussian amplitude on the focal plane and along the optical axis using the C light source. We also use the three sensitivity functions of human eye(CIE 1931) for wavelengths which are from 380nm to 780nm. When the truncation grade of incident beam having Gaussian amplitude decreases, the size of central spot on the focal plane and the depth of focus along the optical axis decrease, and the variation in chromaticity on the focal plane and along the axis increases rapidly. As the illuminance on the focal plane decreases the variation in chromaticity of optical system increases rapidly, and as the depth of focus increases the variation in chromaticity of optical system decreases.

• Korean Journal of Optics and Photonics
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• v.5 no.3
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• pp.349-355
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• 1994

Superrsolution of an optical imaging system, which resolves $\epsilon_O$ (half width of the square top amplitude impulse function) less than the Rayleigh's resolution limit $\epsilon_R$, is theoretically treated by using the diffraction theory, and an experimental system is proposed. Initially superresolution is stated as an inverse problem, and an integral equation is derived as a function of parameter $\beta$, which is positive. The integration is numerically carried out for the given aperture and those given values of $\beta$, which is 1, 5, 10, 15, and 20. 1/2$\times$FWHM's of the amplitude impulse functions are meassured for the cases of diffrent value of {J and in the case of $\beta=5$, the half-width already approaches to $\epsilon_O=0.1$,urn, which is, in the case of the present work, one fifth of the Rayleigh's resolution limit. It is found both the pupil function and the phase of the Huygens wave are to be modified, and theories of the pupil function modulation plate and the phase modulation hologram plate are also presented. The result obtained may be useful in ultrafine optical lithography.graphy.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.13 no.9
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• pp.694-702
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• 2003

A cooling mechanism using acoustic streaming by ultrasonic vibrations and associated convective heat transfer enhancement is investigated experimentally and analytically. Acoustic streaming pattern and associated heat transfer characteristics are presented. Analytical transient temperature profile of the heated plate following Nyborgs theory is accomplished along with experimental measurement. A temperature drop of 30 C is obtained in 4 minutes with vibration amplitude of 10${\mu}{\textrm}{m}$. As the vibration amplitude is further increased to 25${\mu}{\textrm}{m}$ a temperature drop of 40 C is achieved that is the maximum temperature drop obtained with the current experimental apparatus. Analytical heat transfer solutions verified a temperature drop of 4$0^{\circ}C$ with a vibration amplitude of 25${\mu}{\textrm}{m}$ at 28.4 kHz which is experimentally obtained.