• Wind and Structures
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• 제35권4호
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• pp.229-242
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• 2022

Wind tunnel test is often adopted to assess the site-specific wind characteristics for the design of bridges as suggested by current design standards. To investigate the wind characteristics of flat and mountainous terrain, two topographic models are tested in a boundary layer wind tunnel. The wind characteristics, including the vertical and horizontal mean wind speed distributions, the turbulence intensity, and the wind power spectra, are presented. They are investigated intensively in present study with the discussions on the effect of wind direction and the effect of topography. It is indicated that for flat terrain, the wind direction has negligible effect on the wind characteristics, however, the assumption of a homogenous wind field for the mountainous terrain is not applicable. Further, the non-homogeneous wind field can be defined based on a proposed approach if the wind tunnel test or on-site measurement is performed. The calculated turbulence intensities and wind power spectra by using the measured wind speeds are also given. It is shown that for the mountainous terrain, engineers should take into account the variability of the wind characteristics for design considerations.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.612-619
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• 2002

The dynamic behavior of Al-Mg alloys plasma was very unstable and this instability was closely related to the unstable motion of keyhole during laser irradiation. The keyhole fluctuated both in size and shape and its fluctuation period was about 440 ${\mu}{\textrm}{m}$. This instability has been estimated to be caused by the evaporation phenomena of metals with different boiling point and latent heats of vaporization. Therefore, the authors have conducted the spectroscopic diagnostics of plasma induced in the pulsed YAG laser welding of Al-Mg alloys in air and argon atmospheres. In the air environment, the identified spectra were atomic lines of Al, Mg, Cr, Mn, Cu, Fe and Zn, and singly ionized Mg line, as well as strong molecular spectrum of AlO, MgO and AIH. It was confirmed that the resonant lines of Al and Mg were strongly self-absorbed, in particular in the vicinity of pool surface. The self-absorption of atomic Mg line was more eminent in alloys containing higher Mg. These facts showed that the laser-induced plasma was relatively a low temperature and high density metallic vapor. The intensities of molecular spectra of AlO and MgO were different each other depending on the power density of laser beam. Under the low power density irradiation condition, the MgO band spectra were predominant in intensity, while the AlO spectra became much stronger in higher power density. In argon atmosphere the band spectra of MgO and AlO completely vanished, but AlH molecular spectra was detected clearly. The hydrogen source was presumably the hydrogen solved in the base Metal, absorbed water on the surface oxide layer or H$_2$ and $H_2O$ in the shielding gas. The temporal change in spectral line intensities was quite similar to the fluctuation of keyhole. The time average plasma temperature at 1 mm high above the surface of A5083 alloy was determined by the Boltzmann plot method of atomic Cr lines of different excitation energy. The obtained electron temperature was 3, 280$\pm$150 K which was about 500 K higher than the boiling point of pure aluminum. The electron number density was determined by measuring the relative intensities of the spectra1lines of atomic and singly ionized Magnesium, and the obtained value was 1.85 x 1019 1/㎥.

• Nuclear Engineering and Technology
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• 제35권5호
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• pp.497-505
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• 2003

This paper describes a computational approach to the quantification of primary damage under irradiation and demonstrates the effect of neutron energy spectra on the formation of the displacement cascade. The development of displacement cascades in ${\alpha}-Iron$ has been simulated using the MOLDY code - a molecular dynamics code for simulating radiation damage. The primary knock-on atom energy, key input to the MOLDY code, was determined from the SPECTER code calculation on two neutron spectra. The two neutron spectra include; (i) neutron spectrum in the instrumented irradiation capsule of the high-flux advanced neutron application reactor (HANARO), and (ii) neutron spectrum at the inner surface of the reactor pressure vessel steel for the Younggwang nuclear power plant No.5 (YG 5). Minor differences in the normalized neutron spectra between the two spectra produce similar values of PKA energy, which are 4.7 keV for HANARO and 5.3 keV for YG 5. This similarity implies that primary damage to the components of the commercial nuclear reactors should be well simulated by irradiation in the HANARO. Moreover, the application of the MD calculations corroborates this statement by comparing cascades simulation results.

• 한국식품과학회지
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• 제22권4호
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• pp.461-465
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• 1990

양조간장 중의 항산화료력을 가진 물질을 분리하고 그 특성을 알기위하여 Sephadex G-10, 50, 100을 이용하여 양조간장을 겔여과하고 각 획분에 대한 항산화성과 관련된 특성을 측정하여 다음과 같은 결과를 얻었다. 양조간장을 Sephadex G-10에서 분획한 결과 갈색도가 가장 높은 멜라노이딘 획분(MF-I)에서 항산화효력이 가장 컸고, 이 부분은 질소를 함유하고 있었으며 환원력이 매우 큰 물질이었다. Sephadex G-10에서 항산화 효력이 가장 큰 MF-I을 동결 건조하여 Sephadex G-50에서 재분획 한 결과, 갈색물질은 광범위한 범위에서 분획되었고 저분자의 멜라노이딘 부분(MF-II)에서 항산화성 및 환원력이 가장 크게 나타났다. 이 MF-II 획분을 다시 Sephadex G-100에서 재분획하여 항산화효력이 비교적 크고 환원력이 매우 큰 한 획분(MF-III)을 분리하였으나 환원력을 제외한 갈색도 및 항산화력은 상대적으로 낮았다. 그리고 양조간장의 항산화성 관련 물질의 구조적 특성을 알아보기 위해 분리한 각 획분의 UV-VIS 및 IR spectra를 측정한 결과 항산화성이 큰 각 획분(MF-I, MF-II 및 MF-III)의 UV-VIS spectra는 $260{\sim}320\;nm$에서 완만한 peak를 나타내는 전형적인 멜라노이딘의 특성을 나타내었다. 그리고 IR spectra의 경우 특징적인 OH기 및 amine기 구조를 지니고 있었고 이들이 항산화성을 갖는 기능기로 추정되었다.

• Journal of Mechanical Science and Technology
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• 제17권11호
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• pp.1599-1607
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• 2003

The characteristics of elastic waves emanated from crack initiation in 6061 aluminum alloy subjected to fatigue loading are investigated through experiments. The objective of the study is to determine the differences in the properties of the signals generated from fatigue test and also to examine if the sources of the waves could be identified from the temporal and spectral characteristics of the acoustic emission (AE) waveforms. The signals are recorded using nonresonant, flat, broadband transducers attached to the surface of the alloy specimens. The time dependence and power spectra of the signals recorded during the tests were examined and classified according to their special features. Six distinct types of signals were observed. The waveforms and their power spectra were found to be dependent on the crack propagation stage and the type of fracture associated with the signals. The potential application of the approach in health monitoring of structural components using a network of surface mounted broadband sensors is discussed.

• Journal of Advanced Marine Engineering and Technology
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• 제23권4호
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• pp.514-522
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• 1999

The laser-induced plasma affects greatly on the results of welding process. moreover selective evaporation loss of alloying elements leads to change in chemical composition of weld metal as well as the mechanical properties of welded joint. this study was undertaken to obtain a fundamental knowledge of pulsed laser welding phenomena especially evaporation mechanism of different aluminum alloys. The intensities of molecular spectra of AlO and MgO were different each other depeding on the power density of a laser beam Under the low power density condition the MgO band spectrum was predominant in intensity while the AlO spectra became much stronger with an increase in the power density. These behaviors have been attributed to the difference in evaporation phenomena of Al and Mg metals with different boiling points and latent heats of vaporization. The time-averaged plasma temperature and electron number density were determined by spectroscopic methods and consequently the obtained temperature was $3,280{\pm}150K$ and the electron number density was $1.85{\times}10^{19}\;l/m^3$.

• 천문학회지
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• 제28권2호
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• pp.109-117
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• 1995

The morphology and strength of the primordial magnetic field which is generated spontaneously in the early universe are studied for three models: (1) inflation (2) primordial magnetized bubble and (3) primordial turbulence models, We calculate the power spectra of magnetic field that are scale-free and proportional to $k^{1.5},k^{3{\sim}4}$ and $k^{2/3}$, respectively. The configurations of magnetic field having these power spectra are visualized. To constrain the present strength of the primordial magnetic field we calculate the anisotropy of the microwave background radiation in Bianchi type I universe with globally homogeneous magnetic field. From the COBE limit of the quadrupole moment of $({\delta}T/T)_{l=2}$ the present strength of horizen-scale magnetic fields $B_p$ is constrained to be less than $9{\times}10^{-8}G$.

• 전력전자학회논문지
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• 제7권3호
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• pp.224-236
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• 2002

PWM기법의 성능을 비교하고 있다. 랜덤 펄스 위치 PWM기법은 2상 또는 3상 펄스들을 변조 구간내의 임의의 위치에 랜덤하게 배치하고 인버터 스위칭에 의하여 고조파의 스펙트럼을 광역화하는 RPWM기법이다. 시뮬레이션과 실험에 의하여 인버터 출력 전압의 고조파 스펙트럼과 맥동 전류 파형 그리고 모터의 가청 스위칭 소음의 저감 효과를 비교 검토하였다. 스위칭 손실 및 연산시간 그리고 구현의 용이성에서 3상 RPWM보다는 2상 RPWM이 양호하며, 고조파의 광대역화 측면에서는 3상 RPWM이 보다 좋은 성능을 지니고 있음을 알 수 있었다. 또한 가청 스위칭 소음저감은 2상 RPWM과 3상 RPWM이 거의 유사한 성능을 갖고 있음이 확인되었다.

• Wind and Structures
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• 제24권6호
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• pp.579-611
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• 2017

Bora is a strong, usually dry temporally and spatially transient wind that is common at the eastern Adriatic Coast and many other dynamically similar regions around the world. One of the Bora main characteristics is its gustiness, when wind velocities can reach up to five times the mean velocity. Bora often creates significant problems to traffic, structures and human life in general. In this study, Bora velocity and near-ground turbulence are studied using the results of three-level high-frequency Bora field measurements carried out on a meteorological tower near the city of Split, Croatia. These measurements are analyzed for a period from April 2010 until June 2011. This rather long period allows for making quite robust and reliable conclusions. The focus is on mean Bora velocity, turbulence intensity, Reynolds shear stress and turbulence length scale profiles, as well as on Bora velocity power spectra and thermal stratification. The results are compared with commonly used empirical laws and recommendations provided in the ESDU 85020 wind engineering standard to question its applicability to Bora. The obtained results report some interesting findings. In particular, the empirical power- and logarithmic laws proved to fit mean Bora velocity profiles well. With decreasing Bora velocity there is an increase in the power-law exponent and aerodynamic surface roughness length, and simultaneously a decrease in friction velocity. This indicates an urban-like velocity profile for smaller wind velocities and a rural-like velocity profile for larger wind velocities. Bora proved to be near-neutral thermally stratified. Turbulence intensity and lateral component of turbulence length scales agree well with ESDU 85020 for this particular terrain type. Longitudinal and vertical turbulence length scales, Reynolds shear stress and velocity power spectra differ considerably from ESDU 85020. This may have significant implications on calculations of Bora wind loads on structures.

• 한국지구과학회지
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• 제28권7호
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• pp.871-877
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• 2007

최근 발생한 오대산지진(2007년 1월 20일)으로부터 관측된 지반진동 파형을 이용하여 응답스펙트럼을 분석하였으며, 결과를 국내 원자력 관련 구조물의 내진설계 기준과 비교하였다. 연구에 이용된 지반진동 개수는 수평성분 및 수직성분 각각 21개 및 8개이다. 지반진동을 이용하여 주파수별 지반응답을 구하고 정규화 분석 및 통계적 분석을 하였다. 본 연구결과를 국내 원자력시설물의 내진기준에 해당하는 Reg. Guide 1.60과 비교한 결과 특히 약 10 Hz 이상의 고주파수 영역에서 수직 및 수평 성분 모두 MPOSD스펙트럼이 Reg. Guide 1.60보다 높은 값을 보여 주었다. 따라서 향후 국내 지진활동 실정에 적합한 내진설계를 위해 수직 및 수평 성분 모두 10Hz 이상의 고주파수 대역에서 응답스펙트럼 값을 심각하게 고려할 필요가 있다.