• 천문학회보
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• 제39권2호
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• pp.113.1-113.1
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• 2014

We propose a three-year Key Science Program (KSP) consisting of VLBI monitoring observations and single dish (SD) rapid response observations (RRO). The VLBI monitoring observations are comprised of ten 24-hr observations per year (every month) of about 30 gamma-ray brigt active galactic nuclei (AGNs) with Korea VLBI Network (KVN) at 22, 43, 86, and 129 GHz. The SD RROs may consist of twelve 7-hr observations per source (every week for 3 months after triggering) of gamma-ray flaring sources with two KVN SD telescopes at 22, 43, and 86 GHz in dual polarization. We expect one or two sources per year for the SD RROs. Gamma-ray flares of AGNs are known to be occured in innermost regions of relativistic jets which radiate in whole ranges of electromagnetic spectra due to synchrotron radiation, syschrotron self absorption, inverse-compton scttering, doppler boosting etc. Possible explanations of the gamma-ray flares in AGNs are a) shocks-in-jets propagating within jet flow and b) bending of the whole jets. For both cases, we should expect changes in polarization, luminosity, particle distribution, and structures of jets at mas-scale. The multifrequency simultaneous VLBI/SD observations with KVN are the best tool for detecting such changes correlated with gamma-ray flares. This KSP proposal aims to answer the fundamental questions about the basic nature of the flares of AGNs.

• 대한토목학회논문집
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• 제13권5호
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• pp.99-107
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• 1993

본 논문에서는 콘크리트 균열 사이의 인장강성 효과를 고려한 임의의 기하하적 형상을 갖는 철근 콘크리트 쉘을 해석하기 위하여 재료비선형 유한요소 프로그램을 작성하였다. 본 논문은 연속적인 컴퓨터 해석으로 탄성, 비탄성 및 극한 범위에서의 철근과 콘크리트의 응력은 물론, 하중-변위 응답과 균열전파를 추적할 수 있었다. 골재억물림과 철근의 다울작용을 포함하는 유효전단계수를 평가하기 위하여 균열상태의 전단유지계수를 도입하였다. 콘크리트는 인장에서는 취성으로 압축에서는 탄소성으로 가정하였다. 콘크리트의 소성거동은 Drucker-Prager 항복기준과 결합유동법칙에 따르는 것으로 가정하였다. 철근은 Von Mises 항복기준으로 가정하였으며 등가의 두께를 가지는 철근층으로 모델화 하였다. 수치해석을 위하여는 증분형접선강성도 방법을 사용하였다. 수치예제를 제시하여 본 연구결과를 Hedgren의 실험 결과와 Lin의 수치해석과 비교하였다.

• Interaction and multiscale mechanics
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• 제2권4호
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• pp.397-411
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• 2009

The effect of soil-structure interaction on a simple single storeyed and two bay space frame resting on a pile group embedded in the cohesive soil (clay) with flexible cap is examined in this paper. For this purpose, a more rational approach is resorted to using the three dimensional finite element analysis with realistic assumptions. The members of the superstructure and substructure are descretized using 20 node isoparametric continuum elements while the interface between the soil and pile is modeled using 16 node isoparametric interface elements. Owing to viability in terms of computational resources and memory requirement, the approach of uncoupled analysis is generally preferred to coupled analysis of the system. However, an interactive analysis of the system is presented in this paper where the building frame and pile foundation are considered as a single compatible unit. This study is focused on the interaction between the pile cap and underlying soil. In the parametric study conducted using the coupled analysis, the effect of pile spacing in a pile group and configuration of the pile group is evaluated on the response of superstructure. The responses of the superstructure considered include the displacement at top of the frame and moments in the superstructure columns. The effect of soil-structure interaction is found to be quite significant for the type of foundation used in the study. The percentage variation in the values of displacement obtained using the coupled and uncoupled analysis is found in the range of 4-17 and that for the moment in the range of 3-10. A reasonable agreement is observed in the results obtained using either approach.

• 한국전산구조공학회논문집
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• 제29권5호
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• pp.413-419
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• 2016

지진시 사회 인프라시설물의 피해는 시설물 자체의 피해보다 사회 전반에 걸친 2차 피해를 야기한다. 그 중, 지하 공동구 구조물은 통신, 가스, 전기 등 사회의 라이프라인에 해당하여 지진에 대한 취약성을 정확히 평가하여야 할 필요가 있다. 따라서, 본 연구에서는 지하 공동구의 지진 발생 지반가속도에 따른 파괴가능성을 평가하였다. 평가를 위한 입력지반운동은 해외 실측 지진데이터와 한반도에서 발생가능한 인공지진파를 차용하였으며, 지진해석 방법은 응답변위법과 시간이력해석법을 사용하였다. 파괴여부를 판별하는 한계상태는 휨모멘트와 전단 파괴를 바탕으로 하였다. 취약도 함수 도출을 위한 방법은 최우도법이 사용되었으며, 그 분포함수는 대수정규분포로 가정하였다. 이는 지진시 지하 공동구 시설물의 피해 평가는 물론 지하 공동구 시설물의 내진설계를 위한 기초자료로 활용될 수 있다.

• 대한조선학회논문집
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• 제57권2호
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• pp.80-87
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• 2020

As the Arctic sea ice decreases due to various reasons such as global warming, the demand for ships and offshore structures operating in the Arctic region is steadily increasing. In the case of sea ice, the anisotropy is caused by the uncertainty inside the material. For most of the research, nevertheless, estimating the ice load has been treated deterministically. With regard to this, in this paper, a four-point bending strength analysis of an ice specimen was attempted using a stochastic finite element method. First, spatial distribution of the material properties used in the yield criterion was assumed to be a multivariate Gaussian random field. After that, a direct method, which is a sort of stochastic finite element method, and a sensitivity method using the sensitivity of response for random variables were proposed for calculating the probabilistic distribution of ice specimen strength. A parametric study was conducted with different mean vectors and correlation lengths for each material property used in the above procedure. The calculation time was about ten seconds for the direct method and about three minutes for the sensitivity methods. As the cohesion and correlation length increased, the mean value of the critical load and the standard deviation increased. On the contrary, they decreased as the friction angle increased. Also, in all cases, the direct and sensitivity methods yielded very similar results.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1996년도 하계학술대회 논문집 C
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• pp.2004-2006
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• 1996

A thermally driven polysilicon micro actuator has been fabricated using surface micromachining techniques. It consists of P-doped polysilicon as a structural layer and TEOS (tetracthylorthosilicate) as a sacrificial layer. The polysilicon was annealed for the relaxation of residual stress which is the main cause to its deformation such as bending and buckling. And the newly developed HF VPE (vapor phase etching) process was also used as an effective release method for the elimination of sacrificial TEOS layer. The thickneas of polysilicon is $2{\mu}m$ and the lengths of active and passive polysilicon cantilevers are $500{\mu}m$ and $260{\mu}m$, respectively. The actuation is incurred by die thermal expansion due to the current flow in the active polysilicon cantilever, which motion is amplified by lever mechanism. The moving distance of polysilicon micro actuator was experimentally conformed as large as $21{\mu}m$ at the input voltage level of 10V and 50Hz square wave. The actuating characteristics are investigated by simulating the phenomena of heat transfer and thermal expansion in the polysilicon layer. The displacement of actuator is analyzed to be proportional to the square of input voltage. These micro actuator technology can be utilized for the fabrication of MEMS (microelectromechanical system) such as micro relay, which requires large displacement or contact force but relatively slow response.

• Steel and Composite Structures
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• 제26권3호
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• pp.329-345
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• 2018

This paper systematically investigated the mechanical performance of the weak-axis cover-plate connection, including a beam end monotonic loading test and a column top cyclic loading test, and a series of parametric studies for exterior and interior joints under cyclic loading using a nonlinear finite element analysis program ABAQUS, focusing on the influences of the shape of top cover-plate, the length and thickness of the cover-plate, the thickness of the skin plate, and the steel material grade. Results showed that the strains at both edges of the beam flange were greater than the middle's, thus it is necessary to take some technical methods to ensure the construction quality of the beam flange groove weld. The plastic rotation of the exterior joint can satisfy the requirement of FEMA-267 (1995) of 0.03 rad, while only one side connection of interior joint satisfied ANSI/AISC 341-10 under the column top cyclic loading. Changing the shape or the thickness or the length of the cover-plate did not significantly affect the mechanical behaviors of frame joints no matter in exterior joints or interior joints. The length and thickness of the cover-plate recommended by FEMA 267 (1995) is also suitable to the weak-axis cover-plate joint. The minimum skin plate thickness and a design procedure for the weak-axis cover-plate connections were proposed finally.

• Geomechanics and Engineering
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• 제20권5호
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• pp.399-410
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• 2020

Cavities often develop behind the vault during the construction of double-arch tunnels, generally in the form of various defects. The study evaluates the impact of cavities behind the vault on the mechanical and failure behaviors of double-arch tunnels. Cavities of the same sizes are introduced at the vault and the shoulder close to the central wall of double-arch tunnels. Physical model tests are performed to investigate the liner stress variation, the earth pressure distribution and the process of progressive failure. Results reveal that the presence of cavities behind the liner causes the re-distribution of the earth pressure and induces stress concentration near the boundaries of cavities, which results in the bending moments in the liner inside the cavity to reverse sign from compression to tension. The liner near the invert becomes the weak region and stress concentration points are created in the outer fiber of the liner at the bottom of the sidewall and central wall. It is suggested that grouting into the foundation soils and backfilling injection should be carried out to ensure the tunnel safety. Changes in the location of cavities significantly impact the failure pattern of the liner close to the vault, e.g., cracks appear in the outer fiber of the liner inside the cavity when a cavity is located at the shoulder close to the central wall, which is different from the case that the cavity locates at the vault, whereas changes in the location of cavities have a little influence on the liner at the bottom of the double-arch tunnels.

• Biotechnology and Bioprocess Engineering:BBE
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• 제11권2호
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• pp.89-95
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• 2006

Potato plants carrying the Ry gene are extremely resistance to a number of potyviruses, but it is not known which variety expressed the resistance. In this investigation, combined classical and molecular techniques were used to identify virus resistance potatoes. Mechanical inoculation of 32 varieties of Korean potato cultivars, with potato virus Y (PVY), induced various symptoms, such as mosaic, yellowing, necrosis, mottle, vein clearing and vein bending. Different virus spreading patterns were observed, such as highly sensitive, moderate and resistant to $PVY^o$ inoculated leaves in different cultivars. From the results of double antibody sandwich-enzyme links immunosorbant assays (DAS-ELISA), coupled with reverse transcription polymerase chain reaction (RT-PCR), Winter valley and Golden valley were found to be highly susceptible and resistant cultivars to $PVY^o$ respectively. TEM was used as a complementary method to conform the localization of the virus in leaf tissues. TEM detect virus particles in Golden valley, where, ELISA and RT-PCR were unable to detect the CP gene. However, the interior part of the tissues was severely deformed in $PVY^o$ infected Winter valley, than Golden valley The Ry gene is involved in an induced response in $PVY^o$ infected Golden valley plants. The methods described in this study could be applied for the screening and development of antiviral potatoes.

• Tribology and Lubricants
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• 제33권1호
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• pp.9-14
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• 2017

This paper presents a rotordynamic analysis and the operation of a power turbine applied to a 250 kW super-critical $CO_2$ cycle. The power turbine consists of a turbine wheel and a shaft supported by two fluid film bearings. We use a tilting pad bearing for the power turbine owing to the high speed operation, and employ copper backing pads to improve the thermal management of the bearing. We conduct a rotordynamic analysis based on the design parameters of the power turbine. The dynamic coefficients of the tilting pad bearings were calculated based on the iso-thermal lubrication theory and turbine wheel was modeled as equivalent inertia. The predicted Cambell diagram showed that there are two critical speeds, namely the conical and bending critical speeds under the rated speed. However, the unbalance response prediction showed that vibration levels are controlled within 10 mm for all speed ranges owing to the high damping ratio of the modes. Additionally, the predicted logarithmic decrement indicates that there is no unstable mode. The power turbine uses compressed air at a temperature of $250^{\circ}C$ in its operation, and we monitor the shaft vibration and temperature of the lubricant during the test. In the steady state, we record a temperature rise of $40^{\circ}C$ between the inlet and outlet lubricant and the measured shaft vibration shows good agreement with the prediction.