• 한국초전도ㆍ저온공학회논문지
• /
• 제2권1호
• /
• pp.31-34
• /
• 2000

this paper deals with the initial magnetic field decay for a large scale superconducting magnet e.g. NMR/MRI magnet. The high resolution image can not be obained during the periods of the initial field decay. It is known that all superconducting materials have the property of diamagnetism. This diamagnetism is usually explained with the concept of screening current. We assumed that the existence of the screening currebt. we assumed that the existence of the screening current makes the current distribution in the superconducting wire non-uniform. And the initial magnetic field decay is caused steady current state in the view of its pattern. The initial magnetic field decay is caused by the change of the current distribution between the energizing state and persistent current mode. in this paper the theoretical analysis for the current distributions has been introduced for each state. The experiments have been carried out to verify transport currents in order to veperiments, it small at the higher transport current.

• Transactions on Electrical and Electronic Materials
• /
• 제15권2호
• /
• pp.87-90
• /
• 2014

The effects of electric field frequency on the ac electrical treeing phenomena in an epoxy/reactive diluent/layered silicate (1.5 wt%) were carried out, in needle-plate electrode arrangement. A layered silicate was exfoliated in an epoxy base resin, by using our ac electric field apparatus. To measure the treeing propagation rate, constant alternating current (AC) of 10 kV with three different electric field frequencies (60, 500 and 1,000 Hz) was applied to the specimen, in needle-plate electrode arrangement, at $30^{\circ}C$ of insulating oil bath. As the electric field frequency increased, the treeing propagation rate increased. At 500 Hz, the treeing propagation rate of the epoxy/PG/nanosilicate system was $0.41{\times}10^{-3}$ mm/min, which was 3.4 times slower than that of the epoxy/PG system. The electrical treeing morphology was dense bush type at 60 Hz; however, as the frequency increased, the bush type was changed to branch type, having few branches, with very slow propagation rate.

• Structural Engineering and Mechanics
• /
• 제5권6호
• /
• pp.853-865
• /
• 1997

The design of robust plate and shell elements has been a very challenging area for several decades. The main difficulty has been the shear locking phenomenon in plate elements and the shear and membrane locking phenomena together in the shell elements. Among the various artifices or devices which are used to develop elements free of these problems is the field-consistency approach. In this paper this approach is reviewed, It turns out that not only Mindlin type elements but also elements based on higher-order theories could be developed using the technique.

• 대한기계학회논문집B
• /
• 제25권12호
• /
• pp.1667-1675
• /
• 2001

Numerical simulations are carried out for the liquid encapsulant Czochralski(LEC) by imposing a magnetic field. The use of a magnetic field to the crystal growth is to suppress melt convection and to improve the homogeneity of the crystal. In the present numerical investigation, we focus on the range of 0-0.3Tesla strength for the axial and cusped magnetic field and the effect of the magnetic field on the melt-crystal interface, flow field and temperature distribution which are the major factors to determine the quality of the single crystal are of particular interest. For both axial and cusped magnetic field, increase of the magnetic field strength causes a more convex interface to the crystal. In general, the flow is weakened by the application of magnetic field so that the shape of the melt-crystal interface and the transport phenomena are affected by the change of the flow and temperature field.

• 한국초전도학회:학술대회논문집
• /
• 한국초전도학회 2000년도 High Temperature Superconductivity Vol.X
• /
• pp.248-251
• /
• 2000

The fabrication and characteristics of HTS race-track type field coil for generators was carried out. Field coils are composed of 3 pancake coils wound by 37-filamental Bi-2223/Ag-alloy tapes. The winding machine is horizontal type. The critical currents (I$_c$) of the superconducting tapes were measured with variation of bending strain and external magnetic fields. I$_c$ of both whole field coils and 3 pancake coils were measured as a function of temperature. At 77K under the self-field, I$_c$ of whole field coils was 12A, while in the case of middle pancake coil, I$_c$ was 15A. The distribution of magnetic field B was obtained, using 3-D FEM. Our simulation showed that maximums of B${\bot}$A in x-y plane were locally distributed in both the upper and the lower coils. In addition, the fabrication processes and the characteristics of field coil are described.

• Journal of Biosystems Engineering
• /
• 제26권6호
• /
• pp.517-524
• /
• 2001

Steep and narrow ramp installed in the paddy field makes it difficult and sometimes dangerous to drive agricultural machines on it. Because agricultural machinery become larger and heavier, it is necessary to revise design standard for ramp to provide safe travel. This study was conducted to revise the design standard fur paddy field ramp, especially for its width and slope. A mathematical model predicting travelling path of a tractor on paddy field ramp and farm road was developed and simulated. To verify this model, field tests were carried out. The model could predict trvelling path with RMS Error of 12.5cm and 8.2cm, which were judged to be adoptable fur determining the width of paddy field ramp. By applying this model to 110ps tractor, which was assumed to be the largest tractor in Korea, width of paddy field ramp was determined as over 3.6m. The slope of paddy field ramp, which provided tractor with 10% of the total weight of the tractor and the mounted implement as reaction force at front wheel against ground was analyzed by considering weight transfer effect on a ramp. The result showed that adequate slope of paddy field ramp was 14°.

• 한국환경복원기술학회지
• /
• 제11권4호
• /
• pp.57-66
• /
• 2008

This study, with teachers form elementary schools in Chungju-si as its subjects, has investigated influence of school forest on satisfaction with school greenspace and environmental education. It has reached the following conclusions. As for recognition of and satisfaction of school greenspace there were meaningful differences between teachers form school forest model schools and ones from common schools, which was thought to be due to positive effects of school forest movement. It appeared that environmental education was carried out through audio-visual materials once a week in most schools. On the other hand, as for class places, 'classroom in parallel with outdoor class' and 'classroom education' appeared to be carried out most frequently in the case of school forest model schools, and 'classroom education' in the case of common schools. However, considering the fact that 'field learning' the most important element in environmental education, appeared to be carried out least frequently in both of the groups, which suggests that we should improve it for future through introduction of various field-experience learning programs. As a result of the survey of satisfaction with environmental education, some meaningful differences were recognized between school forest schools and common schools, and 'presence or absence of field learning spaces' was the most frequently answered reason for 'satisfied' and 'unsatisfied'. Thus, 'schools' and related institutions' enthusiastic efforts are needed for providing field-experience spaces where children can directly access to and explore into nature.

• 한국산업융합학회 논문집
• /
• 제12권2호
• /
• pp.79-83
• /
• 2009

In this study, actual state of operation of rotary tillage was surveyed in the central part of Korea which includes 29 rural community around Asan city, Choongnam principal. 29 heads of the village and 44 farmers were surveyed at the field with a questionnaire on the operation of rotary tillage with farm tractors. Power requirement of rotary tillage operation was measured at the field of each farmers during rotary tilling operation in the central part of Korea. Summaries of this study are summarized as follows. 1) Operations of tillage and land preparation were found to be carried out with farm tractors by 98 percents which are results of the survey on 29 heads of the village and 44 farmers. 2) More than half of the farms had tractors with horsepower more than 38 kW which states that most tractors were supplied in middle and large size. 3) The case of applying secondary tillage after the first primary tillage were 79 percent among the most of operations of tillage and field preparation. Rotary tillage were carried out on the most of wet paddy fields and only 14 percent of dry fields were cultivated with rotary tillage. 4) 63.6 percent of farm tractors were surveyed to meet the recommended field capacity on tillage operation by the government which were ranged from 15.1 to 34.9 ha. And the field capacity of these farm tractors were greater than that of annual total area of operation field, 13.2ha (data from Information of agricultural machinery, National agricultural products quality service, Korea), which states the utilization of agricultural machinery was high in the area surveyed. 5) 46 percent of farmers changed the blade of rotary implement when they used it over the field capacity of rotary blade of 33 ha which is the area of field for rotary operation per a blade change. 55 percent of farmers changed the rotary blade more than once per two years. 6) The required power for rotary operation of each farm tractors were measured as 17.206 kW at untilled paddy field and 34.989 kW at untilled dry field on the average, respectively. The required power for rotary operation was measured as 28.248 kW on the average at the paddy field which had been plowed once and 28.015 at the paddy field that had rotary tillage operation. Untilled dry field showed the highest value of the required power for rotary operation.

• 한국구조물진단유지관리공학회 논문집
• /
• 제12권3호
• /
• pp.127-134
• /
• 2008

교량의 거동은 교량의 지간 및 노면조도, 주행차량의 중량, 주행속도등 여러 요인들에 의해 달라지므로 교량의 동적거동을 정확히 예측하여 반영하기는 매우 어렵다. 우리나라 표준 시방 기준은 이러한 동적효과를 충격계수로 정의하고, 활하중에 충격계수를 곱하여 설계하중을 증가시키는 방법으로 동적효과를 고려하고 있다. 그러나 각 나라별로 충격계수를 적용하는 방법이 다르고, 아직까지도 명확하게 규명하지 못하고 있다. 그러므로 본 연구에서는 현재 사용 중인 PSC-I 거더교에 대하여 주행차량에 의한 동적 특성을 규명하기 위하여 동적재하, 정적재하, 의사정적재하시험을 이용하였다. 현장 재하시험에서 얻어진 결과를 통해 재하 속도에 따른 변위와 Strain관계를 찾을 수 있었으며, 이를 바탕으로 현재 논란이 있는 국내의 충격계수에 관한 경험식에 대하여 보다 합리적이고 실제적인 거동이 포함되도록 일조하는데 본 연구의 목적이 있다.

• Structural Engineering and Mechanics
• /
• 제81권3호
• /
• pp.379-394
• /
• 2022

Shallow footings usually belonged to the category of thick plate structures. For accurate analysis of thick plates, the contribution of out-of-plane components of the stress tensor should be considered in the formulation. Most of the available shallow footing models are based on the classical plate theories, which usually neglect the effects of the out-of-plane stresses. In this study, a mixed-field plate finite element model (FEM) is developed for the analysis of shallow footings rested on soil foundations. In addition to displacement field variables, the out-of-plane components of the stress tensor are also assumed as a priori unknown variables. For modeling the interaction effect of the soil under and outside of the shallow footings, the modified Vlasov theory is used. The tensionless nature of the supporting soil foundation is taken into account by adopting an incremental, iterative procedure. The equality requirement of displacements at the interface between the shallow footing and soil is fulfilled using the penalty approach. For validation of the present mixed FEM, the obtained results are compared with the results of 3D FEM and previous results published in the literature. The comparisons show the present mixed FEM is an efficient and accurate tool for solving the problems of shallow footings rested on subsoil.