• Journal of the Korean earth science society
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• v.43 no.1
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• pp.30-40
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• 2022

Wildfires, which occur sporadically and irregularly worldwide, are distinct natural disturbances in combustible vegetation areas, important parts of the global carbon cycle, and natural disasters that cause severe public emergencies. While many previous studies have investigated the variability and changes in wildfires globally based on fire emissions, burned areas, and fire weather indices, studies on East Asia are still limited. Here, we explore the characteristics of variability and changes in wildfire danger over East Asia by analyzing the fire weather index for the 40 years-1981-2020. The first empirical orthogonal function (EOF) mode of fire weather index variability represents an increasing trend in wildfire danger over most parts of East Asia over the last 40 years, accounting for 29% of the total variance. The major contributor is an increase in the surface temperature in East Asia associated with global warming and multidecadal ocean variations. The effect of temperature was slightly offset by the increase in soil moisture. The second EOF mode exhibits considerable interannual variability associated with the El Nino-Southern Oscillation, accounting for 17% of the total variance. The increase (decrease) in precipitation in East Asia during El Nino (La Nina) increases (decreases) soil moisture, which in turn reduces (increases) wildfire danger. This dominant soil moisture effect was slightly offset by the temperature increase (decrease) during El Nino (La Nina). Improving the understanding of variability and changes in wildfire danger will have important implications for reducing social, economic, and ecological losses associated with wildfire occurrences.

• Journal of Korean Tunnelling and Underground Space Association
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• v.19 no.1
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• pp.1-9
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• 2017

In this study, a test-bed system simulated a tunnel and concrete track is tested on cite and invested an allowed limit of multi-layered sound absorbing panel for reducing noise reflected on the concrete track in train tunnel considering the criteria and limitation on the theoretical back ground. The studied results are an effective evaluating system of the sound absorbing coefficient influenced fluid effects depending on the vehicle speed in the urban train tunnel and measuring not only structural behaviors of maximum displacement and acceleration of the panel but also dynamic characteristics of damping ratio and natural frequency.

• Journal of Navigation and Port Research
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• v.40 no.6
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• pp.361-368
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• 2016

A ship's rolling motion can make crew and passengers sick and/or apply forces to the structure that cause damage.. Therefore bilge keels are equipped in most ships for anti-rolling. In special cases, anti-rolling tanks (ARTs), fin stabilizers, or gyroscopes can be installed. However, ARTs require a large area to install, and fin stabilizers and gyroscopes are costly to install and expensive to operate. This paper suggests a Anti-rolling pendulum (ARP) to reduce roll motion. ARPs acts like ARTs. However, the ARP has a circular shaped guidance arc instead of the string or wire of a simple pendulum. The device suggested has about 1/ 8 the weight and 1/ 6 the volume of a ART and is more effective. This study derives the nonlinear and linear differential equations of system motion.

• The Journal of the Korea institute of electronic communication sciences
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• v.6 no.6
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• pp.889-895
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• 2011

PMLSM is structurally simple and it have a lot of merits such high speed, high thrust force etc., but cogging force by slot-teeth structure of armature and cogging force by outlet edge effect occurs. This is the cause of thrust force ripple and generate the noise and vibration. Therefore, in this paper we proposed installation of an auxiliary pole to mover of the PMLSM in order to decrease cogging force by the outlet edge which came necessarily into being discontinuous arrangement of the armature. Also, outlet edge cogging force designed a form of the auxiliary pole which the minimum became, and we compared a outlet edge cogging force characteristic along a winding method of an armature as we used 2-D of finite element analysis.

• Journal of Korean Association for Spatial Structures
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• v.11 no.4
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• pp.109-119
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• 2011

To date, a smart base isolation system has been developed in high seismicity region such as Japan, USA etc. Smart base isolation systems developed for structures in high seismicity region cannot directly applied to structures in regions of low-to-moderate seismicity such as Korea. Therefore, problems that occur by applying the smart base isolation system developed for the structures in high seismicity region to the structures in regions of low-to-moderate seismicity have been investigated in this study. To this end, a spacial arch structure was used as an example structure and MR dampers and low damping elastomeric bearings were used to compose a smart base isolation system. Artificial earthquakes were used for ground motions in regions of high and low-to-moderate seismicity. Based on numerical simulation results, it has been known that the capacity of smart base isolation system for the regions of low-to-moderate seismicity should be carefully designed because the base isolation effects of the smart base isolation system for high seismicity region is deteriorated when it is applied to the structures in regions of low-to-moderate seismicity.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.21 no.3
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• pp.265-273
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• 2008

The problem controlling lateral drift by the wind and the earthquake is very important in high rise buildings. But, outrigger system, generally used for residential tall buildings in Korea, has weak points with the occupancy of special space, the difficult construction and the long duration of works. On the other hand, the damper reduces story drifts of building structure by absorbing vibration energy induced by the dynamic loads and the application of damper systems is relatively simple. Also, the lateral drift control system such as outrigger system may raise the wind vibration problem of serviceability like human comfort and this problem may need another vibration control devices. Accordingly, we analyze the effect of the drift control using various dampers to substitute for outrigger system as the efficient system in residential tall buildings.

• Journal of the Korean Vacuum Society
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• v.19 no.4
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• pp.256-264
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• 2010

In this paper, a digital controller of magnetic bearing system for a high vacuum turbomolecular pump (TMP) is designed and examined. For stabilizing and providing damping in magnetic bearing, the digital PID controller is applied for each 5 control axes, and the inter-axis cross feedback controller is also applied to suppress low frequency vibration caused by gyroscopic moment of the rotor at high speed of rotation. The fabricated rotor-shaft has its first flexible natural frequency lower than maximum speed, about 614Hz, so the two lead filters are applied to increase damping of flexible mode. Notch filters with rotating frequency were selected to reduce vibration of the pump housing caused by unbalance load. The implemented controllers are verified by examination of frequency response and rotating test up to 40,000 rpm, which is higher than critical speed of backward flexible mode.

• Journal of the Earthquake Engineering Society of Korea
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• v.7 no.4
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• pp.31-37
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• 2003

This paper presents the methodologies for seismic retrofitting of cabinet equipment which can be employed to resolve the USI A-46 problem related to seismic qualification of old nuclear power plant. To obtain accurate dynamic characteristics of a cabinet structure, three types of structural modeling are introduced and the their free vibration modes are compared. Three types of seismic retrofitting scenarios, such as 1) the installation of bracing, 2) installation of damper, 3) installation of tuned mass damper(TMD), are established and evaluated for the decrease of ICRS(In Cabinet Reponse Spectrum). In the cases of 1) ＆ 2), since the retrofitted structures show larger ICRS than that of the original structure, the careful considerations are need in the application of these methods. Though the installation of TMD shows the best retrofitting result, the construction of analysis model that indicate the accurate vibration modes of real structure is estimated the essential step of this retrofitting method.

• Korean Journal of Optics and Photonics
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• v.32 no.6
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• pp.259-265
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• 2021

Recently, lens manufacturing and assembly technology has greatly improved. However, tight requirements of manufacturing and assembly lead to an increase in cost and manufacturing time, and in some cases the performance of an optical system may deteriorate depending on the operating environment's conditions, such as temperature or vibration. In addition, the use of a compensator is an effective method to reduce sensitivity in an ultra-precision optical system, but in the case of a small lens, such as that in an endoscope, it is difficult to use a compensator due to the size limitation of the lens barrel. Therefore, minimizing lens sensitivity is the most important technology in lens design. For this reason, there have been various attempts to reduce the lens sensitivity, and there is a trend to add functions to reduce the sensitivity in the lens design S/W. In this paper, we introduce a design technology that minimizes lens sensitivity. We first design a lens with quite good performance, then analyze the sensitivity of this lens, make a multi-configuration with high-sensitivity element error, and then reoptimize it. We prove with an example that this design technique is very effective.

• Journal of the Korean GEO-environmental Society
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• v.18 no.4
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• pp.13-21
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• 2017

Ballasted track has been used as track system for more than 100 years. Ballasted track has advantages of low construction cost, flexible maintenance, low noise and vibration, and so on. However, ballasted track leads to continuous settlement which causes maintenance. Recently, increase in speed, traffic volume, and weight of train requires more frequent maintenance. Fouling, well-known phenomenon of accumulation of fine materials due to intrusion of subgrade and breakage of ballast materials, expedites the settlement (i.e., irregular settlement) of track. Ground Penetrating Radar (GPR) can be one of non-destructive tools that can evaluate fouling level of ballast. In this paper, a gain function based on the attenuation characteristics of ballast material is suggested in conjunction with Hilbert transform. Lab box tests and full-scale tests indicate that the suggested method reasonably classifies clean, fouled layers, and subgrade. However, additional study to eliminate effect of sleeper and to include the scattering features of the electromagnetic wave in ballast voids should be required in order to enhance the accuracy.