• Smart Structures and Systems
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• 제17권3호
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• pp.471-489
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• 2016

Modal identification based on ambient vibration data has attracted extensive attention in the past few decades. Since the excitation for ambient vibration tests is mainly from the environmental effects such as wind and traffic loading and no artificial excitation is applied, the signal to noise (s/n) ratio of the data acquired plays an important role in mode identifiability. Under ambient vibration conditions, certain modes may not be identifiable due to a low s/n ratio. This paper presents a study on the mode identifiability of an instrumented cable-stayed bridge with the use of acceleration response data measured by a long-term structural health monitoring system. A recently developed fast Bayesian FFT method is utilized to perform output-only modal identification. In addition to identifying the most probable values (MPVs) of modal parameters, the associated posterior uncertainties can be obtained by this method. Likewise, the power spectral density of modal force can be identified, and thus it is possible to obtain the modal s/n ratio. This provides an efficient way to investigate the mode identifiability. Three groups of data are utilized in this study: the first one is 10 data sets including six collected under normal wind conditions and four collected during typhoons; the second one is three data sets with wind speeds of about 7.5 m/s; and the third one is some blind data. The first two groups of data are used to perform ambient modal identification and help to estimate a critical value of the s/n ratio above which the deficient mode is identifiable, while the third group of data is used to perform verification. A couple of fundamental modes are identified, including the ones in the vertical and transverse directions respectively and coupled in both directions. The uncertainty and s/n ratio of the deficient mode are investigated and discussed. A critical value of the modal s/n ratio is suggested to evaluate the mode identifiability of the deficient mode. The work presented in this paper could provide a base for the vibration-based condition assessment in future.

• 한국환경과학회지
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• 제18권9호
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• pp.1011-1015
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• 2009

The vegetated porous pavement can be installed as an alternative way to replace the traditional pavement, which contributes less to the water circulation system in the urban area. This study aims to an investigation based on the shadow and pressure of the vehicle system, where the turfgrass get grown and the green block get constructed on the grassy parking lot. This study might achieve these conclusions, in the case of use 'green block' makes grass parking lot, plant a kind of 'zenith' and takes sod thickness 40 mm are more efficient for turfgrass growth in the early times. In the case of parking over 8 hours in a day, after 5 weeks turfgrass growth would come into reduce. So over 4 hours parking and after 9 weeks, we need consider to setting up green block in grassy parking lot. The grassy ground would get pressured by the vehicles' load and it would bring into some damage due to the load after 3 weeks. So we should put the grass's growth root point under the designed a top of 'green block' level. When the vehicle amounts and parking density is in a low level, it could be an environmentally friendly product.

• Journal of Communications and Networks
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• 제7권3호
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• pp.325-336
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• 2005

Mobile ad hoc networks (MANET) are composed of moving wireless hosts which, within range of each other, form wireless networks. For communication to occur between hosts that are not within each other's range, routes involving intermediate nodes must be established; however, since the hosts may be in motion, a host that was part of a route may move away from its upstream and downstream partners, thus breaking the route. In this paper, we propose anticipated route maintenance (ARM) protocol with two extensions to route discovery based routing scheme: Extend the route when nodes on a link move apart from each other and they have common neighbor that can be 'inserted' in the path, and shrink route when a node discovers that one of its neighbor which is not the next hop is also on the same route several hops later on. By utilizing only local geographic information (now a part of some route finding algorithms), a host can anticipate its neighbor's departure and, if other hosts are available, choose a host to bridge the gap, keeping the path connected. We present a distributed algorithm that anticipates route failure and performs preventative route maintenance using location information to increase a route lifespan. The benefits are that this reduces the need to find new routes (which is very expensive) and prevents interruptions in service. As the density of nodes increases, the chance to successfully utilize our route maintenance approach increases, and so does the savings. We have compared the performance of two protocols, pure dynamic source routing (DSR) protocol and DSR with ARM. The simulation results show how ARM improves the functionality of DSR by preventing the links in the route from breaking. Packets delivery ratio could be increased using ARM and achieved approximately $\100%$ improvement. The simulations clarify also how ARM shows a noticeable improvement in dropped packets and links stability over DSR, even though there is more traffic and channel overhead in ARM.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2007년도 추계학술대회 논문집
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• pp.1287-1296
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• 2007

Recently, the high speed railway comes into the spotlight as the important and convenient traffic infrastructure. In Korea, Kyung-Bu high speed train service began in about 400km section at 2004, and the Ho-Nam high speed railway will be constructed by 2017. The high speed train will run with a design maximum speed of 300-350km/hr. Since the trains are operated at high speed, the differential settlement of subgrade under the rail is able to cause a fatal disaster. Therefore, the differential settlement of the embankment must be controlled with the greatest care. Furthermore, the characteristics and causes of settlements which occurred under construction and post-construction should be investigated. A considerable number of studies have been conducted on the settlement of the natural ground over the past several decades. But little attention has been given to the compression settlement of the embankment. The long-term settlement of compacted fills embankments is greatly influenced by the post-construction wetting. This is called 'hydro collapse' or 'wetting collapse'. In spite of little study for this wetting collapse problem, it has been recognized that the compressibility of compacted sands, gravels and rockfills exhibit low compressibility at low pressures, but there can be significant compression at high pressures due to grain crushing by several researchers(Marachi et al. 1969, Nobari and Duncan 1972, Noorany et al. 1994, Houston et al. 1993, Wu 2004). The characteristics of compression of fill materials depend on a number of factors such as soil/rock type, as-compacted moisture, density, stress level and wetting condition. Because of the complexity of these factors, it is not easy to predict quantitatively the amount of compression without extensive tests. Therefore, in this research I carried out the wetting collapse tests, with focusing in various soil/rock type, stress levels, wetting condition more closely.

• 전기전자학회논문지
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• 제22권3호
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• pp.651-657
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• 2018

IEEE 802.11 무선랜은 20MHz부터 160MHz의 전송 대역폭을 지원한다. 일반적으로 전송 시 대역폭이 증가함에 따라 전송속도가 증가하지만, 동시에 단위 주파수당 전송파워가 낮아져 오류율이 증가하고 재전송을 유발할 수 있다. 본 논문에서는 이러한 현상이 전송 에너지 소비에 미치는 영향을 분석하기 위한 수학적 모델링을 수행한다. 그리고, 이를 바탕으로 전송 대역폭을 결정하는 기법을 제시한다. 제시된 기법은 기존 전송 결과를 기반으로 대역폭별 오류율을 추정하고 이를 바탕으로 대역폭별 에너지 소비 테이블을 업데이트 후 이들 간의 비교를 통해 최적 대역폭을 결정한다. VoIP 트래픽을 가정한 시뮬레이션을 통해 다양한 환경에서 제안한 기법의 에너지 소비 성능을 평가한다.

• 한국항행학회논문지
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• 제18권6호
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• pp.594-599
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• 2014

현재 스마트폰과 같은 무선 환경 기기의 일상화는 IEEE802.11 그룹의 무선 네트워크 사용 환경을 한층 더 확장시키고 있으며, Wi-Fi와 같은 사용자 중심의 상용화 무선 네트워크 요구가 급증하면서 이의 효과적인 활용과 사용자 중심의 끊김없는 핸드오버는 가장 중요한 논점 중에 하나이다. 그리고 현재 이슈화되고 있는 SDN은 데이터센터 내에서 OpenFlow 스위치 간의 비용 및 복잡도를 줄이는 각 플로우 라우팅을 제공한다. 이는 관리자에게 직관적인 제어를 제공하면서 사용자에게는 지연시간을 줄여주는 이점을 제공하고 있다. 본 논문에서는 SDN이 지원하는 네트워크 서비스 재구성 능력을 밀집된 무선네트워크 환경에 적용하여 끊김없는 핸드오버 매커니즘을 연구하고 성능을 검증한다.

• 한국통신학회논문지
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• 제17권1호
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• pp.1-10
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• 1992

새로운 정보 통신 서어비스의 출현 및 가입자측에서 이에 대한 욕구가 서로 부합되어 가입자 영역에는 기존의 단말기외에 새로운 형태의 데이터 단말의 수요가 증가되고 있는 추세이다. 이에 따라 전송로의 경제성을 고려하여 SMUX에 대한 연구가 진행되고 있다. 따라서 본 논문에서는 SMUX에 Go-back N ARQ 재전송 방식을 적용하여 queueing 모델을 설정한 후 이에 따른 수학적 해석 방법을 제시하였고, 평균 waiting 시간 및 overflow 확률의 일반식을 산출하였다. SMUX의 설계시에 영향을 미치는 파라미터로서는 입력비율, 출력 비율 및 접속되는 단말기들의 batch 크기로 표현되는 트래픽 밀도나 버퍼가 수용할 수 있는 최대 버퍼 크기에 따른 overflow 확률 및 평균 버퍼 크기, 평균 waiting 시간을 비교,분석하였다. 또한 수학적 해석에 따른 각 파라미터들이 SMUX에 미치는 영향은 평균 메세지 크기는 40characters로 고정하였을 경우 전송 선로상의 에러율이 10­$^3$이하에서 운용되어야 하고, 트래픽 밀도가 낮은 (0.5이하)상태에서 운용이 효울적이라 사료된다. 향후 본 논문에서 제시한 자료를 기초로 하여 SMUX를 설계시에 참고 자료가 되리라 사료된다.

• 한국지하수토양환경학회지:지하수토양환경
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• 제8권2호
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• pp.19-35
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• 2003

In order 새 investigate the spatial and seasonal variations of heavy metal pollution in heavily industrialized urban area, urban roadside sediments were collected for five years from gully pots in Seoul City. A series of studies have been carried out concerning the physicochemical characteristics of the sediments in order to evaluate the contamination of heavy metals such as Cd, Co, Cr, Cu, Ni, Pb and Zn. Roadside sediments and uncontaminated stream sediments were analyzed for total metal concentrations using acid extraction. The roadside sediments are characterized by very high concentrations of Zn (2,665.0$\pm$1,815.0 $\mu\textrm{g}$/g), Cu (445.6$\pm$708.0 $\mu\textrm{g}$/g), Pb (214.3$\pm$147.9 $\mu\textrm{g}$/g) and Cr (182.1$\pm$268.8 $\mu\textrm{g}$/g), indicating an artificial accumulation of these metals to the sediment chemistry. Comparing with average contents of uncontaminated stream sediments, roadside sediments were shown zinc 14 times (up to 64.4), copper 9 times (up to 181.7), lead 6 times (up to 63.7), cobalt 6 times (up to 168.7), nickel 4 times (up to 98.4), cadmium 2 times (up to 12.8) and chrome 2 times (up to 40.2) high content. The relative degree of heavy metal pollution for roadside sediments collected from each district in Seoul City is evaluated using the “geoaccumulation index”. As a result, heavy-metal contamination is highest centering the oldest residential district and industry area, and contamination level decreases as go to outer block of the city. The factor analysis results indicate that the levels of Cu, Ni, Fe and Cr are strongly related to numbers of factories, whereas the concentrations of Cr, Zn and Cd dependant on pollution index, indicating artificial contamination due to site-specific traffic density.

• 대한교통학회지
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• 제34권2호
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• pp.168-179
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• 2016

본 연구는 보행자도로의 서비스수준 산정방법을 개선하는 것으로 보행점유공간을 주 척도로 하고 기존의 방법에서 고려되지 않았던 양방향 보행류에 따른 상충개념을 도입하여 분석을 하였다. 분석 시 보행상충 공간을 산정하였으며, 방향별 보행자 수에 따른 상충공간과 평균보행시간을 산정하였다. 그리고 시간흐름에 따른 유효보도면적을 산정하여 시공간개념의 보행 점유공간을 통해 서비스수준을 재조정하였다. 산출된 보행점유공간은 0.68상태에서 용량상태를 보이며 이때의 보행교통류율은 80인/분/m로 산정되었고 추가적으로 보행점유공간을 크기순으로 나열하여 기울기가 변하는 지점을 기준으로 서비스수준을 분류하였다. 서비스 수준별 통계적 검증을 수행한 결과 유의수준 5%에서 모든 서비스수준 범주별 유의미한 차이가 있는 것으로 나타났다.

• 대한교통학회지
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• 제27권1호
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• pp.169-177
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• 2009

본 연구에서는 SLM(Square Lattice Model)과 HLM(Hexagonal Lattice Model)을 개선하여 만든 RHLM(Rotated Hexagonal Model)을 제시하고, 이를 이용하여 양방향 보행 교통류의 시뮬레이션 모형을 개발하였다. $20{\times}20$의 400개 셀에서 진행되었으며, 시간의 변화에 따른 속도, 밀도, 교통류율로 평가하였다. 본 모형을 같은 조건에서 SLM과 비교해본 결과, 기존의 모형이 보행자의 움직임이 불필요한 동작과 비논리적 방향으로 이동하는 것을 모형에 반영하였다면, 본 모형은 보행자의 직진 움직임과 자연스러운 회피 방법을 통해 보행교통류의 최단경로 움직임을 표현할 수 있었다. 또한 보행교통류의 더 높은 곳에서 용량을 가지고 보행을 할 수 있는 현상을 반영할 수 있었고, 보행교통류의 경우 밀도가 높아지더라도 속도가 0이 되는 상황은 잘 일어나지 않는 것을 반영할 수 있음을 알 수 있었다.