• 대한기계학회논문집B
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• 제22권6호
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• pp.860-873
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• 1998

In a low speed open-type wind tunnel, a group of parallel wakes downstream of two dimensional grid model consisting of several circular cylinders were experimentally investigated to study the response of the wake flows to the acoustic excitation, in hoping to promote the understanding of the underlying mechanism behind the gross flow change due to artificial excitation. In the unexcited wake flows, the development of the individual wakes behind cylinders was almost uniform for the ratio of the spacing to the cylinder diameter of s/d.geq.1.5. For smaller s/d, however, the jet streams issued through the gaps between the cylinders became biased in one side and the cylinders had wakes of different sizes. At s/d=1.25, the gap flow directions change in time, leading to unstable wake patterns. Further reduction in s/d made this unstable flip-flopping of the jets stable. The most effective excitation frequency was found to be in the Strouhal number range of St=0.5-0.6. This frequency was related to the vortex shedding. At s/d=1.75, the excitation frequency was 2 or 4 times the vortex shedding frequency. When the flow was excited at this frequency, the vortex sheddings were energized, and pairings between neighboring vortices were generated. Also, the merging process between individual wakes was accelerated. The unstable and unbalanced wake patterns at s/d=2.15 were made stable and balanced. The unstable and unbalanced wake patterns at s/d=2.15 were made stable and balanced. For smaller spacing of s/d .leq,1.0, the acoustic excitation became less effective in controlling the flow.

• 터널과지하공간
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• 제13권4호
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• pp.321-329
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• 2003

채굴적의 붕괴에 따른 침하에 의해 발생하는 변형률 의존 수리전도도 변화를 조사하였다. 수직 및 전단벨변형률, 탄성계수감소비, 절리간격 등은 수리전도도 변화를 좌우하는 주요 인자들이다. 탄성계수감소비는 RMR에 의해 절리간격은 RQD로 표현함으로서 심하게 파쇄된 암반에서부터 무결암까지의 모든 조건을 나타내는 현지암반의 수리전도도 변화를 결정할 수 있다. 지표 부근에서의 수리전도도 변화는 별로 나타나지 않으며 채굴적 주변에서의 변화가 큰 것으로 나타났다. 침하에 의해 수리전도도가 1보다 커지는 지역은 채굴적 상부 약 20m 구간까지 인 것으로 조사되었다. 전단변형률도 채굴적 주변에서의 수리전도도 증가에 큰 역할을 하는 것으로 나타났다. RMR이 감소함에 따라 채굴적 주변의 수리전도도는 증가하는 것으로 나타났는데 이는 RMR이 낮은 불량 암반에서의 침하가 수리전도도에 큰 영향을 미친다는 것을 의미한다.

• 터널과지하공간
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• 제28권6호
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• pp.651-669
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• 2018

본 연구에서는 유전자 프로그래밍과 개체군집최적화기법을 이용하여 픽 커터의 비에너지를 예측하기 위한 모델을 제안하였다. 기계굴착장비의 굴진성능을 평가하는 것은 터널의 설계 초기 단계에서 매우 중요하며, 비에너지를 이용한 기계 굴착장비의 굴진성능평가방법은 모든 기계굴착공법에 적용될 수 있는 표준화된 방법이다. 본 연구에서는 코니컬형상의 픽 커터가 암석을 절삭할 때 요구되는 비에너지와 암석의 강도특성, 절삭조건 간의 상관관계를 분석하고자 하였으며, 선행연구를 통해 총46개의 선형절삭시험 결과를 수집하여 분석에 활용하였다. 본 연구에서 제안한 예측모델을 이용하여 산정된 픽 커터의 비에너지는 다중선형회귀분석에 비해 작은 평균제곱오차를 나타내었으며, 결정계수 또한 본 연구에서 제안한 모델이 다중선형회귀분석에 비해 우수한 예측결과를 나타내는 것을 확인할 수 있었다.

• 한국지반공학회논문집
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• 제39권11호
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• pp.23-31
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• 2023

콘크리트 궤도의 침하는 고속열차의 안전한 운행을 크게 저해할 수 있으며 복구에 상당한 비용이 소요되므로 최근 큰 문제로 대두되고 있다. 침하의 가장 큰 원인 중 하나는 터널공사의 부산물로 발생하는 암석재료를 적정한 입도분를 가진 토사 혼합 없이 하부노반 성토에 사용하기 때문으로 보고되고 있다. 본 논문에서는 암석재료로 성토된 하부노반의 부분 보강으로 인한 침하억제 효과를 수치해석으로 평가하였다. 하부노반에 일정하게 배치된 기둥 형태의 보강 영역은 상부노반에 지반아칭을 유발하게 되며 성토하중이 보강된 영역에 집중하게 되어 침하가 억제되는 효과가 있다. 유한요소해석을 통하여 보강된 영역의 크기, 강성, 간격이 하부노반의 침하억제에 미치는 효과를 평가하였으며, 해석 결과를 토대로 침하를 허용치 이내로 억제하기 위한 최소 보강 간격을 결정하는 설계 방법을 제안하였다.

• 지질공학
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• 제19권3호
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• pp.287-294
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• 2009

퇴적암인 셰일이나 이암이 광역변성작용을 받아 형성된 운모편암들이 분포하는 지역에서 도로나 철도 등의 건설 시 발생하는 사면은 이들 암석들이 가지는 층리나 엽리 혹은 절리와 같은 불연속면들의 발달 방향과 도로나 철로의 진행 방향의 조건에 따라 자주 사면의 붕괴 등에 의한 시공 상 어려움이 발생하고 있다. 운모편암은 변성작용 과정에서나 변형작용 과정을 거치면서 형성된 엽리 및 벽계면들이 사면에 영향을 미친다. 일반적으로 운모편암 중에는 팽창성광물인 스멕타이트. 버미쿨라이트, 몬모리오나이트 등이 함유 되어 사면이 형성된 후 지표에 노출되면서 공기 중의 수분과 반응하여 사면의 안정성에 불리하게 작용한다. 경부 고속철도 건설 시 수원부근의 운모 편암 지대에서 터널 시공 중 터널의 종점부 사면에서 붕괴가 일어난 곳의 지반 조건 및 붕괴 원인을 분석하였다. 이곳은 1-5cm의 절리 간격을 가지며 역단층과 주향이동단층 들이 발당한다. 이로 인해 사면이 심히 파쇄 되어 있으며 시추 코어에서도 전 구간의 RQD가 0-20%값을 가진다.

• 대한기계학회논문집B
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• 제27권11호
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• pp.1618-1629
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• 2003

The effects of the interaction between flow field and heat transfer caused by the longitudinal vortices are experimentally investigated using a five hole probe and a transient liquid crystal technique. The test facility consists of a wind tunnel with vortex generators protruding from a bottom surface and a mesh heater. In order to control the strength of the longitudinal vortices, the angle of attack of vortex generators used in the present experiment is 20$^{\circ}$, and the spacing between the vortex generators is 25mm. The height and cord length of the vortex generator is 20mm and 50mm, respectively. Three-component mean velocity measurements are made using a f-hole probe system, and the surface temperature distribution is measured by the hue capturing method using a transient liquid crystal technique. The transient liquid crystal technique in measuring heat transfer has become one of the most effective ways in determining the full surface distributions of heat transfer coefficients. The key point of this technique is to convert the inlet flow temperature into an exponential temperature profile using the mesh heater set up in the wind tunnel. The conclusions obtained in the present experiment are as follows: The two maximum heat transfer values exist over the whole domain, and as the longitudinal vortices move to the farther downstream region, these peak values show the decreasing trends. These trends are also observed in the experimental results of other researchers to have used the uniform heat flux method.

• Wind and Structures
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• 제24권5호
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• pp.501-528
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• 2017

In this work, wind tunnel tests of pressure measurements are carried out to assess the global aerodynamic interference factors, the local wind pressure interference factors, and the local lift spectra of an square high-rise building interfered by an identical cross-sections but lower height building arranged in various relative positions. The results show that, when the interfering building is located in an area of oblique upstream, the RMS of the along-wind, across-wind, and torsional aerodynamic forces on the test building increase significantly, and when it is located to a side, the mean across-wind and torsional aerodynamic forces increase; In addition, when the interfering building is located upstream or staggered upstream, the mean wind pressures on the sheltered windward side turn form positive to negative and with a maximum absolute value of up to 1.75 times, and the fluctuating wind pressures on the sheltered windward side and leading edge of the side increase significantly with decreasing spacing ratio (up to a maximum of 3.5 times). When it is located to a side, the mean and fluctuating wind pressures on the leading edge of inner side are significantly increased. The three-dimensional flow around a slightly-shorter disturbing building has a great effect on the average and fluctuating wind pressures on the windward or cross-wind faces. When the disturbing building is near to the test building, the vortex shedding peak in the lift spectra decreases and there are no obvious signs of periodicity, however, the energies of the high frequency components undergo an obvious increase.

• Wind and Structures
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• 제30권6호
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• pp.617-631
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• 2020

Wake-induced aerodynamics of yawed circular cylinders with smooth and grooved surfaces in a tandem arrangement was studied. This pair of cylinders represent sections of stay-cables with smooth surfaces and high-voltage power conductors with grooved surfaces that are vulnerable to flow-induced structural failure. The study provides some insight for a better understanding of wake-induced loads and galloping problem of bundled cables. All experiments in this study were conducted using a pair of stationary section models of circular cylinders in a wind tunnel subjected to uniform and smooth flow. The aerodynamic force coefficients and vortex-shedding frequency of the downstream model were extracted from the surface pressure distribution. For measurement, polished aluminum tubes were used as smooth cables; and hollow tubes with a helically grooved surface were used as power conductors. The aerodynamic properties of the downstream model were captured at wind speeds of about 6-23 m/s (Reynolds number of 5×10⁴ to 2.67×10⁵ for smooth cable and 2×10⁴ to 1.01×10⁵ for grooved cable) and yaw angles ranging from 0° to 45° while the upstream model was fixed at the various spacing between the two model cylinders. The results showed that the Strouhal number of yawed cable is less than the non-yawed case at a given Reynolds number, and its value is smaller than the Strouhal number of a single cable. Additionally, compared to the single smooth cable, it was observed that there was a reduction of drag coefficient of the downstream model, but no change in a drag coefficient of the downstream grooved case in the range of Reynolds number in this study.

• Geomechanics and Engineering
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• 제19권6호
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• pp.485-498
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• 2019

Pre-tensioned rock bolts can be classified into fully anchored, lengthening anchored and point anchored bolts based on the bond length of the resin or cement mortar inside the borehole. Bolts in varying anchoring methods may significantly affect the supporting effect of surrounding rock around a tunnel. However, thus far, the theoretical basis of selecting a proper anchoring method has not been thoroughly investigated. Based on this problem, 16 schemes were designed while incorporating the effects of anchoring length, pretension, bolt length, and spacing, and a systematic numerical experiment was performed in this paper. The distribution characteristics of the stress field in the surrounding rock, which corresponded to various anchoring scenarios, were obtained. Furthermore, an analytical approach for computing the active and passive strengthening index of the anchored surrounding rock is presented. A new fully anchoring method with pretension and matching technology are also provided. Then, an isolated loading model of the anchored surrounding rock was constructed. The physical simulation test for the bearing capacity of the model was performed with three schemes. Finally, the strengthening mechanism of varying anchoring methods was validated. The research findings in this paper may provide theoretical guidelines for the design and construction of bolting support in tunnels.

• Wind and Structures
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• 제26권2호
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• pp.57-68
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• 2018

Wind tunnel tests are conducted to investigate wake-induced vibrations of two circular cylinders with a center-to-center spacing of 4 diameters and attack angle varying from $0^{\circ}$ to $20^{\circ}$ for Reynolds numbers between 18,000 and 168,800. Effects of structural damping, Reynolds number, attack angle and reduced velocity on dynamic responses are examined. Results show that wake-induced vortex vibrations of the downstream cylinder occur in a wider range of the reduced velocity and have higher amplitudes in comparison to the vortex-induced vibration of a single circular cylinder. Two types of wake-induced instability phenomena with distinct dynamic characteristics are observed, which may be due to different generation mechanisms. For small attack angles like $5^{\circ}$ and $10^{\circ}$, the instability of the downstream cylinder characterizes a one-degree-of-freedom (1-DOF) oscillation moving in the across-wind direction. For a large attack angle like $20^{\circ}$, the instability characterizes a two-degree-of-freedom (2-DOF) oscillation with elliptical trajectories. For an attack angle of $15^{\circ}$, the instability can transform from the 1-DOF pattern to the 2-DOF one with the increase of the Reynolds number. Furthermore, the two instabilities show different sensitivity to the structural damping. The 1-DOF instability can be either completely suppressed or reduced to an unsteady oscillation, while the 2-DOF one is relatively less sensitive to the damping level. Reynolds number has important effects on the wake-induced instabilities.