• KSII Transactions on Internet and Information Systems (TIIS)
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• 제18권6호
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• pp.1659-1674
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• 2024

Conventional maritime communication struggles to provide high data rate services for Internet of Things (IoT) devices due to the variability of maritime environments, making it challenging to ensure consistent connectivity for onboard sensors and devices. To resolve this, we perform mathematical modeling of the maritime channel and compare it with real measurement data. Through the modeled channel, we verify the received beam gain at buoys on the ocean surface. Additionally, leveraging the modeled wave motions, we estimate future angles of the buoy to use the Extended Kalman Filter (EKF) for design beamforming strategies that adapt to the evolving maritime environment over time. We further validate the effectiveness of these strategies by assessing the results from an outage probability perspective. focuses on improving maritime communication by developing a dynamic model of the maritime channel and implementing a Kalman filter-based buoy motion tracking system. This system is designed to enable precise beamforming, a technique used to direct communication signals more accurately. By improving beamforming, the aim is to enhance the quality of communication links, even in challenging maritime conditions like rough seas and varying sea states. In our simulations that consider realistic wave motions, you've observed significant improvements in link quality due to the enhanced beamforming technique. These improvements are particularly notable in environments with high sea states, where communication challenges are typically more pronounced. The progress made in this area is not just a technical achievement; it has broad implications for the future of maritime communication technologies. This paper promises to revolutionize the way we approach communication in maritime environments, paving the way for more reliable and efficient information exchange on the seas.

• 한국도로학회논문집
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• 제2권1호
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• pp.135-145
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• 2000

최근 포장도로의 역학적 상태를 평가하는 방법으로 비파괴 시험인 FWD(Falling Weight Deflectometer)와 탄성파시험이 많이 이용되고 있다. 그러나 기존의 방법들은 공용중인 도로에서 차량을 통제시킨 후 시험을 실시해야 하는 제한이 있다. 그러므로 실제 주행하중 통과시 아스팔트 콘크리트 포장구조체의 물성을 추정하여 잔존수명 예측 및 이동하중에 대한 포장체 거동을 분석하는 경우에는 FWD와 같이 표면처짐으로부터 아스팔트 콘크리트 포장구조체 각 층의 물성을 추정하는 방법의 사용이 곤란하다. 이런 경우에 MDD (Multi-Depth Deflectometer)를 통해 얻어진 깊이별 처짐을 사용하여 아스팔트 콘크리트 포장구조체 각 층의 물성을 역산 추정하고자 본 연구에서는 다층 탄성이론의 반복적인 역산과 충격하중의 영향을 고려하여 깊이별 처짐으로부터 아스팔트 콘크리트 포장구조체 각층의 물성을 추정할 수 있는 역산반복기법을 개발한 후 이를 수치검증하였다. 수치모델을 통하여 검증한 결과, 역산추정된 탄성계수와 실제탄성계수 사이의 오차는 최대 0.114%로 신뢰성 있는 결과를 얻었다. 또한 본 연구에서는 주행하중의 속도에 따른 아스팔트 콘크리트 포장구조체의 동적특성을 파악하여 실제적인 포장구조체의 거동을 분석하고자 수도권 외곽 순환고속도로 김포구간에서 실제 트럭주행을 통한 현장시험을 실시하였다. 주행하중에 대한 아스팔트 콘크리트 포장구조체의 거동을 깊이별 처짐 측정장비인 MDD를 이용하여 깊이별 상대처짐을 측정하고, 주행속도에 따라 포장구조체의 거동을 해석하여 차량의 속도와 포장체 거동을 역학적으로 분석하였고, 주행속도별 층별 동적물성을 개발된 역해석 프로그램으로부터 산정하였다. 주행속도별 동적특성 분석결과, 차량의 주행속도가 증가할수록 깊이별 상대처짐은 감소하였고, 실측된 깊이별 처짐으로부터 포장구조체의 층별 물성을 역해석한 결과 속도가 증가할수록 탄성계수가 증가하였다. 따라서 주행속도가 줄어들수록 포장체의 구조적 능력 저하에 크게 영향을 주는 것을 알 수 있었다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제6권2호
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• pp.219-235
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• 2014

With the development of the technology of underwater moving bodies, the need for developing the knowledge of surface effect interaction of free surface and underwater moving bodies is increased. Hence, the two-phase flow is a subject which is interesting for many researchers all around the world. In this paper, the non-linear free surface deformations which occur during the water-exit of a circular cylinder due to its buoyancy are solved using finite volume discretization based code, and using Volume of Fluid (VOF) scheme for solving two phase flow. Dynamic mesh model is used to simulate dynamic motion of the cylinder. In addition, the effect of cylinder mass in presence of an external force is studied. Moreover, the oblique exit and entry of a circular cylinder with two exit angles is simulated. At last, water-exit of a circular cylinder in six degrees of freedom is simulated in 3D using parallel processing. The simulation errors of present work (using VOF method) for maximum velocity and height of a circular cylinder are less than the corresponding errors of level set method reported by previous researchers. Oblique exit shows interesting results; formation of waves caused by exit of the cylinder, wave motion in horizontal direction and the air trapped between the waves are observable. In 3D simulation the visualization of water motion on the top surface of the cylinder and the free surface breaking on the front and back faces of the 3D cylinder at the exit phase are observed which cannot be seen in 2D simulation. Comparing the results, 3D simulation shows better agreement with experimental data, specially in the maximum height position of the cylinder.

• Structural Engineering and Mechanics
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• 제36권1호
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• pp.111-127
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• 2010

This paper studies the effects of spatially varying ground motions on the responses of a bridge frame located on a canyon site. Compared to the spatial ground motions on a uniform flat site, which is the usual assumptions in the analysis of spatial ground motion variation effects on structures, the spatial ground motions at different locations on surface of a canyon site have different intensities owing to local site amplifications, besides the loss of coherency and phase difference. In the proposed approach, the spatial ground motions are modelled in two steps. Firstly, the base rock motions are assumed to have the same intensity and are modelled with a filtered Tajimi-Kanai power spectral density function and an empirical spatial ground motion coherency loss function. Then, power spectral density function of ground motion on surface of the canyon site is derived by considering the site amplification effect based on the one dimensional seismic wave propagation theory. Dynamic, quasi-static and total responses of the model structure to various cases of spatially varying ground motions are estimated. For comparison, responses to uniform ground motion, to spatial ground motions without considering local site effects, to spatial ground motions without considering coherency loss or phase shift are also calculated. Discussions on the ground motion spatial variation and local soil site amplification effects on structural responses are made. In particular, the effects of neglecting the site amplifications in the analysis as adopted in most studies of spatial ground motion effect on structural responses are highlighted.

• 대한기계학회논문집A
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• 제36권12호
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• pp.1489-1495
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• 2012

본 논문에서는 부유식 플랫폼의 동적 거동을 고려하여 해상 풍력 발전기 타워의 구조 해석을 수행하였다. 풍력 발전기는 플랫폼, 타워, 낫셀, 허브 그리고 3 개의 블레이드로 구성된다. 타워는 3 차원 빔 요소를 사용하여 탄성체로 모델링하여 탄성 다물체계 동역학을 기반으로 한 운동 방정식을 구성하였다. 회전하는 블레이드에는 블레이드 요소 운동량 이론에 따라 계산된 공기역학적 힘이 적용되었고, 부유식 플랫폼에는 유체정역학적 힘, 유체동역학적 힘 그리고 계류력이 적용되었다. 타워의 구조 동역학적 거동을 수치적으로 시뮬레이션하였다. 시뮬레이션 결과를 이용하여 굽힘 모멘트와 응력을 산출하고 허용치와 비교하였다.

• International Journal of Fluid Machinery and Systems
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• 제2권4호
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• pp.269-277
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• 2009

The current paper focuses on the analysis of transient cavitating flow in pressurised polyethylene pipes, which are characterized by viscoelastic rheological behaviour. A hydraulic transient solver that describes fluid transients in plastic pipes has been developed. This solver incorporates the description of dynamic effects related to the energy dissipation (unsteady friction), the rheological mechanical behaviour of the viscoelastic pipe and the cavitating pipe flow. The Discrete Vapour Cavity Model (DVCM) and the Discrete Gas Cavity Model (DGCM) have been used to describe transient cavitating flow. Such models assume that discrete air cavities are formed in fixed sections of the pipeline and consider a constant wave speed in pipe reaches between these cavities. The cavity dimension (and pressure) is allowed to grow and collapse according to the mass conservation principle. An extensive experimental programme has been carried out in an experimental set-up composed of high-density polyethylene (HDPE) pipes, assembled at Instituto Superior T$\acute{e}$cnico of Lisbon, Portugal. The experimental facility is composed of a single pipeline with a total length of 203 m and inner diameter of 44 mm. The creep function of HDPE pipes was determined by using an inverse model based on transient pressure data collected during experimental runs without cavitating flow. Transient tests were carried out by the fast closure of the ball valves located at downstream end of the pipeline for the non-cavitating flow and at upstream for the cavitating flow. Once the rheological behaviour of HDPE pipes were known, computational simulations have been run in order to describe the hydraulic behaviour of the system for the cavitating pipe flow. The calibrated transient solver is capable of accurately describing the attenuation, dispersion and shape of observed transient pressures. The effects related to the viscoelasticity of HDPE pipes and to the occurrence of vapour pressures during the transient event are discussed.

• 대한조선학회논문집
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• 제36권2호
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• pp.61-71
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• 1999

본 연구에서는 조수간만의 차가 큰 항만에 적합한 새로운 하역 시스템을 제안하였다. 새로운 하역 시스템은 기존의 갑문 이용 방식 대신에 일종의 유압 엘리베이터를 사용함으로써 조수간만의 차에 따라 변하는 선박의 높이로 실시간 제어하여 선적 흑은 하역을 하는 방식이다. 한번에 대량의 컨테이너를 처리하기 위해 컨테이너 팔레트 운송타를 도입함으로써 기존 시스템에 비해 시간당 처리능력을 높일 수 있고, 선적 및 하역의 전 과정을 자동화함으로써 작업 효율을 극대화할 수 있도록 하였다. 새로운 하역 시스템의 개념 입증을 위해 소형 하역 시스템 모델을 설계 제작하였다. 제작된 모델은 컨테이너 팔레트 운송차, 플랫폼과 리프트 칼럼의 복합 구조물, 그리고 전용 운반선으로 구성된다. ER(electro-rheological) 밸브-실린더에 의해 구동되는 플랫폼은 조수간만의 차와 과도에 의해 변하는 선박의 높이를 능동적으로 추적 제어하게 된다. 동적 모델성을 통해 플랫폼의 운동 지배 방정식을 도출하였고, 파도 가진으로 인한 선박의 거동 및 조수간만의 차를 모델링하였다. 선적 및 하역의 자동화와 플랫폼의 정확한 위치 제어를 위해 시퀀스 제어기와 PID(proportional-integral-derivative) 위치 제어기가 연계된 제어 시스템을 구성하였다. 컴퓨터 시뮬레이션과 실험을 통해 제안된 하역 시스템의 유용성을 입증하였다.

• 한국지진공학회논문집
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• 제11권4호
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• pp.25-31
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• 2007

For areas such as the Korean Peninsula, which have moderate seismic activity but no available records of strong ground motion, synthetic seismograms can be used to evaluate ground motion without waiting for a strong earthquake. Such seismograms represent the estimated ground motions expected from a set of possible earthquake scenarios. Local site effects are especially important in assessing the seismic hazard and possible ground motion scenarios for a specific fault. The earthquake source and rupture dynamics can be described as a two-step process of rupture initiation and front propagation controlled by a frictional sliding mechanism. The seismic wavefield propagates through heterogeneous geological media and finally undergoes near-surface modulations such as amplification or deamplification. This is a complex system in which various scales of physical phenomena are integrated. A unified approach incorporates multi-scale problems of dynamic rupture, radiated wave propagation, and site effects into an all-in-one model using a three-dimensional, fourth-order, staggered-grid, finite-difference method. The method explains strong ground motions as products of complex systems that can be modified according to a variety of fine-scale rupture scenarios and friction models. A series of such deterministic earthquake scenarios can shed light on the kind of damage that would result and where it would be located.

• Journal of Advanced Marine Engineering and Technology
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• 제8권1호
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• pp.85-99
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• 1984

The stability of the two-phase flow in a heated channel is of great importance in the design and operation of the boilers and light water nuclear reactors, because it can cause flow oscillations and lead to a violation of thermal limits with resultant overheating of the channels and cladding. This paper presents a systematic evaluation to the variation effects of the basic four (4) dimensionless parameters in a homogeneous equilibrium model. The flow stability is examined on the ground of static characteristic curves. The complicated transfer function of flow dynamics which gives consideration to the transport lag of density wave is derived, and the transient flow stability is analysed by applying the Nyquist stability criterion in control engineering. The analysis results summed up as follows 1. The coolant flow becomes stable in large friction number and specific flow, while it is unstabale in small friction number and flow. 2. Large phase-change number and Froude number destabilize the two-phase flow, but small numbers stabilize it. The effect to variation of phase-change number is more dominant compared with Froude number. 3. The dynamic analysis is required to hold the sufficient safety of heated channels since only static results does not keep it. The special attention could be payed in the design and operation of heat engines, because the unstaable region exists within the stable boundary at small and middle phase-change number and Froude number.

• 한국해양공학회지
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• 제30권3호
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• pp.161-168
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• 2016

This paper presents a methodology through which the time series of the dynamic response of mooring line tension can be predicted without relying on a time-consuming nonlinear time-domain analysis. The mooring line tension for the target short-term sea states was predicted using a Hammerstein-Wiener model, a popular system identification scheme, based upon the pre-calculated motion-tension time history data for some selected short-term sea states that do not overlap with the targeted ones. The obtained mooring line tension was further processed, and a fatigue damage comparison was made between the predicted and calculated values. The results showed that the predicted time series of the mooring line tension matched the calculated one fairly well. Thus, it is expected that the methodology may be employed to enhance the efficiency of mooring line tension analysis.