• International Journal of Naval Architecture and Ocean Engineering
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• 제8권6호
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• pp.589-601
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• 2016

The hybrid grid was adopted and numerical prediction analysis of propeller unsteady bearing force considering free surface was performed for mode and full-scale KCS hull-propeller-rudder system by employing RANS method and VOF model. In order to obtain the propeller velocity under self-propulsion point, firstly, the numerical simulation for self-propulsion test of full-scale ship is carried out. The results show that the scale effect of velocity at self-propulsion point and wake fraction is obvious. Then, the transient two-phase flow calculations are performed for model and full-scale KCS hull-propeller-rudder systems. According to the monitoring data, it is found that the propeller unsteady bearing force is fluctuating periodically over time and full-scale propeller's time-average value is smaller than model-scale's. The frequency spectrum curves are also provided after fast Fourier transform. By analyzing the frequency spectrum data, it is easy to summarize that each component of the propeller bearing force have the same fluctuation frequency and the peak in BFP is maximum. What's more, each component of full-scale bearing force's fluctuation value is bigger than model-scale's except the bending moment coefficient about the Y-axis.

• 한국재료학회지
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• 제30권3호
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• pp.142-148
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• 2020

Graphene has attracted the interest of many researchers due to various its advantages such as high mobility, high transparency, and strong mechanical strength. However, large-area graphene is grown at high temperatures of about 1,000 ℃ and must be transferred to various substrates for various applications. As a result, transferred graphene shows many defects such as wrinkles/ripples and cracks that happen during the transfer process. In this study, we address transfer-free, large-scale, and high-quality monolayer graphene. Monolayer graphene was grown at low temperatures on Ti (10nm)-buffered Si (001) and PET substrates via plasma-assisted thermal chemical vapor deposition (PATCVD). The graphene area is small at low mTorr range of operating pressure, while 4 × 4 ㎠ scale graphene is grown at high working pressures from 1.5 to 1.8 Torr. Four-inch wafer scale graphene growth is achieved at growth conditions of 1.8 Torr working pressure and 150 ℃ growth temperature. The monolayer graphene that is grown directly on the Ti-buffer layer reveals a transparency of 97.4 % at a wavelength of 550 nm, a carrier mobility of about 7,000 ㎠/V×s, and a sheet resistance of 98 W/□. Transfer-free, large-scale, high-quality monolayer graphene can be applied to flexible and stretchable electronic devices.

• Advances in nano research
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• 제7권2호
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• pp.99-108
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• 2019

Higher-order theories are very important to investigate the mechanical properties and behaviors of nanoscale structures. In this study, a free vibration behavior of SiNW resting on elastic foundation is investigated via Eringen's nonlocal elasticity theory. Silicon Nanowire (SiNW) is modeled as simply supported both ends and clamped-free Euler-Bernoulli beam. Pasternak two-parameter elastic foundation model is used as foundation. Finite element formulation is obtained nonlocal Euler-Bernoulli beam theory. First, shape function of the Euler-Bernoulli beam is gained and then Galerkin weighted residual method is applied to the governing equations to obtain the stiffness and mass matrices including the foundation parameters and small scale parameter. Frequency values of SiNW is examined according to foundation and small scale parameters and the results are given by tables and graphs. The effects of small scale parameter, boundary conditions, foundation parameters on frequencies are investigated.

• 한국정보과학회논문지:시스템및이론
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• 제34권5_6호
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• pp.202-213
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• 2007

네트워크는 공학이나 자연과학은 물론이고 사회과학의 여러 분야를 연구하는데 중요하게 사용되는 모델이다. 이런 네트워크를 좀 더 쉽게 분석하기 위해서는 시각적으로 네트워크의 특징을 잘 나타내는 것이 필요하다. 이러한 그래프 레이아웃 연구는 컴퓨터 기술이 발달함에 따라 많이 연구되고 있다. 그 중에서 요즘 새롭게 부각되고 있는 척도 없는(Scale-free) 네트워크는 다양한 분야에서 복잡한 현상들을 분석하고 이해하는데 유용하게 쓰이고 있다. 이 네트워크의 특징은 링크의 수(Degree)가 멱함수(power law) 분포를 보이고, 다수의 링크를 가지는 허브가 존재함이 알려졌다. 따라서 척도 없는 네트워크에서는 허브를 시각적으로 잘 표현하는 것이 중요하지만 기존의 그래프 레이아웃 알고리즘은 클러스터를 잘 표현하는 정도이다. 그래서 본 논문에서는 척도 없는 네트워크를 잘 표현하는 그래프 레이아웃 알고리즘을 제안한다. 본 논문에서 제안한 알고리즘에서 허브들 간에 작용하는 허브성 척력이 거리에 반비례하고, 허브들의 degree가 a배 증가하면, 허브사이에 작용하는 척력의 크기는 ${\alpha}^{\gamma}({\gamma}$는 연결선 지수)배가 된다. 또한, 전체 노드수와 전체 링크수에 따라 적용되는 힘의 크기를 조정하는 계수를 두어서 네트워크의 규모에 관계없이 허브성 척력이 적용되는 특성이 있다. 제안한 알고리즘이 허브를 잘 표현하는 그래프 레이아웃 알고리즘인지를 기존의 방식과 실험을 통해서 비교하였다. 실험의 절차는 먼저 네트워크에 허브가 존재하는지를 식별한다. 허브의 존재를 식별하기 위한 방법은 연결선 지수를 확인하고, 연결선 지수의 값이 2와 3사이에 있으면 허브가 존재하는 척도 없는 네트워크로 판단한다. 다음은 이 네트워크의 레이아웃 작성에 제안한 알고리즘을 사용한다. 그 결과, 제안한 그래프 레이아웃 알고리즘이 기존의 Noack등의 클러스터중심의 알고리즘에 비해서 척도 없는 네트워크의 허브를 확실히 잘 보여주고 있음을 확인할 수 있었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집D
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• pp.759-764
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• 2001

Heat (mass) transfer characteristics have been investigated on the endwall of a large-scale linear turbine cascade passage under a combustor-level high free-stream turbulence with a large length scale. Local heat (mass) transfer coefficients are measured by using the naphthalene sublimation technique. The result shows that local heat (mass) transfer on the endwall is greatly enhanced in the central region of the turbine passage, but there is no noticeable change in the local heat (mass) transfer in the region suffering severe heat load. Under the high free-stream turbulence, the local heat (mass) transfer coefficient shows more uniform distribution and its average value across the whole endwall region is increased by 26% of that at low turbulence condition. The heat (mass) transfer data on the endwall strongly supports that well-organized vortices near the endwall tends to suffer an suppression by the high free-stream turbulence.

• Advances in nano research
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• 제8권2호
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• pp.115-134
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• 2020

In this paper, the free vibrations analysis of the nanoplates made of three-directional functionally graded material (TDFGM) with small scale effects is presented. To study the small-scale effects on natural frequency, modified strain gradient theory (MSGT) has been used. Material properties of the nanoplate follow an arbitrary function that changes in three directions along the length, width and thickness of the plate. The equilibrium equations and boundary conditions of nanoplate are obtained using the Hamilton's principle. The generalized differential quadrature method (GDQM) is used to solve the governing equations and different boundary conditions for obtaining the natural frequency of nanoplate made of three-directional functionally graded material. The present model can be transformed into a couple stress plate model or a classic plate model if two or all parameters of the length scales set to zero. Finally, numerical results are presented to study the small-scale effect and heterogeneity constants and the aspect ratio with different boundary conditions on the free vibrations of nanoplates. To the best of the researchers' knowledge, in the literature, there is no study carried out into MSGT for free vibration analysis of FGM nanoplate with arbitrary functions.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2558-2561
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• 2008

In the present study, a parallel Taylor-Galerkin/level set based two-phase flow code was developed using finite element discretization and domain decomposition method based on MPI (Message Passing Interface). The proposed method can be utilized for the analysis of a large scale free surface problem in a complex geometry due to the feature of FEM and domain decomposition method. Four-step fractional step method was used for the solution of the incompressible Navier-Stokes equations and Taylor-Galerkin method was adopted for the discretization of hyperbolic type redistancing and advection equations. A Parallel ILU(0) type preconditioner was chosen to accelerate the convergence of a conjugate gradient type iterative solvers. From the present parallel numerical experiments, it has been shown that the proposed method is applicable to the simulation of large scale free surface flows.

• Structural Engineering and Mechanics
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• 제82권6호
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• pp.747-756
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• 2022

This study presents the nonlinear free and forced vibrations of a cracked atomic force microscopy (AFM) cantilever by using the modified couple stress. The cracked section of the AFM cantilever is considered and modeled as rotational spring. In the frame work of Euler-Bernoulli beam theory, Von-Karman type of geometric nonlinear equation and the modified couple stress theory, the nonlinear equation of motion for the cracked AFM is derived by Hamilton's principle and then discretized by using the Galerkin's method. The semi-inverse method is utilized for analysis nonlinear free oscillation of the system. Then the method of multiple scale is employed to investigate primary resonance of the system. Some numerical examples are presented to illustrate the effects of some parameters such as depth of the crack, length scale parameter, Tip-Mass, the magnitude and the location of the external excitation force on the nonlinear free and forced vibration behavior of the system.

• Structural Engineering and Mechanics
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• 제65권5호
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• pp.573-585
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• 2018

This article presents strain gradient elasticity-based procedures for static bending, free vibration and buckling analyses of functionally graded rectangular micro-plates. The developed method allows consideration of smooth spatial variations of length scale parameters of strain gradient elasticity. Governing partial differential equations and boundary conditions are derived by following the variational approach and applying Hamilton's principle. Displacement field is expressed in a unified way to produce numerical results in accordance with Kirchhoff, Mindlin, and third order shear deformation theories. All material properties, including the length scale parameters, are assumed to be functions of the plate thickness coordinate in the derivations. Developed equations are solved numerically by means of differential quadrature method. Proposed procedures are verified through comparisons made to the results available in the literature for certain limiting cases. Further numerical results are provided to illustrate the effects of material and geometric parameters on bending, free vibrations, and buckling. The results generated by Kirchhoff and third order shear deformation theories are in very good agreement, whereas Mindlin plate theory slightly overestimates static deflection and underestimates natural frequency. A rise in the length scale parameter ratio, which identifies the degree of spatial variations, leads to a drop in dimensionless maximum deflection, and increases in dimensionless vibration frequency and buckling load. Size effect is shown to play a more significant role as the plate thickness becomes smaller compared to the length scale parameter. Numerical results indicate that consideration of length scale parameter variation is required for accurate modelling of graded rectangular micro-plates.

• 대한조선학회논문집
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• 제58권6호
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• pp.415-423
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• 2021

There have been many experimental studies on the manoeuvrability of KRISO Container Ship (KCS). However, the scale ratio of the model ship and the test procedure for each institute are slightly different, so direct comparison for the data is technically difficult to perform. This paper presents the manoeuvrability of the ship with different scale ratios: 1/65.8, 1/42.0, and 1/31.6 in model scale. KRISO conducted Free Running Model Tests (FRMT): 35° turning circle tests and 20/20(10/10) zigzag manoeuvring tests. The test results indicated that advance and tactical diameter in turning circle tests were similar, and overshoot angles in two zigzag manoeuvring tests increased as the model ship size increased. In addition, a basic concept for the FRMT method with an auxiliary X-thrust device was proposed so that the scale effect could be considered in model ship tests.