• International Journal of Naval Architecture and Ocean Engineering
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• 제2권3호
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• pp.139-145
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• 2010

In the present study, we conducted resistance test, propeller open water test and self-propulsion test for a ship's resistance and propulsion performance, using computational fluid dynamics techniques, where a Reynolds-averaged Navier-Stokes equations solver was employed. For convenience of mesh generation, unstructured meshes were used in the bow and stern region of a ship, where the hull shape is formed of delicate curved surfaces. On the other hand, structured meshes were generated for the middle part of the hull and the rest of the domain, i.e., the region of relatively simple geometry. To facilitate the rotating propeller for propeller open water test and self-propulsion test, a sliding mesh technique was adopted. Free-surface effects were included by employing the volume of fluid method for multi-phase flows. The computational results were validated by comparing with the existing experimental data.

• 한국군사과학기술학회지
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• 제12권6호
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• pp.704-710
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• 2009

In this study, a comparison study of flutter analysis for the AGARD 445.6 wing with wind turnnel test data has been conducted in the subsonic, transonic and supersonic flow regions. Nonlinear aeroelastic using FSIPRO3D which is a generalized user-friendly fluid-structure analyses have been conducted for a 3D wing configuration considering shockwave and turbulent viscosity effects. The developed fluid-structure coupled analysis system is applied for aeroelastic computations combining computational structure dynamics(CSD), finite element method(FEM) and computations fluid dynamics(CFD) in the time domain. MSC/NASTRAN is used for the vibration analysis of a wing model, and then the result is applied to the FSIPRO3D module. the results for dynamic aeroelastic response using different turbulent models are presented for several Mach numbers. Calculated flutter boundary are compared with the wind-tunnel experimental and the results show very good agreements.

• Structural Engineering and Mechanics
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• 제1권1호
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• pp.87-102
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• 1993

A $C^{\circ}$ continuous finite element formulation of a higher order displacement theory is presented for predicting linear and geometrically non-linear in the sense of von Karman transient responses of composite and sandwich plates. The displacement model accounts for non-linear cubic variation of tangential displacement components through the thickness of the laminate and the theory requires no shear correction coefficients. In the time domain, the explicit central difference integrator is used in conjunction with the special mass matrix diagonalization scheme which conserves the total mass of the element and included effects due to rotary inertia terms. The parametric effects of the time step, finite element mesh, lamination scheme and orthotropy on the linear and geometrically non-linear responses are investigated. Numerical results for central transverse deflection, stresses and stress resultants are presented for square/rectangular composite and sandwich plates under various boundary conditions and loadings and these are compared with the results from other sources. Some new results are also tabulated for future reference.

• International Journal of Naval Architecture and Ocean Engineering
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• 제8권2호
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• pp.188-197
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• 2016

Offshore substructures have been developed to support structures against complex offshore environments. The load at offshore substructures is dominated by waves, and deformation of waves caused by interactions with the current is an important phenomena. Wave load simulation of fixed offshore substructures in waves with the presence of uniform current was carried out by numerical wave tank technique using the commercial software, FLUENT. The continuity and Navier-Stokes equations were applied as the governing equations for incompressible fluid motion, and numerical wavemaker was employed to reproduce offshore wave environment. Convergence test against grids number was carried out to investigate grid dependency and optimized conditions for numerical wave generation were derived including investigation of the damping effect against length of the damping domain. Numerical simulation of wave and current interactions with fixed offshore substructure was carried out by computational fluid dynamics, and comparison with other experiments and simulations results was conducted.

• 한국전산구조공학회논문집
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• 제23권2호
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• pp.209-215
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• 2010

본 논문에서는 CFD와 FEM에 기초한 진보된 전산해석 기법을 적용하였고 지진에 의한 수평 및 수직방향 기반가진 및 풍하중 효과를 동시에 고려하여 MW급 풍력발전기의 지진응답 해석을 수행하였다. 본 연구에서는 실용적인 응답특성 파악을 위해 시간영역 수치해석기법을 적용하였으며, 지진기반하중 및 풍하중에 의한 영향을 상호 비교하였다. 본 연구의 결과를 통해 관성 특성이 매우 큰 초대형 풍력발전기가 정상작동 조건 중 지진이 유발되는 경우가 타워에 유발되는 응력특성에 큰 영향을 미칠 수 있기 때문에 중요하게 고려될 필요성이 있음을 알 수 있었다.

• Wind and Structures
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• 제20권2호
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• pp.169-189
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• 2015

To investigate the effects of "sudden change" of wind fluctuations on vehicle running performance, which is caused by the artificial discrete simulation of wind field, a three-dimensional vehicle model is set up with multi-body dynamics theory and the vehicle dynamic responses in crosswind conditions are obtained in time domain. Based on Hilbert Huang Transform, the effects of simulation separations on time-frequency characteristics of wind field are discussed. In addition, the probability density distribution of "sudden change" of wind fluctuations is displayed, addressing the effects of simulation separation, mean wind speed and vehicle speed on the "sudden change" of wind fluctuations. The "sudden change" of vehicle dynamic responses, which is due to the discontinuity of wind fluctuations on moving vehicle, is also analyzed. With Principal Component Analysis, the comprehensive evaluation of vehicle running performance in crosswind conditions at different simulation separations of wind field is investigated. The results demonstrate that the artificial discrete simulation of wind field often causes "sudden change" in the wind fluctuations and the corresponding vehicle dynamic responses are noticeably affected. It provides a theoretical foundation for the choice of a suitable simulation separation of wind field in engineering application.

• International Journal of Aeronautical and Space Sciences
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• 제8권1호
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• pp.140-147
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• 2007

Whenever the hinge axis of aircraft wing rotates, its stiffness varies. Also, there are nonlinearities in the connection of the actuator and the hinge axis, and it is necessary to inspect the coupled effects between the actuator dynamics and the hinge nonlinearity. Nonlinear aeroelastic characteristics are investigated by using the iterative V-g method. Time domain analyses are also performed by using Karpel's minimum state approximation technique. The doublet hybrid method(DHM) is used to calculate the unsteady aerodynamic forces in subsonic regions. Structural nonlinearity located in the load links of the actuator is assumed to be friction. The friction nonlinearity of an actuator is identified by using the describing function technique. The nonlinear flutter analyses have shown that the flutter characteristics significantly depends on the structural nonlinearity as well as the dynamic stiffness of an actuator. Therefore, the dynamic stiffness of an actuator as well as the nonlinear effect of hinge axis are important factors to determine the flutter stability.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1366-1371
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• 2003

Molecular dynamics (MD) simulations are conducted to investigate the thermophysical characteristics and the stability of liquid threads for various conditions. A cylindrical thread in the simulation domain is made of Lennard-Jones molecules. The surface tension of liquid threads can be determined from local densities, local normal and transverse components of the pressure force. In order to understand the effects of thread radii on surface tensions, the Tolman equation is modified on the basis of the cylindrical coordinates for prediction of surface tensions. Surface tensions calculated from the MD simulation agree with the prediction from the modified Tolman equation. In addition, surface tensions decrease linearly with increasing system temperature. For a binary system, the surface tension decreased linearly compared to that for a pure system with increasing binary ratio of solute molecules which have relatively large value of the affinity coefficient. For a fixed binary ratio, the surface tension increased slightly with the affinity coefficient and the maximum value appear around where the affinity coefficient is 1.5 and decreased rapidly for upper value of 1.5. In addition, the critical wavelengths of perturbations are proven to be directly proportional to the equimolar dividing radii of the liquid threads.

• Composites Research
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• 제29권6호
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• pp.375-378
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• 2016

Characteristics of nanocrystalline materials are known substantially dependent on the microstructure such as grain size, crystal orientation, and grain boundary. Thus it is desired to have systematic characterization methods on the various nanomaterials with complex geometries, especially in low dimensional nature. One of the interested nanomaterials would be a pure two-dimensional material, graphene, with superior mechanical, thermal, and electrical properties. In this study, mechanical properties of "polycrystalline" graphene were numerically investigated by molecular dynamics simulations. Subdomains with various sizes would be generated in the polycrystalline graphene during the fabrication such as chemical vapor deposition process. The atomic models of polycrystalline graphene were generated using Voronoi tessellation method. Stress strain curves for tensile deformation were obtained for various grain sizes (5~40 nm) and their mechanical properties were determined. It was found that, as the grain size increases, Young's modulus increases showing the reverse Hall-Petch effect. However, the fracture strain decreases in the same region, while the ultimate tensile strength (UTS) rather shows slight increasing behavior. We found that the polycrystalline graphene shows the reverse Hall-Petch effect over the simulated domain of grain size (< 40 nm).

• 설비공학논문집
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• 제15권4호
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• pp.318-328
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• 2003

Numerical simulation using computational fluid dynamics (CFD) is performed to calculate the velocities and temperature profiles of air in adjacent to a worker within the individual local air conditioning system. The calculation domain is the space of ㄴ between walls and a worker in the climate room. The fresh air is supplied from the three different inlets located on the right, left and center wall in the climate room. In this study, the calculated data of velocities and temperature profiles of air in the nearest the skin of a worker are used to calculate the PMV (Predicted Mean Vote) for evaluation of thermal comfort of a worker in the local air conditioning system. Because the data of veto-cities temperature profiles of air in adjacent to a worker and the PMV of a worker are the design parameters of the local air conditioning system. The results of calculation show that the fresh air velocity and injection position are closely related to the PMV value. In individual air condition system of ㄴ, the appropriate PMV are obtained when the fresh air velocity and position are 1.0 m/s, throat of a worker and are 1.5 m/s, head of a worker, respectively. The method of numerical calculation is effective to obtain the optimum velocity and position of the fresh air for optimum the PMV and energy saving in individual local air conditioning system.