• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2000.11a
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• pp.3-8
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• 2000

Chemical nature of nascent metal surfaces which is one of the important active sources for tribochemical reactions was investigated using a newly developed method. Some enhanced activities were observed. For example, organic compounds chemisorbed on nascent gold surfaces and aromatic compounds decomposed on nascent nickel surfaces resulting in hydrogen evolution. Non-polar compounds such as organic sulfides had a higher chemisorption activity on nascent steel surfaces than polar compounds such as fatty acids and phosphates. Organic sulfides reacted directly with nascent steel surfaces and the surface was covered with metal sulfides. The activity for the chemisorption of organic compounds was closely dependent on the electronic structure of metals. Although benzene chemisorbed very easily on nascent surfaces of transition metals, it did not chemisorb ell nascent surfaces of simple metals. Boundary lubricating behaviors of extreme pressure additives were explained on the bases of the chemical activities of nascent surfaces obtained in this investigation. Under mild conditions, polar compounds such as fatty acids and phosphates were effective for boundary lubrication, because surfaces are covered with oxide layers. On the other hand, sulfides were more effective under severe conditions where the oxide layers were removed and the nascent surfaces were formed.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.5
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• pp.1052-1060
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• 1989

Using a vortex stretching invariant term, the two-layer k-.epsilon. model has been modified to account for the extra staining of turbulence due to the mean-flow convergence and divergence. The calculations of turbulent boundary layers in a plane of symmetry are compared for experimental cases which are an axisymmetric body at an incidence of 15.deg.. The comparisons between the calculations and experimental data show that additional modifications to the dissipation rate equation have brought the significant improvement to the prediction of plane of symmetry boundary layers in the strong mean-flow convergence and divergence.

• Structural Engineering and Mechanics
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• v.25 no.6
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• pp.637-652
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• 2007

This paper presents stacking sequence optimization of laminated angle-ply cylindrical panel based on natural frequency. Finite element method (FEM) is used to obtain the vibration characteristic of an anisotropic panel using the first order shear deformation theory(FSDT) and genetic algorithm (GA) is used to obtain the optimal stacking sequence of the layers. Cylindrical panel has finite length and arbitrary boundary conditions. The thicknesses of the layers are assumed constant and their angles are specified as design variables. The effect of the number of plies and boundary conditions in the fitness function is considered. Numerical examples are presented for four, six and eight layered anisotropic cylindrical panels.

• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.379-381
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• 2011

Direct numerical simulations (DNSs) of spatially developing turbulent boundary layers (TBLs) over two-dimensional (2-D) and rod and three-dimensional (3-D) cube rough walls were performed to investigate the effects of streamwise spacing on the properties of the TBL The 2-D and 3-D roughness were periodically arranged in the downstream direction with pitches of px/k=2, 3, 4, 6, 8 and 10 and for the cube, the spanwise spacing is fixed to pz/k=2 with staggered array, where px and pz are the streamwise and spanwise spacings of the roughness and k is the roughness height. Inspection of the Reynolds stresses showed that except for px/k=2 and 3 over the 2-D rough walls, the effects of the surface roughness extend to the outer layer over the 2-D and 3-D rough walls and the magnitude of the Reynolds shear stress in the outer layer is increased with proportion to px/k. However, such results are contrary to the trends of form drag, roughness junction and roughness length against px/k, which showed the maximum values at px/k=8 and 4 over the 2-D and 3-D rough walls respectively.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.9
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• pp.805-815
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• 2011

Nonlinear Parabolized Stability Equations(NSPE) can be effectively used to study more throughly the transition process. NPSE can efficiently analyze the stability of a nonlinear region in transition process with low computational cost compared to Direct Numerical Simulation(DNS). In this study, NPSE in general coordinate system is formulated and a computer code to solve numerically the equations is developed. Benchmark problems for incompressible and compressible boundary layers over a flat plate are analyzed to validate the present code. It is confirmed that the NPSE methodology constructed in this study is an efficient and effective tool for nonlinear stability analysis.

• Journal of the Institute of Electronics and Information Engineers
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• v.51 no.12
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• pp.140-149
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• 2014

Image enhancement has become an important area of study with the recent development of hi-fidelity devices, such as UHD displays. While conventional methods are able to enhance the image contrast and detail, this sometimes results in contrast reversion in boundary region. Therefore, this paper proposes the use of multi-layers and intensity deviation in boundary areas to enhance the perceived image quality. First, the image contrast of individual blocks is enhanced using multi-layers with different sizes. After calculating the block boundaries, weights are then determined based on the intensity deviation and used to enhance the image detail. Experiments with several test images confirm that the proposed algorithm is superior that image contrast and detail to conventional methods.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.7
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• pp.834-841
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• 2004

Experimental study on the three-dimensional topology of hairpin packet structures in turbulent boundary layers were carried out. Two different Reynolds number based on momentum thickness, Re$\sub$$\theta$/=514 and 934 were generated in a blowing type wind tunnel under the condition of zero pressure gradient. Simultaneous measurements of velocity fields at a wall-normal plane and wall-parallel plane by a plane PIV and a Stereo-PIV systems. The two Nd:Yag laser systems and three CCD cameras were synchronized to obtain instantaneous velocity fields at the same time. To avoid optical noise at the crossing line by the two laser light sheets, a new optical arrangement using polarization was applied. The obtained velocity fields show the existence of hairpin packet structure vividly and the idealized hairpin vortex signature is confirmed by experiment. Two counter-rotating vortex pair which reflects the cutting plane of hairpin legs are found both side of a strong streaky structure when the wall-normal plane cuts the hairpin head.

• Advances in nano research
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• v.6 no.4
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• pp.299-321
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• 2018

A layerwise shear deformation theory is applied in this paper for buckling analysis of piezoelectric truncated conical shell. The core is a multiphase nanocomposite reinforced by carbon nanotubes (CNTs) and carbon fibers. The top and bottom face sheets are piezoelectric subjected to 3D electric field and external voltage. The Halpin-Tsai model is used for obtaining the effective moisture and temperature dependent material properties of the core. The proposed layerwise theory is based on Mindlin's first-order shear deformation theory in each layer and results for a laminated truncated conical shell with three layers considering the continuity boundary condition. Applying energy method, the coupled motion equations are derived and analyzed using differential quadrature method (DQM) for different boundary conditions. The influences of some parameters such as boundary conditions, CNTs weight percent, cone semi vertex angle, geometrical parameters, moisture and temperature changes and external voltage are investigated on the buckling load of the smart structure. The results show that enhancing the CNTs weight percent, the buckling load increases. Furthermore, increasing the moisture and temperature changes decreases the buckling load.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.7
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• pp.945-956
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• 2000

The axisymmetric bodies considered in this study have hemispherical and ellipsoidal noses. The near-field pressure fluctuations over each nose model at $Re_D=2.43{\times}10^5$ were investigated in the laminar separation region and developing turbulent boundary layers using a 1/8' pin-holed microphone sensor. The wall pressure fluctuations were also measured in an axisymmetric boundary layer on a cylinder parallel to mean flow at a momentum thickness Reynolds number of 850 and a boundary layer thickness to cylinder radius ratio of 1.88.

• Bulletin of the Society of Naval Architects of Korea
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• v.18 no.2
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• pp.7-20
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• 1981

A method to predict the three-dimensional turbulent boundary layer on ship forms is introduced. The present differential method is in the scope of thin boundary layer theory and adopting the eddy-viscosity turbulence model. Two different numerical schemes are taken in this paper to handle the sign-changing cross-flows. The method is applied to predict the boundary layer development on real ship forms; SSPA Model 720($C_B$=0.675) and HSVA Tanker Model($C_B$=0.85). The results are qualitatively in good agreements with measurements except at the very stern. Therefore the method seems to be very promising if further developments are accomplished to handle the thick stern boundary layer effectively.