• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.1
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• pp.41-52
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• 2021

During atmospheric ascent of a launch vehicle, airborne acoustic loads act on the vehicle and its effect becomes pronounced at transonic speed. In the present study, acoustic loads acting on a hammerhead launch vehicle at a transonic speed have been analyzed using ��-ω SST based IDDES and the results including mean Cp, Cprms, and PSD are compared to available wind-tunnel test data. Mesh dependency of IDDES results has been investigated and it has been concluded that with an appropriate turbulence scale-resolving computational mesh, the characteristic flow features around a transonic hammerhead launch vehicle such as separated shear flow at fairing shoulder and its reattachment on rear body as well as large pressure fluctuation in the region of separated flow behind the boat-tail can be predicted with reasonable accuracy for engineering purposes.

• Journal of the Korea Concrete Institute
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• v.14 no.1
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• pp.126-135
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• 2002

Plate bonding technique has been widely used in strengthening of existing concrete structures, although it has often a serious problem of premature falure such as interface separation and rip-off. However, this premature failure problem has not been well explored yet especially in view of local failure mechanism around the interface of plate ends. The purpose of the present study is, therefore, to identify the local failure of strengthened plates and to derive a separation criterion at the interface of plates. To this end, a comprehensive experimental program has been set up. The double lap pull-out tests considering pure shear force and half beam tests considering combined flexure-shear force were performed. The main experimental parameters include plate thickness, adhesive thickness, and plate end arrangement. The strains along the longitudinal direction of steel plates have been measured and the shear stress were calculated from those measures strains. The effects of plate thickness, bonded length, and plate end treatment have been also clarified from the present test results. Nonlinear finite element analysis has been performed and compared with test results. The Interface properties are also modeled to present the separation failure behavior of strengthened members. The cracking patterns as well as maximum failure loads agree well with test data. The relation between maximum shear and normal stresses at the interface has been derived to propose a separation failure criterion of strengthened members. The present study allows more realistic analysis and design of externally strengthened flexural member with steel plates.

• Journal of the Korea institute for structural maintenance and inspection
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• v.19 no.4
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• pp.92-100
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• 2015

The aim of this study is to evaluate the fracture characteristics of ductile fiber reinforced cement based composites with 1.5 volume ratio of polyvinyl alcohol and steel fiber by high velocity impact of steel projectile. We used gunpowder impact facility to evaluate the fracture characteristics of ductile fiber reinforced cement based composites by collision of steel projectile, and the impact velocity was from about 150 to 1,000m/s. The results of evaluation on the fracture characteristics of ductile fiber reinforced cement based composites were penetration grade, which is the kinetic energy more than three times of no-fiber reinforced specimen (Plain). In addition, ductile fiber reinforced cement based composites did not occurred critical damage other than the debris. In the case of mass loss, Plain specimen was proportional to kinetic energy of steel projectile, while ductile fiber reinforced cement based composites was not significantly affected by kinetic energy of steel projectile. In particular, this tendency had a close relationship with the fracture characteristics of back side of specimens, and the scabbing inhibiting efficiency of PVA specimen was higher than S specimen. In the results of verifying relationship between front and back side calculated by local damage, scabbing occurred at the region close to the back side in the ductile fiber reinforced cement based composites unlike Plain specimen. Thus, in this study, we examined principal fracture behaviors of ductile fiber reinforced cement based composites under collision of steel projectile, and verified that impact resistance performance was improved as compared to Plain specimen.

• Applied Microscopy
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• v.30 no.1
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• pp.11-20
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• 2000

The morphological changes in normal and weathering hair shafts of the human scalp were investigated by using the transmission and scanning electron microscopes. The hair shaft composed of cuticular layer, cortex and medula. The surface of normal hairs are smooth and covered by imbricated cuticular scales. The cuticular layer consists of five to seven cuticle cells. These cells, which are flat and thin, measuring about $100{\mu}m$ long and $0.4{\mu}m$ thick, appears intercellular membrane complex in diameter 25 nm. The cortex composed of melanin granules and cornified cells, which multicomponent concentric microfibrils in diameter about 8 nm give rise to macrofibrils in diameter $0.5{\mu}m$ to $0.8{\mu}m$ encased in limiting membrane. The melanin granules are spherical shaped about $0.5{\mu}m$ in size and scattered between macrofibrils. The medulla in the normal hairs are $16{\mu}m$ in diameter centrally region of cortex. Normal hair shafts undergo progressive degenerative changes due to a variety of environmental insults. In the initial weathering process of hair, the cuticular scales became irregularly raised and broken, and then cuticle cells formed cytoplasmic vacuolation, following dissociated intercellular membrane complex, ultimately entirely lost and nuded cortex. Occasionally, transverse fissures were seen at hair shafts indicating that the hairs were deteriorated. Complete removal of the cuticular layer in the heavily damaged cortex portions appeared splitting of the cortical cell into its macrofibrils and scattering of melanin granules.

• Korean Journal of Heritage: History & Science
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• v.45 no.1
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• pp.70-85
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• 2012

Uiseong Tamni-ri Five-storied Stone Pagoda is composed of andesitic tuff and partially combined with tuff breccia and fine-grained granite. The andesitic tuff is identical to basement rock of Geumseongsan Mountain based on lithological, mineralogical and geochemical characteristics. The pagoda has suffered physical weathering such as crack and scaling, discoloration and biological colonization with complex reaction. Expecially, dark gray and brown discoloration appeared whole over the surface of the pagoda, and three to five-layered exfoliation and granular disintegration dominantly occurred in the fourth and fifth roof stones. It is assuming that the stone elements of the pagoda are evaluated as third to forth grades (average third grade) of weathering compared to fresh rock in Geumseongsan Mountain. The physical strength of the stone elements shows low values in the south and west sides of the pagoda that corresponds high weathering degree of the west side due to exfoliation, crack and granular disintegration. It is necessary to investigate the pagoda for precise deterioration assessment, monitoring and conservation treatment.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.561-564
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• 2008

Concrete scaling is the progressive surface deterioration of susceptible subjected to freeze-thaw cycling in the presence of moisture. Particularly, it has been recognized that chlorides present in deicing agents can significantly increase concrete surface scaling. Domestic area of most be happened chloride deicer damage. Because daily mean temperature is below 0$^{\circ}C$ from the area of domestic most. Use of deicing chemicals has been and will continue to be a major part of highway snow and ice control methods. Chloride-containing chemicals such as calcium chloride or rock salt are main deicers for the road. Extensive use of chloride deicers is, however, not only the source of substantial cost penalties due to their corrosive action and ability to deterioration roadway surface materials but also the source of environmental damages. In this study, Use of (40, 27, 21MPa) pavement concrete analyze freeze-thaw test and scaling on the chlorides present.

• Journal of the Korean Society of Earth Science Education
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• v.15 no.1
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• pp.91-102
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• 2022

Sinsu Island in Korea National Marine Park is suitable for geological learning and geotourism as various geological structures, geological activities, and fossils can be observed. In this study, eight geosites were developed by analyzing 2015 revision science curriculum. The 3D panoramic geological virtual field trips were developed according to the three-step outdoor learning model. The 3D panoramic geological virtual field trips, which consist of 8 classes, are composed of cooperative learning by group, enabling autonomous inquiry activities. It is designed to realize convergence education that can learn not only geology but also creativity and humanity through nodular Limestones, exfoliation, salt weathering, perforated shell holes, sedimentary structures and environments, dinosaurs habitats, and volcanic activity. Five experts revised and supplemented the Delphi analysis method to verify the validity of the development purpose and direction. The satisfaction with the geological field for Sinsu Island course was 4.52, indicating that the overall satisfaction with the field course was high. By reflecting on the opinions of each student and reinforcing safety education, we completed the 3D panoramic geological virtual field trips for Sinsu Island. The 3D panoramic geological virtual field trips in Sinsu Island in Korea National Marine Park will be a good example of geology learning tourism where you can make memories and enjoy while studying geology.

• Journal of the Korean Society of Marine Environment & Safety
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• v.18 no.4
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• pp.352-359
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• 2012

An experiment was carried out to figure out the turbulence flow characteristics in the wake of the transom stern's 2-dimensional section by 2-frame grey level cross correlation PIV method at Re= $3.5{\times}10^3$, Re= $7.0{\times}10^3$. The angles of transom stern are $45^{\circ}$(Model "A"), $90^{\circ}$(Model "B") and $135^{\circ}$(Model "C") respectively. The depth of wetted surface is 40mm from free surface. Strong turbulence intensity appears at the interaction between the flow separation of the bottom of a model and the free surface. This study provides statistic flow information such as turbulence intensity, Reynolds stress and turbulence kinetic energy. Model C type (Raked transom) has low Reynolds stress and turbulence kinetic energy.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.8
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• pp.693-702
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• 2015

This paper presents the performance characteristics of a Darrieus-type vertical-axis wind turbine (VAWT) with National Advisory Committee for Aeronautics (NACA) airfoil blades. To estimate the optimum shape of the Darrieus-type wind turbine in accordance with various design parameters, we examine the aerodynamic characteristics and separated flow occurring in the vicinity of the blade, the interaction between the flow and blade, and the torque and power characteristics that are derived from it. We consider several parameters (chord length, rotor diameter, pitch angle, and helical angle) to determine the optimum shape design and characteristics of the interaction with the ambient flow. From our results, rotors with high solidity have a high power coefficient in the low tip-speed ratio (TSR) range. On the contrary, in the low TSR range, rotors with low solidity have a high power coefficient. When the pitch angle at which the airfoil is directed inward equals $-2^{\circ}$ and the helical angle equals $0^{\circ}$, the Darrieus-type VAWT generates maximum power.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.19 no.5
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• pp.484-491
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• 2007

Numerical experiments using a numerical wave tank have been performed to verier the nonlinear interaction between monochromatic waves and a submerged horizontal plate. As a model for numerical wave tank, we used a higher-order Boundary Element Method(BEM) based on fully nonlinear potential flow theory and CADMAS-SURF for solving Navier Stokes equations and exact free surface conditions. Both nonlinear models are able to predict the higher harmonic generation in the shallow water region over a submerged horizontal plate. CADMAS-SURF, which involves the viscous effect, can evaluate the higher harmonic generation by flow separation and vortices at the each ends of plate. The comparison of reflection and transmission coefficients with experimental results(Patarapanich and Cheong, 1989) at different lengths and submergence depths of a horizontal plate are presented with a good agreement. It is found that the transfer of energy from the incident fundamental waves to higher harmonics becomes larger as the submergence depth ratio decreases and the length ratio increases.