• 한국기계연구소 소보
• /
• s.17
• /
• pp.167-176
• /
• 1987

It is well known that, about 70-90% of the total drag of a ship is due to the viscous effects. Large amount of theoretical and/or experimental studies on the topic have already been performed. More studies are needed, however, to develop efficient numerical methods which are useful for practical ship design. The present study deals with the theories and numerical methods essential for understanding of real fluid characteristics. Actual ships are not considered because enough computer capacities were not available at the time. Numerical methods, however, are developed to describe complicated ship geometries, transition processes and turbulent boundary layers. The present study can serve as a good start for estimations of viscous ship resistance if an high speed computer is available.

• Korean Journal of Materials Research
• /
• v.7 no.4
• /
• pp.287-294
• /
• 1997

(NdㆍDy)-(FeㆍCoㆍAIㆍM)-B 합금에 Sn,Mo등을 첨가하여 그에 따른 미세구조와 열적안정성 및 자기적 특성 변화를 조사하였다. Sn과 Mo의 첨가는 (NdㆍDy)-(FeㆍCoㆍAIㆍM)-B 합금리본의 큐리온도를 크게 향상시켰으며 자기특성, 특히 보자력을 1KOe이상 증가시켰다. 그리고 이러한 현저한 보자력 증가는 입계형 defect인 disturbed grain boundary defect에 기인하는 것이라 판단되었다. 또한 Sn과 Mo 첨가원소는 irreversible loss를 각각 4%와 6% 감소시켜 리본자석의 열적안정성을 향상시켰다. 이는 Sn과 Mo의 첨가가 보자력을 크게 증가시켰기 때문이다. 한편 (NdㆍDy)-(FeㆍCoㆍAIㆍM)-B 리본자석들의 열저항온도(heat resistance temperature)는 irreversible loss와 직선관계를 이루었다.

• Advances in materials Research
• /
• v.2 no.1
• /
• pp.1-13
• /
• 2013

Lead-free compound $(Na_{0.5}Bi_{0.5})(Zr_{0.75}Ti_{0.25})O_3$ was prepared using conventional ceramic technique at $1070^{\circ}C$/4h in air atmosphere. X-ray diffraction analysis showed the formation of single-phase orthorhombic structure. Permittivity data showed low temperature coefficient of capacitance ($T_{CC}{\approx}5%$) up to $100^{\circ}C$. Complex impedance studies indicated the presence of grain boundary effect, non-Debye type dielectric relaxation and evidences of a negative temperature coefficient of resistance. The ac conductivity data were used to evaluate the density of states at Fermi level and apparent activation energy of the compound.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.8 no.4
• /
• pp.451-462
• /
• 1996

In this study, which focuses on ice storage, a fundamental study in cooling and solidification was performed, including the interesting phenomena of density inversion, supercooling and dendritic ice. A numerical study was performed for natural convection and ice formation considering existence of subcooling and dendritic ice were analyzed numerically by using finite difference method and boundary fixing method. In the mesh, the solid fraction was introduced with adding as a term to the energy conservation equation. A flow in the dendrite was modelled as a flow in a porous medium, and the momentum conservation equation was modified to incorporate resistance forces involved in flows through porous media. A numerical solution of the time dependencies of dendrite area and dense ice front was successfully obtained, and the numerical results were good agreement with experimental results. Based on this methodology, a discussion was made of phenomena and characteristics of cooling and freezing processes under various conditions.

• Journal of Sensor Science and Technology
• /
• v.16 no.2
• /
• pp.91-96
• /
• 2007

The characteristic of tellurium thin films was studied for detecting nitrogen dioxide gas at room temperature. The film was deposited on $Al_{2}O_{3}$ substrate by using thermal evaporator. The subsequent process was heat treatment by several conditions. (temperature, flowed gases) Surface and grain boundary was investigated using SEM. The results showed that resistance of the tellurium film decreases reversibly in the presence of nitrogen dioxide. The sensitivity of this device depends on the gas concentration and detect lower concentrations less than 10 ppm.

• Transactions of the Korean Society for Noise and Vibration Engineering
• /
• v.18 no.9
• /
• pp.952-960
• /
• 2008

Fan noise prediction method is presented for air conditioning, automobile and electronic cooling system applications where fan acts as an internal equipment having very complicated flow interaction with other various system components. The internal flow paths and distribution in the fan-applied systems such as computer or air conditioner are analyzed by using the FNM(flow network modeling). Fan noise prediction method comprises two models for the discrete frequency noise due to rotating steady aerodynamic lift and blade interaction and for the broadband noise due to turbulent boundary layer and wake vortex shedding. Based on the fan operation point predicted from the FNM analysis results and fan design parameters, the present far noise model predicts overall sound pressure level and spectrum. The predictions for the flow distribution, the fan operation and the noise level in air cooling system by the present method are well agreed with 3-D CFD and actual noise test results.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2002.05a
• /
• pp.891-896
• /
• 2002

Flat plate is susceptible to punching shear failure at the slab-column connection, which may cause catastrophic structural collapse. To prevent such brittle failure, strength and ductility of the connection should be ensured. However, since it is very difficult to experimentally simulate the actual load and boundary conditions of the flat plate system, it is not easy to obtain reliable information and data regarding to the strength and ductility of the flat plate-column connection. In the present study, numerical studies were performed for interior connections of continuous flat plate. The results were compared with the existing experiments, and the variations of bending moment, shear, torsional moment around the connection were investigated. Based on the findings of the numerical studies, the disadvantages of current design methods were discussed.

• The Transactions of the Korean Institute of Electrical Engineers P
• /
• v.60 no.4
• /
• pp.214-219
• /
• 2011

The substation grounding design,"IEEE Guide for Safety in AC Substation Grounding (ANSI / IEEE Std 80)"has been widely used. Substation grounding design and substation grounding resistance of grounding network site to predict the voltage at the risk of a very important task, which is a ground fault current due to the influx of the ground network and due to rise in the Earth's potential can be applied to human dangerous Voltage within safe tolerances be configured to be the ground because the network. IEEE Std. 80 for the substation construction safety equipment on the ground securing the ground electrode and the mesh around the boundary potential distribution in terms of risk analysis by the touch voltage and the reference was to clean up the definition and the basic steps of the voltage of the voltage limits the risk of peripheral grounding electrode Suppression by the simulator through a new secure from dangerous voltage design techniques were presented.

• Tribology and Lubricants
• /
• v.18 no.3
• /
• pp.211-218
• /
• 2002

The effect of characteristic of surface roughness and roughness patterns on friction was studied experimentally in boundary lubrication with reciprocating tribometer. Roughness was changed from Ra=0.2($\mu\textrm{m}$)to Ra=1.2($\mu\textrm{m}$). Three roughness pattern-transverse, oblique, longitudinal- were tested for various load and velocity. The experimental results show that the scuffing resistance of surfaces with transversal roughness pattern is higher than that of surface with longitudinal and obliq pattern. under the conditions of the roughness values of Ra=0.2, 0.5,1.0 and 1.2. surfacer roughness (Ra) was decreased with the normal load increased before scuffing occurred. oblique pattern and longitudinal pattern with Ra=0.2 and Ra=1.0 was higher scuffing load under low sliding velocity, but with Ra=0.5 was higher scuffing load under high sliding velocity.

• Steel and Composite Structures
• /
• v.15 no.1
• /
• pp.1-14
• /
• 2013

Predicting the fire resistance of structures has been significantly advanced by full scale fire tests in conjunction with improved understanding of compartmental fire. Despite the progress, application of insulation is still required to parts of structural steelwork to achieve over 60 minutes of fire rating. It is now recognised that uncertainties on insulation properties hinder adaptation of performance based designs for different types of structures. Intumescent coating has recently appeared to be one of most popular insulation types for steel structures, but its design method remains to be confirmed by empirical data, as technical difficulties on the determination of the material properties at elevated temperatures exist. These need to take into account of further physiochemical transitions such as moving boundary and endothermic reaction. The impetus for this research is to investigate the applicability of the conventional differential equation solution which examines the temperature rise on coated steel members by an inorganic intumescent coating, provided that the temperature-dependent thermal/mechanical insulation properties are experimentally defined in lab scale tests.