• Journal of the Society of Naval Architects of Korea
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• v.36 no.3
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• pp.1-7
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• 1999

This paper describes the modification of hull bottom for the air lubrication technique to the passenger boat in service at the Chung-Ju lake, which has a large beam-draft ratio. From numerical analysis of 2-D cavity problem by potential theory, the cavity shape, length and the pressure in cavity are estimated for the simplified geometry of hull bottom, and the non-dimensional parameters affecting air cavity phenomena are investigated. Extensive resistance tests for the model ship which has variation of step height and side strip have been performed to investigate the formation of air cavity and the drag reduction effectiveness. And also, the development of attached cavity to the bottom were observed from the flat bottom made by transparent acrylic plate. From this survey on the modification of bottom shape and the air lubrication technique, the total resistance of model ship could be reduced by about 25% at the design speed compared to the proto type hull form.

• Journal of the Society of Naval Architects of Korea
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• v.50 no.6
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• pp.399-406
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• 2013

In the present study, the flat plate model test method is developed to evaluate the skin friction of the marine coating in the cavitation tunnel. Six-component force balance is used to measure the profile drag of the flat plate and strut. LDV(laser Doppler velocimetry) technique is also employed to evaluate the drag and to figure out the reason of the drag reduction. The flow velocities above the surface can be used to assess the skin friction, combined with direct force measurement. Since the vortical structure in the coherent turbulence structure influences on the skin friction in the high Reynolds number regime, the interaction between the turbulence structure and the surface wall is paying more attention. This sort of thing is important in the passive control of the turbulent boundary layer because the skin friction can't be determined only by wall condition. As complicated flow phenomena exist around a paint film, systematic measurement and analysis are necessary to evaluate the skin friction appropriately.

• Journal of The Korean Association For Science Education
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• v.8 no.2
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• pp.43-52
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• 1988

과학교육에서의 문제해결력의 강조는 그 긴 역사를 가지고 있으나, 인지 심리학에서의 정보처리 모형을 사용한 문제해결과정의 분석이 사용되면서 그 교수가능성이 높아지고 있다. 본 연구는 하나의 탐색연구로써 학습자들이 물리문제를 해결하려는 과정에서 그 문제를 자기나름으로 이해하여 만든 내적표상과 동원한 해결방안이 문제해결에 어떤 관련이 있는지를 알아내보려고 한다. 물리전공 박사과정 학생 3명을 전문가로, 고등학생 2명과 대학 1년생 4명, 모두 6명을 초보자로 삼아 역학내용을 다룬 세 문제를 소리내어 푸는 과정을 개인별로 녹음하여 그 문제해결과정들을 분석하였으며, 학생들의 사고수준을 알기위해 사고 수준검사가 실시 되었다. 주로 질적 분석을 사용했으나 그 결론을 뒷받침하기위해 비모수통계방법이 사용되었다. (유의수준 . 10) 밝혀진 결론은 다음과 같다. 1) 내적표상은 피험자와 문제에 따라 각각 달랐다. 초보자들은 모두 한가지 표상을 세 문제에 걸쳐 계속 사용한데 반해, 2명의 전문가는 문제에 따라 다른 표상을 사용하였다. 이러한 표상의 형태에 따라 문제해결결과가 달랐다. 즉,일-에너지 표상형태를 사용한 피험자가 더 나은 결과를 얻는것으로 나타났다. 2) 문제해결방안에 있어서는 전문가들은 세문제에 걸쳐 계속하여 지식-개발 방안을 사용하였으나 초보자들은 문제에 따라 다른 방안들을 동원하였다. 지식-개발 방안을 사용한 경우가 다른 것들에 비해 더 나은 결과를 얻는 것으로 나타났다. 3) 사고 수준검사(하위검사 또는 전체)의 접수와 문제해결과정 변인들-특히 내적표상의 형태, 문제해결방안의 종류, 목표확인 그리고 문제 해결력-간에는 유의미한 관련이 있는 것으로 나타났다. 4) 그외 속도와 가속도 개념의 혼동, 마찰력 개념의 부정확 등이 공통적으로 범하는 실수였다. 본 연구가 과학교육 실제에 주는 함의로는 내적표상, 문제해결방안의 훈련을 통한 문제해결력의 향상을 들 수 있겠으며 이를 위한 세부연구가 실행되어야 할 것이다.

• Geotechnical Engineering
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• v.14 no.4
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• pp.5-16
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• 1998

The ground of Taegu area consists mainly of shales with elastic sedimentary rocks. These shales have a nonhomogeneous and anisotropic characteristics. So their physical and mechanical properties are very different due to the angles($\beta$ value) of bedding planes of sedimentary rock. In this study, the physical and mechanical characteristics of shales in Taegu area are studied by performing all kinds of rock test. According to results of test, apparent specific gravity of shale decreases as the $\beta$ value increases. On the contrary, porosity and absorption increase. Elastic wave velocity shorts the highest value at the $90^{\circ}$. And Young's modulus shows the maximum value at the $30^{\circ}$. The uniaxial strength, triaxial strength, cohesion and angle of friction show the minimum value at the $60^{\circ}$respectively.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.6
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• pp.789-795
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• 2010

For the reliable clamping of a robotic colonoscope inside the colon, we propose a clamping module consisting of six legs at the front and a trigger at the rear. In addition, a pneumatic-line based locomotive mechanism, which was developed previously for in-pipe inspection, is adopted to reduce the friction force between the pneumatic lines and the locomotion environment. In order to evaluate locomotion performance, a robot with a diameter of 15 mm and a length of 110.250 mm is used. Based on control signal from LabVIEW, it is tested in acrylic pipe and pig's colon. The proposed robot is able to move in the curved path which has a radius of over 25 mm. The speed of the robot is 33 mm/s in a straight path and 12.1 mm/s on a vertical path. The proposed robot, which has one pneumatic line and two clamping modules, conclusively shows reliable locomotion performance under in vitro condition.

• Nuclear Engineering and Technology
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• v.24 no.3
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• pp.263-273
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• 1992

The study on the velocity distribution and the pressure drop characteristic of the nuclear fuel assembly is of importance for the thermal hydraulic design and safety analysis. The purpose of this experimental study is to investigate the hydraulic mixing behind the different kinds of spacer grids in the now or rod bundles. In this study, the detailed hydraulic characteristics in subchannels of 5$\times$5 PWR(Pressurized Water Reactor) rod bundles were measured using one-component He-Ne LDV(Laser Doppler Velocimeter). Measurements of the axial velocity, turbulent intensities and pressure drops were peformed Lateral velocity, turbulent intensities and Reynolds shear stress were also measured by adjust-ing LDV alignment. Friction factors in rod bundles and loss coefficients for spacer grids were evaluated from the measured pressure drops. Hydraulic mixing performance for different kinds of spacer grids could be investigated by estimating the turbulent cross-flow mixing rates between neighboring subchannels.

• Tunnel and Underground Space
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• v.21 no.3
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• pp.205-215
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• 2011

In this study, we analyzed physical properties of granites and their correlation with rock mass classification methods. The granite samples were obtained from field survey, in-situ borehole tests and laboratory tests for a design phase of various roads, railways and other civil engineering works in Korea. Among the measured physical properties, the results of unit weight, compressive strength, tensile strength, seismic velocity, cohesion, friction angle, elastic modulus and deformation modulus were introduced. We also correlated these properties with the compressive strength. The results of different rock classification method of RQD, RMR, and Q-system against the granites in Korea were compared with each other, and the correlation equations were proposed in a more simplified form. We also derived RMR values using the compressive strength as well as the RQD values of in-situ drilled cores, and estimated the deformation modulus of in-situ rock mass in terms of the RMR values.

• Journal of Navigation and Port Research
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• v.34 no.6
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• pp.435-440
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• 2010

A numerical model, which can compute deformation of rubble mound structures composed with various size materials, was proposed. In the numerical model, wave field into the mound structures was computed by CADMAS-SURF and the deformations of mound structures were computed by DEM. Interaction between wave field and sectional deformation of structure was considered and to present the variation of behaviors caused by various properties of materials, computation was carried out with random coefficients by Monte Carlo simulation method for contact stiffness and friction angle. The experiments were carried out with rubbles and glass balls with radius of 2.9cm, 2.6cm and 1.5cm. And the deformation characteristics of rubble mounds composed with various size materials were clarified. Furthermore the validity and the applicability of the model were discussed by comparing with the experimental results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.8
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• pp.799-807
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• 2010

Direct numerical simulations (DNS) of turbulent channel flow for which the drag is reduced by using polymer additives have been performed by a pseudo-spectral method. The Reynolds number based on the friction velocity and half-channel height is 395, and the polymeric stresses due to the polymer additives are evaluated using the FENE-P (finitely extensible nonlinear elastic-Peterlin) model. The numerical results show that the drag reduction rate is significantly affected by the parameters used in the FENE-P model, such as the maximum extensibility and relaxation time of the polymer molecules. The turbulence data for both low- and high-drag reduction regimes are analyzed. In addition, the effects of FENE-P model parameters on the flow characteristics have been investigated for the same drag reduction rate due to the polymer additives. Finally, the present DNS results have been used to verify the correlation between rheological parameters and the extent of drag reduction, which was suggested by Li et al. (2006).

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.3
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• pp.1253-1258
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• 2014

The tribological properties of TiC, TiN and TiC/TiN coatings on steels prepared by the cathodic-arc (CA) ion plating technique were investigated. Experiments were carried out on a tribo-test machine using a Falex journal V block system. The friction and wear characteristics of the coatings were determined by varying the applied load and sliding speed. The TiC, TiN and TiC/TiN coatings markedly increased the tribological characteristics of the surface. As far as a single layer coating was concerned, TiN goes better results than TiC. However, the TiC/TiN multilayer coating performed better than either single layer coating. The major factor in the improved performance of the multilayer coating was the role of TiC in improving the adhesion between the external TiN layer and the substrate steel.