• 대한기계학회논문집B
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• 제29권2호
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• pp.189-196
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• 2005

The physical model considered here is a horizontal layer of fluid heated below and cold above with a conducting body placed at the center of the layer. The body has dimensionless thermal conductivities to the fluid of 0.1, 1 and 50. Two-dimensional solution for unsteady natural convection is obtained using an accurate and efficient Chebyshev spectral methodology for different Rayleigh numbers. Multi-domain technique is used to handle a square-shaped conducting body. The results for the case of a conducting body are also compared to those of adiabatic and neutral isothermal bodies. When the dimensionless thermal conductivity is 0.1, a pattern of fluid flow and isotherms and the corresponding time-averaged surface Nusselt number are almost the same as the case of an adiabatic body. When the dimensionless thermal conductivity is 50, a pattern of flow and isotherm and the corresponding surface and time-averaged Nusselt number are similar to those of neutral body. The results for the case of dimensionless thermal conductivity of unity are also compared to those of pure natural convection.

• 한국해안·해양공학회논문집
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• 제20권1호
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• pp.25-33
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• 2008

파랑은 주로 바람에 의해서 발생하여 많은 에너지를 해안으로 전달하며 각종 수리현상을 야기하고 물질의 이송 등에 의하여 연안환경 뿐만 아니라 인간의 활동에도 큰 영향을 미친다. 또한, 해안 구조물과 파랑의 상호작용에 의한 효과를 정확히 예측하는 것은 구조물의 설계 및 거동특성 파악에 매우 중요하다. 본 논문에서는 이층 유체에서 수표면과 저층에 설치되어 있는 얇은 연직벽에 의한 표면파와 내부파의 분산을 선형파 이론을 이용하여 이차원으로 해석하였다. 반사계수를 계산하여 여러 경우에 대하여 효과를 분석한 결과 반사계수는 구조물의 형상과는 별도로 경계층의 위치와 유체간의 밀도차에 크게 영향을 받는 것으로 밝혀졌다.

• Structural Engineering and Mechanics
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• 제34권4호
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• pp.449-474
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• 2010

A smart hollow cylinder consisting of a host functionally graded elastic core layer and two surface homogeneous piezoelectric layers is presented in this paper. The bonding between the layers can be perfect or imperfect, depending on the parameters taken in the general linear spring-layer interface model. The effect of such weak interfaces on free vibration and steady-state response is then investigated. Piezoelectric layers at inner and outer surfaces are polarized axially or radially and act as a sensor and an actuator respectively. For a simply supported condition, the state equations with non-constant coefficients are obtained directly from the formulations of elasticity/piezoelasticity. An approximate laminated model is then introduced for the sake of solving the state equations conveniently. It is further assumed that the hollow cylinder is embedded in an elastic medium and is simultaneously filled with compressible fluid. The interaction between the structure and its surrounding media is taken into account. Numerical examples are finally given with discussions on the effect of some related parameters.

• Geomechanics and Engineering
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• 제5권1호
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• pp.1-15
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• 2013

Pressure transducers are increasingly used within soil mass or at soil-structure interface for appraisal of stresses acting at point of installation. Calibration of pressure transducers provides a unique relationship between applied pressure and voltage or strain sensed by transducer during various loading conditions and is crucial for proper interpretation of results obtained from pressure transducers. In the present study an in-house calibration device is used to calibrate pressure transducers and the study is divided into two parts: 1) demonstration of developed calibration device for fluid and in-soil calibration of pressure transducers; 2) effect of soil layer thickness on the earth pressure cell (EPC) output. Results obtained from the present study revealed successful performance of the developed calibration device, and significant effect of sand layer thickness on the calibration results. The optimum sand layer thickness is obtained as 1.5 times the diameter of EPC.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 1995년도 추계학술대회 논문집
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• pp.542-545
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• 1995

This paper provides an investigation of torque converter system using ERF (Electro-Rheological Fluid). The torque converter system using ERP is a new concepting device because we can change an apparent viscosity of ERF by adapting an electric field. The device was designed by using the equations which were proposed by Carlson et al. The devices based on ERF generally assume one two possible forms. One is the parallel plate type in which the device elements are facing circular disks separated by a flat layer of ERF, The other is coaxial cylinder or Couette types in which the ERF file the annular apace between a pair of coaxial cylindrical electrode. The discussion on this study is specifically for coaxial cylinder gemetry and experiment results show that the measured torque was rapidly increased with the increase of the eletric field.

• Nuclear Engineering and Technology
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• 제47권3호
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• pp.267-283
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• 2015

The main objective of this study is to investigate experimentally the two-phase flow characteristics in terms of the direct contact condensation of a steam-water stratified flow in a horizontal rectangular channel. Experiments were performed for both air-water and steam-water flows with a cocurrent flow configuration. This work presents the local temperature and velocity distributions in a water layer as well as the interfacial characteristics of both condensing and noncondensing fluid flows. The gas superficial velocity varied from 1.2 m/s to 2.0 m/s for air and from 1.2 m/s to 2.8 m/s for steam under a fixed inlet water superficial velocity of 0.025 m/s. Some advanced measurement methods have been applied to measure the local characteristics of the water layer thickness, temperature, and velocity fields in a horizontal stratified flow. The instantaneous velocity and temperature fields inside the water layer were measured using laser-induced fluorescence and particle image velocimetry, respectively. In addition, the water layer thickness was measured through an ultrasonic method.

• 한국공작기계학회논문집
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• 제16권2호
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• pp.100-105
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• 2007

Electro-rheological(ER) fluid and valve are fabricated and evaluated experimentally in its durability to utilize the hydraulic control systems for long term operation. The two-ports ER valve used in the experiment consist of twelve parallel multi-layer electrodes and provide a restriction to the passage of ER fluid because of the viscous pressure drop and a component induced by the electric field. The durability test of ER valve are performed by measuring the surface roughness of electrodes with variation of an electric field strength and test time(1000 or 1800min.). Also, the shear stress and shear rate are measured to evaluate the durability of ER fluid as function of time. After durability test, ER shear stress increases approximately proportional to the shear rate with applied electric field intensity, In the ER valve, the center line average height roughness(Ra) of copper electrode increases about 1.56 times and ten-point median height roughness(Rz) increases about 2.2 times after the durability test. An understanding of these durability is essential to predicting the service life of ER fluid and valves.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
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• pp.445-448
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• 2006

The effects of surface roughness on a spatially-developing turbulent boundary layer (TBL) were investigated by performing direct numerical simulations of TBLs over rough and smooth walls. The Reynolds number based on the momentum thickness was varied in the range $Re_{\theta}=300{\sim}1400$. The roughness elements used were periodically arranged two-dimensional spanwise rods, and the roughness height was $k=1.5{\theta}_{in}$, which corresponds to $k/{\delta}=0.045{\sim}0.125$. To avoid generating a rough wall inflow, which is prohibitively difficult, a step change from smooth to rough was placed $80{\theta}_{in}$ downstream from the inlet. The spatially-developing characteristics of the rough-wall TBL were examined. Along the streamwise direction, the friction velocity approached a constant value and a self-preserving form of the turbulent stress was obtained. Introduction of the roughness elements affected the turbulent stress not only in the roughness sublayer but also in the outer layer. Despite the roughness-induced increase of the turbulent stress in the outer layer, the roughness had only a relatively small effect on the anisotropic Reynolds stress tensor in the outer layer. Inspection of the triple products of the velocity fluctuations revealed that introducing the roughness elements onto the smooth wall had a marked effect on vertical turbulent transport across the whole TBL. By contrast, good surface similarity in the outer layer was obtained for the third-order moments of the velocity fluctuations.

• 한국해양학회지
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• 제30권1호
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• pp.57-63
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• 1995

$\beta$-효과를 포함한 원통 용기 내의 상하층유체의 하충운동을 하충수와 같은 밀도를 갖는 외부유체를 주입하여 야기시키고 상층수와 같은 유체를 유입 및 유출시켜 지형류 조절에 의한 경계면의 변형 및 이에 따른 상층운동 특성을 실험적으로 관찰하였다. 특 히 유입-유출 위치를 변경시키고 동시에 유입과 유출양을 다르게 하여 상층수에 해상 풍의 효과를 첨가하였다. 실험의 목적은 극전선 형성과 관련하여 유입-유출 경로(대만 난류의 유입-쓰가루, 쏘야 유출)를 결정짓는 외부 인자를 파악하고자 함이다. 유입, 유출양이 같은 경우 유입수의 경로는 서안에 따라 북상하다가 이안후 위도에 평행하게 유출된 반면 상층수에 해상풍의 영향을 주었을시 서안 경계류의 분기(negative curl of wind stress 효과시), 남향의 서안 경계류에 의한 유입수의 내부로 침투(positive curl of wind stress 효과시) 등의 현상을 보여준다. 한편 유입-유출의 위치를 변경 시켜 유입수의 위치를 서안경계에 두었을 시 유입수의 경로가 경계면의 지형류 조절에 의한 서안에 근접하게 북상후 유출 (negative curl), 동안에 근접하게 북상후 유출 (positive curl)의 특성을 보여준다. 북부의 반시계방향의 흐름이 유입수의 북방 한계 를 결정짓는 역할을 하고 있음도 관찰되었다.

• 한국해양공학회지
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• 제37권3호
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• pp.89-98
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• 2023

Analyzing the interactions of free surface waves caused by a submerged-body movement is important as a fundamental study of submerged-body motion. In this study, a two-dimensional mini-towing tank was used to tow an underwater body for analyzing the generation and propagation characteristics of free surface waves. The magnitude of the maximum wave height generated by the underwater body motion increased with the body velocity at shallow submerged depths but did not increase further when the generated wave steepness corresponded to a breaking wave condition. Long-period waves were generated in the forward direction as the body moved initially, and then short-period waves were measured when the body moved at a constant velocity. In numerical simulations based on potential flow, the fluid pressure changes caused by the submerged-body motion were implemented, and the maximum wave height was accurately predicted; however, the complex physical phenomena caused by fluid viscosity and wave breaking in the downstream direction were difficult to implement. This research provides a fundamental understanding of the changes in the free surface caused by a moving underwater body.