• Proceedings of the Acoustical Society of Korea Conference
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• 한국음향학회 2001년도 추계학술발표대회 논문집 제20권 2호
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• pp.389-392
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• 2001

To investigate the generation mechanism of the shock-associated noise, an underexpanded supersonic jet from an axisymmetic nozzle is simulated under the conditions of the Nozzle exit Mach number of 2 and the exit pressure ratio of Pe/Pe =1.5. The present simulation is performed based on the high-order accuracy and high-resolution ENO (Essentially Non-Oscillatory) scheme to capture the time-dependent flow structure representing the sound source. It was found that the shock-associated noise is generated by the weak interaction between the downstream propagating large turbulence structures of the jet flow and the quasi-periodic shock cell structure during the one is passing through the other. The directivity of propagating waves to the upstream is clearly shown in the visualization of pressure field. It is shown that the present calculation of the centerline pressure distribution is in fare agreement with the experimental data at the location of first shock cell.

• Journal of the Korean Society for Precision Engineering
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• 제16권6호
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• pp.197-208
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• 1999

This paper presents an operation sequence-based approach for determining machine cell layout in a cellular manufacturing environment. The proposed model considers the sequence of operations in evaluating the intercell and intracell movements. In this paper, design of cellular layout has an objective of minimization of total material flow among facilities, where the total material flow is defined as a weighted sum of both intercell and intracell part movements. The proposed algorithm is developed by using genetic algorithm and can be used to design an optimal cellular layout which can cope with changes of shop floor situation by considering constraints such as the number of machine cells and the number of machines in a machine cell.

• Transactions of the Korean hydrogen and new energy society
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• 제15권4호
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• pp.291-300
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• 2004

Performance of PEMFC is affected by many factors such as that of MEA, configuration of flow field, operating conditions, humidification, cooling and so on. In this study, in order to improve the performance of fuel cell, a small area fuel cell stack was made and its performance was tested under various operating conditions. Stack consists of 3 single PEM fuel cells. Channel is serpentine type and the active area of the electrode is $50cm^2$. The test results show that the peak power is 60W at $70^\circ{C}$ of stack temperature with humidification condition.

• Proceedings of the KSME Conference
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• 대한기계학회 2004년도 춘계학술대회
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• pp.1885-1890
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• 2004

In this paper the numerical method with a zonal embedded grid system was applied to the spin-up flow within a semi-circular container. Flow visualization was also performed on a rotating table. The results show that at a low level of damping (i.e. low viscosity and high liquid depth) a cyclonic cell produced initially near the left-hand-side corner of the container moves along a wall and merged with the cell on the right-hand-side.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.240-244
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• 2004

Characteristics of steady and unsteady cavitation in a turbopump inducer were investigated in this paper. To see the effect of tip clearance on the inducer performance, three cases of tip clearance were tested. The helical inducer, which has two blades with inlet tip blade angle of 7.8 degree and tip solidity of 2.7, was tested in the water. In the non-cavitating condition, the inducer head decreased with increase in the tip clearance. Rotating cavitation and cavitation surge were observed through unsteady pressure measurements at the inducer inlet. The cell number and propagation speed of the rotating cavitation were determined through cross-correlation analysis. During the rotating cavitation one cell rotated at the same rotational speed as that of the inducer rotation and the cavitation surge did not rotate. The critical cavitation number increased with increase in the tip clearance at the same flow rate, but the change of critical cavitation number was small at the nominal flow rate.

• Transactions of the Korean Society of Mechanical Engineers B
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• 제26권2호
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• pp.194-200
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• 2002

A method for simulating flow of extremely low Reynolds number in a vortex-in-cell (VIC) was studied. The viscous diffusion was represented by the random walk method. The validity of this method was proven by applying it to the flows passing over a fence placed vertically at an extremely low Reynolds number. The effects of parameters of the VIC method on the random walk method, such as the number of point vortices, the mesh density and the time increment, were investigated by numerical analysis using a one-dimensional diffusion equation. Changes in the relative error of vorticity depending on those parameters were clarified.

• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
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• 한국지하수토양환경학회 2001년도 추계학술발표회
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• pp.27-30
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• 2001

Flow and transport at fracture intersections, and their effects on network scale transport, are investigated in three-dimensional random fracture networks. Fracture intersection mixing rules complete mixing and streamline routing are defined in terms of fluxes normal to the intersection line between two fractures. By analyzing flow statistics and particle transfer probabilities distributed along fracture intersections, it is shown that for various network structures with power law size distributions of fractures, the choice of intersection mixing rule makes comparatively little difference in the overall simulated solute migration patterns. The occurrence and effects of local flows around an intersection (local flow cells) are emphasized. Transport simulations at fracture intersections indicate that local flow circulations can arise from variability within the hydraulic head distribution along intersections, and from the internal no flow condition along fracture boundaries. These local flow cells act as an effective mechanism to enhance the nondiffusive breakthrough tailing often observed in discrete fracture networks. It is shown that such non-Fickian (anomalous) solute transport can be accounted for by considering only advective transport, in the framework of a continuous time random walk model. To clarify the effect of forest environmental changes (forest type difference and clearcut) on water storage capacity in soil and stream flow, watershed had been investigated.

• Transactions of the Korean Society of Mechanical Engineers B
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• 제30권11호
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• pp.1027-1034
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• 2006

In order to investigate flow characteristics of chicken blood in a micro tube of 100$\mu$m in diameter, in-vitro experiments were carried out using a micro-PIV technique. The micro-PIV system consists of a microscope, 2-head Nd:YAG laser, 12 bit cooled CCD camera and a delay generator. Chicken blood with 40% hematocrit was supplied into a micro tube using a syringe pump. The blood flow shows clearly the cell free layer near the tube wall and its thickness is increased with increasing the flow speed. The hemorheological characteristics of chicken blood, including shear rate and shear stress were estimated from the PIV velocity field data obtained. Since the aggregation index of chicken blood is less than 50% of human blood, non-Newtonian flow characteristics of chicken blood are smaller than those of human blood. As the flow rate increases, the degree of flatness in the velocity profile at the center region is decreased and the parabola-shaped shear stress distribution becomes to have a linear profile. Under the same flow rate, chicken blood shows higher shear stress, compared with human blood.

• Proceedings of the KSME Conference
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• 대한기계학회 2000년도 춘계학술대회논문집A
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• pp.900-907
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• 2000

The three-dimensional turbulent flow in curved pipes susceptible to flow-accelerated corrosion has been analyzed numerically to predict the pressure and shear stress distributions on the inner surface of the pipes. The analysis employs the body-fitted non-orthogonal curvilinear coordinate system and a standard $ {\kappa}-{\varepsilon}$ turbulence model with wall function method. The finite volume method is used to discretize the governing equations. The convection term is approximated by a high-resolution and bounded discretization scheme. The cell-centered, non-staggered grid arrangement is adopted and the resulting checkerboard pressure oscillation is prevented by the application of a modified version of momentum interpolation scheme. The SIMPLE algorithm is employed for the pressure and velocity coupling. The numerical calculations have been performed for two curved pipes with different bend angles and curvature radii, and discussions have been made on the distributions of the primary and secondary flow velocities, pressure and shear stress on the inner surface of the pipe to examine applicability of the present analysis method. As the result it is seen that the method is effective to predict the susceptible systems or their local areas where the fluid velocity or local turbulence is so high that the structural integrity can be threatened by wall thinning degradation due to flow-accelerated corrosion.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• 제15권10호
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• pp.809-820
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• 2003

The present study investigates effects of flow velocity on the convective heat/mass transfer characteristics in wavy ducts of a primary surface heat exchanger application. Local heat/mass transfer coefficients on the wavy duct sidewall are determined by using a naphthalene sublimation technique. The flow visualization technique is used to understand the overall flow structures inside the duct. The aspect ratio and corrugation angle of the wavy duct is fixed at 7.3 and 145$^{\circ}$ respectively, and the Reynolds numbers, based on the duct hydraulic diameter, vary from 100 to 5,000. The results show that there exist complex secondary flows and transfer processes resulting in non-uniform distributions of the heat/mass transfer coefficients on the duct side walls. At low Re (Re<1000), relatively high heat/mass transfer regions like cell shape appear on both pressure and suction side wall due to the secondary vortex flows called Taylor-Gortler vortices perpendicular to the main flow direction. However, at high Re (Re>1000), these secondary flow cells disappear and boundary layer type flow characteristics are observed on pressure side wall and high heat/mass transfer region by the flow reattachment appears on the suction side wall. The average heat/mass transfer coefficients are higher than those of the smooth circular duct due to the secondary flows inside wavy duct. And also friction factors are about two times greater than those of the smooth circular duct.