• Journal of Korea Water Resources Association
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• v.34 no.5
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• pp.427-438
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• 2001

A numerical model to analyze the unsteady flow in water distribution system was developed by using wave adjustment method. When analyzing the unsteady flow in the real water distribution system, the computational procedures are very complex due to the various boundary condition. Wave adjustment method, which can solve the boundary condition more simply and accurately, was introduced to overcome this difficulty and related equations to solve external flow directly were presented. Using these equations, the numerical model was developed to analyze water hammer. The suggested model was applied to a hypothetical distribution system and a real system with 26 pipes with various external flow boundary condition to evaluate the applicability of the developed model. The simulation results by this model agree with those by Karney's analysis in terms of discharge and pressure.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.1
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• pp.139-149
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• 1994

A numerical simulation of unsteady axisymmetric turbulent flow was performed for a reciprocating engine including port/valve assembly. The governing equations based on a nonorthogonal coordinate formulation with Cartesian velocity components were used and discretised by the finite volume method with non-staggered variable arrangements. The modified $\kappa-\xi$. turbulence model which included the effect of compressibility was used. The results of twodimensional transient calculation for the axisymmetric configuration were compared with the experimental data. Although slightly low rms velocity was predicted compared to the experimental data, predicted velocity distributions at the valve exit and in-cylinder region showed good agreements with the experimental data. The flow at the valve exit was separated at the same valve lift position with the experimental data. Two vortices incylinder region were generated during the initial intake process. The clockwise main vortex became strong and moved upward to the top wall. The counter-clockwise second vortex became weak and stick to the upper left corner of the cylinder. After middle intake process, new vortex adjacent to upper cylinder wall appeared by the piston motion and therefore, the in-cylinder flow was formed into three vortices. The cylinder pressure just before bottom dead center of piston was higher than inlet pressure and then the reverse flow occured at the valve exit. The in-cylinder flow characteristics were strongly dependent on piston motion, but insensitive to valve motion.

• International Journal of Automotive Technology
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• v.6 no.1
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• pp.1-8
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• 2005

Recently, the HSDI (High Speed Direct Injection) diesel engine has been spotlighted as a next generation engine because it has a good potential for high thermal efficiency and fuel economy. This study was carried out to investigate the in-cylinder flow characteristics generated in a HSDI diesel engine with a 4-valve type cylinder head. The four kinds of cylinder head were manufactured to elucidate the effect of intake port geometry on the in-cylinder flow characteristics. The steady flow characteristics such as coefficient of flow rate $(C_{f})$, swirl ratio (Rs), and mass flow rate (m,) were measured by the steady flow test rig and the unsteady flow velocity within a cylinder was measured by PIV. In addition, the in-cylinder flow patterns were visualized by the visualization experiment and these results were compared with simulation results calculated by the commercial CFD code. The steady flow test results indicated that the mass flow rate of the cylinder head with a short distance between the two intake ports is $13\%$ more than that of the other head. However, the non-dimensional swirl ratio is decreased by approximately $15\%$. As a result of in-cylinder flow characteristics obtained by PIV and CFD calculation, we found that the swirl center was eccentric from the cylinder center and the position of swirl center was changed with crank angle. As the piston moves to near the TDC, the swirl center corresponded to the cylinder center and the velocity distribution became uniform. In addition, the results of the calculation are in good agreement with the experimental results.

• Journal of Advanced Marine Engineering and Technology
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• v.34 no.1
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• pp.76-83
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• 2010

The Characteristics of the flow field through cylinders with in-line and staggered arrangements were calculated by vortex method. Vortex distributions and velocity profiles around the cylinders with in-line and staggered arrangements were simulated at the pitch ratio of Pt/D=1.25~2.0 and Reynolds number of Re=$4.0{\times}10^1{\sim}4.0{\times}10^4$. As the results the vortices of clockwise at the upper separation point cylinder and the vortices of anticlockwise at the lower separation point of each cylinder were generated at both in-line and staggered arrangements. The generation of the reverse flow in the rear region of the cylinders was caused by the pitch ratio and Reynolds number, the boundary region was at the pitch ratio of Pt/D=1.5 and Reynolds number of Re=$4.0{\times}10^2{\sim}4.0{\times}10^3$ in case of in-line arrangement and was at the pitch ratio of Pt/D=1.4 and Reynolds number of Re=$4.0{\times}10^1{\sim}4.0{\times}10^2$ in case of staggered arrangement.

• Journal of Korea Water Resources Association
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• v.34 no.1
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• pp.57-65
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• 2001

A numerical model for analyzing transcritical flow in open channel is tested to various cases of channel shape. As the numerical models developed for transcritical flow until now mainly focused on the application to only prismatic or hypothetical channels, there are some restrictions to apply the nonprismatic channels. In this study, to verify the accuracy and stability of second-order implicit ENO scheme, the numerical model was applied to the channels which haute the varying channel bed and width. Also the numerical model was applied to unsteady flow as well as steady flow. The study shows that the numerical model provides good accuracy in the calculation of stage and velocity with no numerical oscillation, particularly in the calculation of hydraulic jump and discontinous flow Then the implicit ENO scheme demonstrated good accuracy as a high-resolution scheme and stability as an implicit scheme.

• Journal of Korea Water Resources Association
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• v.34 no.4
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• pp.289-301
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• 2001

A fractional step finite difference model for the longitudinal dispersion of nonconservative pollutants is applied to the Nakdong River to simulate the phenol spill accident which occurred on March, 1971. Prior to the dispersion calculation, the flow conditions are simulated to provide inputs to the dispersion model. An unsteady flow model based on Preissmann's four-point scheme is used for this purpose. Sensitivities of the dispersion calculation to empirical equations for dispersion coefficient and to the first-order decay coefficient are analyzed. The time to peak concentration at a downstream location is significantly different depending on the formula for the dispersion coefficient. Although the decay coefficient does not affect the shape of the temporal concentration distribution, the concentration values depend on the decay coefficient verb significantly. An optimization technique is used to calibrate the dispersion model as well as the flow model. The time to the peak concentration is simulated for major positions of water intake along the Nakdong River.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.3
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• pp.203-210
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• 2014

In this study, the hydrodynamic characteristics of Weis-Fogh type water turbine were calculated by the advanced vortex method. The wing (NACA0010 airfoil) and both channel walls were approximated by source and vortex panels, and free vortices are introduced away from the body surfaces. The distance from the trailing edge of the wing to the wing axis, the width of the water channel and the maximum opening angle were selected as the calculation parameters, the important design factors. The maximum efficiency and the power coefficient for one wing of this water turbine were 26% and 0.4 at velocity ratio U/V=2.0 respectively. The flow field of this water turbine is very complex because the wing moves unsteadily in the channel. However, using the advanced vortex method, it could be calculated accurately.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1182-1190
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• 2003

A parallelized FEM code based on domain decomposition method has been recently developed for large-scale computational fluid dynamics. A 4-step splitting finite element algorithm is adopted for unsteady flow computation of the incompressible Navier-Stokes equation, and Smagorinsky LES model is chosen for turbulent flow computation. Both METIS and MPI Libraries are used for domain partitioning and data communication between processors, respectively. Tiburon model of Hyundai Motor Company is chosen as the computational model at Re=7.5 $\times$ 10$^{5}$ , which is based on the car height. The calculation is carried out under both the wind tunnel condition and the road condition using IBM SP parallel architecture at KISTI Super Computing Center. Compared with the existing experimental data, both the velocity and pressure fields are predicted reasonably well and the drag coefficient is in good agreement. Furthermore, it is confirmed that the drag under the road condition is smaller than that under the wind-tunnel condition.

• 유체기계공업학회:학술대회논문집
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• 2002.12a
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• pp.504-512
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• 2002

Bileaflet mechanical valves have the complications such as hemolytic and thromboembolic events, leaflet damage, and leaflet break. These complications are related with the fluid velocity and shear stress characteristics of mechanical heart valves. This fact makes clear the importance of determining the fluid velocity and shear stress characteristics of mechanical heart valves, and requires a detailed understanding of these system properties and further substantial research. The first aim of current study is to introduce fluid-structure interaction method for calculation of unsteady and three-dimensional blood flow through bileaflet valve and leaflet behavior interacted with its flow, and to overcome the shortness of previous studies, where the leaflet motion has been ignored or simplified, by using FSI method. To accomplish this goal, a finite volume computational fluid dynamics code and a finite element structure dynamics code have been used concurrently to solve the flow and structure equations, respectively, to investigate the interaction between the blood flow and leaflet. Physiologic ventricular and aortic pressure waveforms were prescribed as flow boundary conditions. The interaction of aortic flow and valve motion were computed.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.1
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• pp.142-155
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• 1992

Numerical analysis is performed for the unsteady axisymmetric two dimensional phase change problem of freezing of water around a vertical tube. Heat conduction in the tube wall and solid phase, natural convection in liquid phase and volume expansion caused by density difference between solid and liquid phases are included in the numerical analysis. Existing correlation is used for estimating density-temperature relation of water, and the effect of volume expansion is reflected as fluid velocity at the interface and the free surface. As pure water has maximum density at 4.deg. C, it is found that there exists an initial temperature at which the flow direction reverses near the interface and by this effect the slope of interface becomes reversed depending on the initial temperature of water. By considering natural convection and solid-liquid density difference in the calculation, their effects on phase change process are studied and the effects of various parameters are also studied quantitatively.