• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.2 no.4
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• pp.268-278
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• 1990

Hydrodynamic stability equations are formulated for natural convection flows adjacent to a heated or cooled, inclined, isothermal surface in pure water at $4^{\circ}C$, where the density variation with temperature becomes nonlinear. The resulting stability equations, when reduced to ordinary differential equations by a similarity transformation, constitute a two-point boundary-value problem, which was solved numerically. It is found from the obtained stability results that the neutral stability curves are systematically shifted to have lower critical Grashof numbers, as the inclination angle of upward-facing plate increases. Also, the nose of the neutral stability curve becomes blunter as the angle increases. It implies that the greater the inclination of the upward-facing plate, the more susceptible of the flow to instability for the wide range of disturbance wave number and frequency.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.2
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• pp.630-643
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• 1991

The hydrodynamic stability equations are formulated for buoyancy-induced flows adjacent to a vertical, planar, isothermal surface in cold pure water. The resulting stability equations, when reduced to ordinary differential equation by a similarity transformation, constitute a two-point boundary-value(eigenvalue) problem, which was numerically solved for various values of the density extremum parameter R=( $T_{m}$ - $T_.inf./) / ( $T_{o}$ - $T_.inf./). These stability equations have been solved using a computer code designed to accurately solve two-point boundary-value problems. The present numerical study includes neutral stability results for the region of the flows corresponding to 0.0.leq. R. leq.0.15, where the outside buoyancy force reversals arise. The results show that a small amount of outside buoyancy force reversal causes the critical Grashof number $G^*/ to increase significantly. A further increase of the outside buoyancy force reversal causes the critical Grashof number to decrease. But the dimensionless frequency parameter $B^*/ at $G^*/ is systematically decreased. When the stability results of the present work are compared to the experimental data, the numerical results agree in a qualitative way with the experimental data.erimental data.

• Journal of the Korean Society for Precision Engineering
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• v.22 no.2
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• pp.188-193
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• 2005

Surface texturing of tribological application is another attractive technology of friction reducing. Also, reduction of friction is therefore considered to be a necessary requirement for improved efficiency of machine. In this paper attempts to investigate the effect of density for micro-scale dimple pattern on bearing steel flat mated with pin-on-disk. We demonstrated the lubrication mechanism for a Stribeck curve, which has a relationship between the friction coefficient and a dimensionless parameter for lubrication condition. It is found that friction coefficient is depended on the density of surface pattern. It was thus verified that micro-scale dimple could affect the friction reduction considerably under mixed and hydrodynamic lubrication conditions from based on friction map. Lubrication condition regime has an influence on the friction coefficient induced the density of micro dimple.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.32A no.3
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• pp.100-109
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• 1995

A new Hydrodynamic model for the high energy tail electrons(Tail Electron Hydrodynamic Model : TEHD) is developed using the moment method. The Monte Carlo method is applied to a $n^{+}-n^{-}-n^{+}$ device to calibrate the TEHD equations. the discretization method and numerical procedures are explained. New models for the impact ionization and injection into the gate oxide using the tail electron density are proposed. The simulated results of the impact ionization rate for a $n^{+}-n^{-}-n^{+}$ device and MOSFET devices, and the gate injection experiment are shown to give good agreement with the Monte Carlo simulation and the measurements.

• Journal of Environmental Science International
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• v.11 no.1
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• pp.25-32
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• 2002

In order to control of water quality in Jeju harbor, variation of physical oceanographic environments was estimated using material cycle model. It is composed of the three-dimensional hydrodynamic model for the simulation at water flow and material cycle model for the simulation of water quality. The three dimensional hydrodynamic model simulation of the circulation and mixing in Jeju Harbor has been conducted forced by Sanzi River Discharge, Tidal elevation, wind and Solar heat in case of August and November, 2000 and February and May, 2001, respectively. The results of numerical model and observation show that the model can produce realistic results of current in the harbor. The monthly variation of velocity pattern are not so much changed are found In Jeju Harbor. The residual current was forced by temperature, salinity, density, wind and tidal current. The residual current of August, 2000 are the strongest among four month. It can be explained that the density effect can be important role in residual current at Jeju Harbor. As the results of salinity distribution simulation, very low concentration of all levels were simulated in August, 2000. The flowrate of Sanzi river was investigated 77,760 ㎥ /d in August, 2000. Therefore, pollutant loadings from Sanzi river should be considered for water quality management in Jeiu harbor.

• Journal of Advanced Marine Engineering and Technology
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• v.21 no.1
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• pp.43-48
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• 1997

The present study is aimed to investigate experimentally on the hydrodynamic behaviours of air ~ pulverized coal flow in rectangular combustor with cavity. Mean velocity, density and tur¬bulent properties of pulverized coal in rectangular entrained flow combustor were measured by PDA. Experimental results show that the flow reattachment point at the lower plane in the com¬buster chamber has been developed near X/D= 15. The similarities at each section are found after the flow reattachment point. The maximum values of turbulent intensity and Reynolds shear stress have been shown near Y/D=6, which is higher than centerline. The maximum density of the pulverized coal sited in the range ofY/D=6~8.

• Journal of the Korean Society for Precision Engineering
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• v.28 no.10
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• pp.1159-1165
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• 2011

The recording density of HDD is increasing in ratio of 100% each year. Because the increasing of recording density requires the feature of high rotation, fixation and low-noise, fluid hydrodynamic bearing(FDB) has been paid attention to overcome a limitation in ball bearing. Most of researches related to improving performance of FDB have been studied in Japan which has 80% more market share of HDD spindle motor assembly. Main subject of studies are about for the design of the groove shape, manufacturing process of fluid dynamic bearing, performance evaluation and measurement. In HDD, non-repeatable runout(NRRO) is most important parameter which determines the performance of HDD spindle system because NRRO is unpredictable that cannot be compensated in head/slider servo system. In this study, performance evaluation system can measure dynamic behaviors were designed and methodology for calculating imbalance, RRO, and NRRO were proposed.

• Ocean Systems Engineering
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• v.4 no.1
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• pp.1-19
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• 2014

Hydrodynamic coefficients strongly affect the dynamic performance of autonomous underwater vehicles (AUVs). A novel kind of underwater vehicle (Heavier-than-water AUV) with higher density than water is presented, which is different from conventional ones. RANS method and overlapping grids are used to simulate the flow field around the vehicle. Lifts, drags and moments of different attack and drift angles in steady state are calculated. The hydrodynamic performances and how the forces change with the attitude are analyzed according to the flow field structure. The steady-state results using overlapping grid method are compared with those of software FLUENT and wind tunnel tests. The calculation results show that the overlapping grid method can well simulate the viscous flow field around the underwater vehicle. Overlapping grid skills have also been used to figure out the planar-motion-mechanism (PMM) problem of Heavier-than-water AUV and forecast its hydrodynamic performance, verifying its effectiveness in dealing with the dynamic problems, which would be quite helpful for design and control of Heavier-than-water AUV and other underwater vehicles.

• International Journal of Naval Architecture and Ocean Engineering
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• v.2 no.1
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• pp.14-23
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• 2010

A numerical model was developed for simulating a three-dimensional multilayer hydrodynamic and thermodynamic model in domains with irregular bottom topography. The model was designed for examining the interactions between flow and topography. The model was based on the three-dimensional Navier-Stokes equations and was solved using the fractional step method, which combines the finite difference method in the horizontal plane and the finite element method in the vertical plane. The numerical techniques were described and the model test and application were presented. For the model application to the northern part of Ariake Sea, the hydrodynamic and thermodynamic results were predicted. The numerically predicted amplitudes and phase angles were well consistent with the field observations.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.65.2-65.2
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• 2014

We present a new hydrodynamic simulation code based on the Voronoi tessellation for estimating the density precisely. The code employs both of Lagrangian and Eulerian description by adopting the movable mesh scheme, which is superior to the conventional SPH (smoothed particle hydrodynamics) and AMR (adaptive mesh refinement) schemes. The code first generates unstructured meshes by the Voronoi tessellation at every time step, and then solves the Riemann problem for all surfaces of each Voronoi cell so as to update the hydrodynamic states as well as to move current meshes. Besides, the IEM (incremental expanding method) is devised to compute the Voronoi tessellation to desired degree of speed, thereby the CPU time is turned out to be just proportional to the number of particles, i.e., O(N). We discuss the applications of our code in the context of cosmological simulations as well as numerical experiments for galaxy formation.