• Journal of the Korean Society of Propulsion Engineers
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• v.21 no.5
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• pp.71-79
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• 2017

A numerical analysis of the liquid-gas two-phase flows has been conducted. The incompressible equations of the two-phase flows were solved by the artificial compressibility method with the CLSVOF interface capturing method. To analyze the grid dependency of CLSVOF, a numerical analysis of Zalesak's disk and three-dimensional liquid deformation problem were carried out, and the reconstruction of deformation was investigated. The Rayleigh-Taylor instability was numerically analyzed by applying the equations of incompressible two-phase flow, and the surface instability was observed.

• Journal of the Korean Society of Visualization
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• v.3 no.2
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• pp.45-50
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• 2005

Electro-osmotic flow (EOF) instability in a microchannel has been experimentally investigated using a micro-PIV system. The micro-PIV system consisting of a two-head Nd:Yag laser and cooled CCD camera was used to measure instantaneous velocity fields and vorticity contours of the EOF instability in a T-shape glass microchannel. The electrokinetic flow instability occurs in the presence of electric conductivity gradients. Charge accumulation at the interface of conductivity gradients leads to electric body forces, driving the coupled flow and electric field into an unstable dynamics. The threshold electric field above which the flow becomes unstable and rapid mixing occurs is about 1000V/cm. As the electric field increases, the flow pattern becomes unstable and vortical motion is enhanced. This kind of instability is a key factor limiting the robust performance of complex electrokinetic bio-analytical devices, but can also be used for rapid mixing and effective flow control fer micro-scale bio-chips.

• Journal of computational fluids engineering
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• v.19 no.1
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• pp.57-63
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• 2014

This study performed numerical analysis for the characteristics of flow-induced forces and the flow instability depending on the cross-sectional shape of the cylinder in laminar flow. To implement the cylinder cross-section, we adopted an Immersed Boundary Method with marker particles. We analyzed flow characteristics based on the radius of corner curvature. Main parameters are corner radius and Reynolds number (Re). With Re = 40, 50, 150 we calculated the flow field, drag coefficient, RMS of lift coefficient, pressure coefficient and Strouhal number in conjunction with the corner radius variation. Also, we calculated critical Reynolds number ($Re_c$) depending on the corner radius variation.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.12
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• pp.1774-1783
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• 1998

The transition from steady laminar to chaotic convection in a glass melting furnace specified by upper surface temperature distribution has been studied by the direct numerical analysis of the two and three-dimensional time dependent Navier-Stokes equations. The thermal instability of convection roll may take place when modified Rayleigh number($Ra_m$) is larger than $9.71{\times}10^4$. It is shown that the basic flows in a glass melting furnace are steady laminar, unsteady periodic, quasi-periodic or chaotic flow. The dimensionless time scale of unsteady period is about the viscous diffusion time, ${\tau}_d=H^2/{\nu}_0$. Through primary and secondary instability analyses the fundamental unsteady feature in a glass melting furnace is well defined as the unsteady periodic or weak chaotic flow.

• Proceeding of EDISON Challenge
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• 2016.03a
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• pp.622-624
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• 2016

교량의 케이블이나 송전선같은 원형 구조물들은 유동의 불안정성에 의해 물체 후방에 Vortex가 발생한다. 이렇게 발생한 Vortex는 구조물에 진동과 소음을 발생시키게 된다. 본 연구에서는 이러한 원형 구조물들의 배열에 따른 해석을 진행하였다. 같은 크기의 두 원형 실린더를 주 유동 방향으로 정렬시킨 배열을 EDISON 전산열유체 시스템을 이용하여 해석하였다. 두 원형 실린더의 중심의 거리를 1.5 D부터 5 D까지 변화시켜가며 거리에 따른 각 실린더의 Drag coefficient에 초점을 맞추어 연구를 진행하였다. 두 원형 실린더 사이의 거리가 감소할수록 후류 쪽에 위치한 실린더의 Drag coefficient의 값이 감소하는 양상을 보였다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.8
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• pp.702-710
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• 2007

Direct numerical simulation (DNS) is performed to investigate suppressed wake transition and occurrence of lock-on in the wake of a circular cylinder disturbed by sinusoidal perturbation at the Reynolds number of 220 (A-mode instability regime). The sinusoidal perturbation, of which the frequency is near twice the natural shedding frequency, is superimposed on the free stream velocity. It is shown that the wake transition behind the circular cylinder can be suppressed due to the perturbation of the free stream velocity. This change causes a jump in the Strouhal number from the value corresponding to A-mode instability regime to the value corresponding to retarded wake transition regime (extrapolated from laminar shedding regime) in the Strouhal-Reynolds number relationship. As a result, vortex shedding frequency is locked on the perturbation frequency depending not on the natural shedding frequency but on the modified shedding frequency.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.6
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• pp.381-388
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• 2000

The temperature and velocity fluctuations occurred by the baroclinic instability in the melt for Czochralski crystal growth method were experimentally investigated. Wood's metal, which has similar Pr number to the silicon melt, was used as the working fluid and azimuthal velocity was measured using incorporated magnet probe. The azimuthal velocities near the free surface are faster than velocities near the bottom and the rotational velocities near the model crystal become very fast. The results of measured temperature fluctuation as increasing rotation rate were shown that baroclinic instability occurred at the region of Ro<1.01, Ta>$9.63{\times}10^8$. In these region, the fluctuations of temperature and velocity have the same frequency.

• The Journal of Engineering Geology
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• v.19 no.2
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• pp.205-215
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• 2009

The conventional numerical models to analyze flow in subsurface porous media under the transient state usually generate numerical oscillation and unstability due to local flux domain for critical cases such as infiltration into initially dry soil during rainfall period. In this case, it is required refined mesh and small time step, but it decrease efficiency of computation. In this study, numerical unstability in discontinuity domain is removed by applying particle tracking algorithm to simulate unsteady subsurface flow with inflow boundary condition. Finally the hybrid LE FEM improving numerical stability is proposed. The hypothetical domains with unsteady uniform and nonuniform flow field were used to demonstrated algorithm verification. In comparison with analytic solution, we obtained reasonable results and conducted simulation of hypothetical 3-D recharge/pumping area. The proposed algorithm can simulate saturated/unsaturated porous media with more practical problems and will greatly contribute to accuracy and stability of numerical computation.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.04a
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• pp.77-78
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• 2011

A multi-step quasi-global mechanism is developed for the kerosene/oxygen combustion analysis including dissociation products. Reaction constants of the global reaction are determined to have agreement with experimental data. The mechanism is used for the numerical analysis of the combustion flow field of the kerosene/oxygen shear coaxial injector. The results from high-resolution numerical analysis confirmed qualitatively that the recess enhance the fuel/air mixing and combustion efficiency by the increased flow instabilities.

• Journal of the Korean Society of Propulsion Engineers
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• v.6 no.2
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• pp.45-52
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• 2002

An experimental investigation on the structure and dynamic behavior of two dimensional over-expanded air jets exiting into water was carried out. The hish speed digital video imaging and static pressure distribution measurement were made to characterize the structure and time-dependant behavior of the jets. Mach number at the jet exit was 2.0 and was slightly less than the value predicted by the ideal nozzle calculation. Variance of jet spreading angle at different stagnation condition was measured as a function of mass flow rate. Periodic nature of the air jet distortion in water was observed and the frequency of the repetition was approximately 5-6 Hz for all cases tested. Three characteristic length scales were defined to characterize jet structure. $L_1$, maximum width of the plume when the periodic instability occurs, $L_2$, width of the jet where secondary reverse flow entrained jet flow and $L_3$, distance from the jet exit to the location where entrainment of the secondary reverse flow occurs. The ratio of $L_1$ and $L_2$ decreased with increasing stagnation pressure, i.e. mass flow rate. $L_3$ increased with increasing stagnation pressure. The temporal behavior of static pressure measurements also showed peak around frequency of 5, which corresponds the frequency obtained by visual measurements