• Journal of Korean Society of Coastal and Ocean Engineers
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• v.9 no.3
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• pp.115-124
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• 1997

We measured the variance of eddy diffusion and associated ‘diffusion coefficients’ in coastal regions of Korea by observing the separation distances among multiple drifters deployed simultaneously at the same initial position. The variance of eddy diffusion was found to be proportional to $t^m$, where t is the time and m is a non-integer scaling exponent between 1.5 and 3.5. The observed scaling exponent of eddy diffusion cannot be reproduced by diffusion models employing constant eddy diffusivity. In this study, we applied fractal theory in simulating exponential increase of variance of eddy diffusion. We employed the fGn(fractional Gaussian noise) as a ‘modified’ random walks corresponding to the oceanic eddy diffusion. The variance of eddy diffusion, which corresponds to the fBm(fractional Brown motion) of our diffusion model, is proportional to $t^{2H}$, where H is Hurst scaling exponent. The temporal increase of the variance. with scaling exponent between 1 and 2, was successfully reproduced by our fractal diffusion model. However, our model cannot reproduce scaling exponent greater than 2. The scaling exponents greater than 2 are associated with the velocity shear of the mean flow.

• Proceedings of the Korea Water Resources Association Conference
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• 2020.06a
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• pp.89-89
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• 2020

본 연구에서는 GPU 가속화 기반의 Boussinesq 모형인 Celeris Advecnt에 수심 적분된 2차원 이송-확산방정식을 추가하여 인터렉티브 시스템 기반의 추적자 이동 모형을 개발하였다. Celeris Advent는 최초로 개발된 인터렉티브 시스템을 갖춘 Boussinesq 모형으로, 시뮬레이션 중에 사용자가 모형의 파라미터뿐 아니라 모델 도메인 내 수위 및 수심을 바꿀 수 있다. 이를 통해 사용자는 모의가 진행되는 도중에 모델의 안정성 및 효율성을 위해 시간 간격을 조정할 수 있을 뿐 아니라 방파제 설치 등과 같은 지형 변화를 고려하기 위해 도메인 내 격자별 수심을 조정할 수 있다. 본 연구에서는 연안에서의 추적자 이동 모의를 위해 Boussinesq 방정식과 더불어 이송-확산방정식을 풀이하는 추적자 이동 모형을 개발하였다. 추적자의 확산항의 경우 분자 자체의 확산과 더불어 쇄파에 따른 난류 확산을 고려하였다. 난류 확산계수는 슈미트 수를 1로 두어 와동점성계수와 동일하게 두었으며, 와동점성은 단순화된 형태의 쇄파모형을 고려하여 계산하였다. 쇄파모형의 고려로 인해 이송-확산방정식과 더불어 운동량 방정식에서도 쇄파에 따른 운동량 소산이 고려되었다. 마지막으로, 추적자 농도에 대한 인터렉티브 시스템을 추가하여, 모델 구동 중에도 사용자가 수심적분된 추적자 농도를 조정할 수 있도록 하였다. 기수행된 2개의 수리실험 조건과 관측값을 이용하여 벤치마크 테스트를 수행하였으며, 관측값과 대체로 일치하는 것을 확인하였다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.9
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• pp.1209-1219
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• 2003

The dynamic behaviors of the single vortex interacting with $CH_4-Air$ jet diffusion flame are investigated numerically. The numerical method is based on a predict-corrector scheme for a low Mach number flow. A two-step global reaction mechanism is adopted as a combustion model. Studies are conducted in fixed initial velocities for the three cases according as where $CO_2$ is added; (1) without dilution, (2) dilution in fuel stream and (3) dilution in oxidizer stream. A single vortex is generated by an axisymmetric jet, which is made by an impulse of a cold fuel when a flame is developed entirely in a computational domain. The simulation shows that $CO_2$ dilution in fuel stream results in somewhat larger vortex radius, and greater amount of entrainment of surrounding fluid than in other cases. Thus, the dilution of $CO_2$ in fuel stream enhances the mixing in single vortex and increases the stretching of the flame surface. The budgets of the vorticity transport equation are examined to reveal the mechanism of vortex formation when $CO_2$ is added. It is found that, in the case of $CO_2$ dilution in fuel stream, the vortex destruction due to volumetric expansion and the vortex production due to baroclinic torque are more dominant than in other cases.

• Proceedings of the Korean Institute of Industrial Safety Conference
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• 1998.11a
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• pp.21-27
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• 1998

비정상, 층류 유동으로 실린더와 같은 블러프(Bluff)물체 주위를 흐르는 유동에서 발생하는 Von Karman 와동은 외부 유체로부터 전달된 힘이 물체 표면에 분포되어 양력이나 항력등에 많은 영향을 주기 때문에 유체역학 분야에 있어서 매우 중요한 분야라 할 수 있다. 이러한 유동은 벽으로부터 점진적으로 전달되고 확산되는 큰 와류에 의해서 특징 지워진다. (중략)

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.1
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• pp.153-164
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• 1997

Flame oscillation phenomena in a co-flow diffusion flame was experimentally studied with periodic fuel supply using a solenoid valve. The degree of excitation was controlled by changing the volume flux of fuel passing through the valve. Flame oscillation frequencies were measured utilizing a photodiode, a spectrum analyzer, video and high speed movies. Laser planar visualization was employed to study the correlation between the flame oscillation and the toroidal vortices. Observed are three regimes of flame oscillation, where the oscillation frequencies are for the multiples of excitation, the excitation itself and the flame natural oscillation. Both periods of natural oscillation and of excitation induced oscillation exist over one cycle of the excitation in the frequency multiplied regime. It is considered as an effect of balancing the influence of buoyancy driven vortex with that of excitation induced vortex near the excitation rate of 0.2. Flame shapes are become monotonous as increasing the excitation frequency to the range of over two fold of the natural oscillation. The flame oscillation can be modulated to the frequency of either multiples of excitation or excitation itself under certain conditions. This implies that the flame oscillation could be modulated to avoid the resonance frequency of the combustor, and shows the possibility of active control of the flame oscillation.

• 한국연소학회:학술대회논문집
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• 2002.06a
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• pp.68-75
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• 2002

The dynamic behaviors of the single vortex and flame-vortex interaction in a $CH_4/Air$ diffusion flame with addition of $CO_2$ were investigated numerically. The numerical method was based on a predictor-corrector for low Mach number flow. A two-step global reaction mechanism was adopted as a combustion model. Through comparison of results by effect of $CO_2$ added either on the fuel or oxidizer side, it was found that the growth of single vortex and entrainment of surrounding fluid by $CO_2$addition on the fuel side are larger than those by $CO_2$ addition on oxidizer side. Also, when $CO_2$ is added on fuel side, flame-vortex interaction becomes more significant than on air side.

• Journal of Advanced Marine Engineering and Technology
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• v.35 no.1
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• pp.82-88
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• 2011

The objective of this study is to investigate the validity of the dynamic sub-grid G-equation model to a complex turbulent premixed combustion such as bluff body stabilized turbulent premixed flames for the considering of the realistic engineering application. In this study, a new turbulent flame speed model, introduced by the sub-grid turbulent diffusivity and the flame thickness, is also proposed and is compared with an usual model using sub-grid turbulent intensity and with the experimental data. The calculated results can predict the velocity and temperature of the combustion flow in good agreement with the experiment data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.8
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• pp.1087-1096
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• 2001

The dynamic structures of unsteady CH$_4$/Air jet diffusion flame with a flame-vortex interaction were numerically investigated. A timed-dependent, axisymmetric computational model and a low mach number approximation were employed in the present calculation. A two-step global reaction mechanism which considers 6 species, was used to calculate the reaction rates. The predicted results including the gravitational effect show that the large outer vortices and the small inner vortices can be well simulated without any additional disturbances near nozzle tip. It was found that the temperature and species concentrations have deviated values even for the same mixture fraction in the flame-vortex interaction region. It was also shown that the flame surface is not deformed by the inner vortex in upstream region, while in downstream region, the flame surface is compressed or stretched by the outer vortex roll-up. The present unsteady jet flame configuration accompanying a flame-vortex interaction is expected to give good implications for the unsteady structures of turbulent flames.

• Journal of the korean academy of Pediatric Dentistry
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• v.28 no.2
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• pp.228-237
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• 2001

It is well known that glass-ionomer cement and compomer can release fluoride which can inhibit the progression of dental caries. The purposes of this study were to evaluate whether the fluoride from fluoride-containing filling materials can penetrate the dentin bonding adhesive and the dentin bonding adhesive can increase the bond strength of resin-modified glass ionomer. The amount of fluoride release from resin-modified glass ionomer and compomer was measured during 28 days of period and statistically analyzed by t-test. The bond strength of each material with/without dentin bonding adhesive was measured and also statistically analyzed by t-test. The distribution of fluoride from each material into the tooth was explored by electron probe microanalysis system(EPMA). The experimental teeth used were second primary molars about to exfoliate. The amount of fluoride release from each material was diminished by dentin bonding adhesive during 28 days(p<0.05) and the bond strength was not increased by dentin adhesive in resin-modified glass ionomer. The bond strength of group VI was not detectable. The distribution of fluoride from each material into teeth was according to dentino-enamel junction and dispersed into pulp from pulpal floor. The widest distribution was shown in resin-modified glass ionomer cement filled without the application of dentin bonding adhesive.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.8 no.2
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• pp.123-136
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• 1996

A three-dimensional Galerkin-FEM model which can handle the temporal and spatial variation of density is presented. The hydrostatic approximation is used and density effects are included by means of conservation equation of heat and the equation of state. The finite difference grids are used in the horizontal plane and a set of linear-shape functions is used for the vertical expansion. The similarity transform is introduced to solve resultant matrix equations. The proposed model was first applied to the density-driven circulation in an idealized basin in the presence of the heat exchange between the air and the sea. The advection terms in the momentum equation were ignored, while the convection terms were retained in the heat equation. Coefficients of the vertical eddy viscosity and diffusivity were fixed to be constant. Calculation in a non-rotating idealized basin shows that the difference in heat capacity with depth gives rise to the horizontal gradient of temperature. Consequently, there is a steady new in the upper layer in the direction of increasing depth with compensatory counter flow .in the lower layer. With Coriolis force, geostrophic flow was predominant due to the balance between the pressure gradient and the Coriolis force. As a test in region of irregular topography, the model is applied to the Yellow Sea. Although the resultant flow was very complex, the character of the flow Showed to be geostrophic on the whole.