• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.3
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• pp.27-36
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• 2002

Numerical stability is studied based on numerics and mathematics. It is frequently observed in the region where velocity is zero. In that region, the Euler equation have numerous solutions and, thus, it is impossible to determine a unique solution with only governing equations. However, a unique solution can be determined by additional outer flow conditions or outer numerical discontinuity calculation since the information or a unique solution under undisturbed conditions is lost by disturbances. In this reason, the numerical scheme comsistent with Euler equations cannot remove shock instability completely.

• Journal of computational fluids engineering
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• v.6 no.4
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• pp.43-53
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• 2001

본 논문은 충격파 불안정성이 나타나지 않는 충격파 안정적인 수치기법의 개발을 목표로 하고 있다. Roe의 수치기법은 유동의 수치계산에 있어 높은 정확도를 보장하지만 carbuncle 현상과 같은 충격파 불안정성이 나타나는 것으로 알려져 있다. Roe의 수치기법과 HLLE 수치기법의 수치점성을 비교하여 충격파 불안정성의 원인을 살펴보았으며, Roe의 수치기법에 나타나는 반감쇠항에 마하수의 함수인 조절함수 f와 g를 도입하여 충격파 안정성을 획득하였다. 본 논문에서 제안된 수치기법을 다양한 유동문제에 적용하여 수치기법의 충격파 안정성과 정확성을 검증하였다

• Korean Chemical Engineering Research
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• v.58 no.2
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• pp.319-324
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• 2020

Electroconvective instability is a non-linear transport phenomenon which can be found in ion-selective transport system such as electrodialysis, Galvanic cell and electrolytic cell. The instability is triggered by the fluctuation of space charge layer in adjacent of ion-selective surface, leading to increase of mass transport rate. Thus, in the aspect of mass transport, the instability has an important meaning. Although recent experimental techniques have opened up an avenue to direct visualize the instability, fundamental investigations have been conducted in limited area due to several experimental limitations. In this work, the electroconvective instability under potentiostatic mode was solved by numerical method in order to demonstrate correlation between current-time curve and the instability behavior. By rigorous time-resolved analysis, the transition behaviors can be divided into three stages; formation of space charge layer - growth of electroconvective instability - steady state. Furthermore, scaling laws of transition time were numerically obtained according to applied voltage as well.

• Journal of the Korean Society of Propulsion Engineers
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• v.9 no.3
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• pp.101-119
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• 2005

For MEMS applications, the effects of the momentum and heat loss on the stability of laminar premixed flames in a narrow channel are investigated by high-fidelity numerical simulations. A general finding is that momentum loss promotes the Saffman-Taylor (S-T) instability which is additive to the Darrieus-Landau (D-L) instabilities, while the heat loss effects result in an enhancement of the diffusive-thermal (D-T) instability. These effects are also valid in nonlinear behavior of the premixed flame. The simulations of multiple cell interactions are also conducted with heat and momentum loss effects.

• Korean Chemical Engineering Research
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• v.59 no.4
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• pp.626-631
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• 2021

One of the nonlinear electrokinetic phenomena around ion exchange membrane is electroconvective instability which can be found in various electrokinetic applications such as electrodialysis, electrochemical battery, microfluidic analysis platform, etc. Such instability acts as a positive transport mechanism for the electrodialysis via amplifying mass transport rate. On the other hands, in the electrochemical battery and the microfluidic applications, the instability provokes unwanted mass transport. In this research, to control the electroconvective instability, the onset of the instability was analyzed as a function of confinement effect as well as applied voltage. As a result, we figured out that the dynamic behavior of electroconvective instability transited as a sequence of stable regime - static regime - chaotic regime depending on the applied voltage and confinement effect. Furthermore, stability curves about the dynamic transition were numerically determined as well. Conclusively, the confinement effect on electroconvective instability can be applied for effective means to control the electrokinetic chaos.

• Proceeding of EDISON Challenge
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• 2012.04a
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• pp.33-36
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• 2012

풍력발전기 성능은 유동의 안정성과 풍속에 의해 결정되는데, 이때 유동 불안정성은 풍력발전기의 성능뿐만 아니라 구조적 문제를 함께 유발시킨다. 본 연구에서는 풍력발전기 타워 후류에서의 불안정성을 최소화시키기 위하여 타워 단면의 기초 형상설계 연구를 수행하였다. 기존의 풍력발전기 타워 형상에 부가 구조물을 설치함으로써 Karman vortex의 생성을 지연시키고 와류 간섭현상을 줄여 풍력발전기의 안정성을 증대시키고자 하였다. 이를 위해 다양한 타워 단면 형상에 대하여 양력계수 및 항력계수를 비교 분석하였다. 그 결과 반지름의 1/2 길이의 자유류 방향 tip과 splitter plate를 후방에 설치하는 것이 후류 불안정성을 억제하는데 가장 효율적인 것으로 나타났다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1999.10a
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• pp.25-25
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• 1999

응답 함수는 추진제의 연소 불안정성을 예측하는데 사용되는 중요한 측정 방법중 하나이다. 복사 열속과 압력은 연소 불안정을 일으키는 주요인으로 알려져 있다. 최근에는 레이저 기술의 발달로 비접촉 방식으로 외부에서 연소장을 교란하여 불안정성을 판단하는 실험 연구가 많이 이뤄지고 있다. 이러한 실험 연구에 대한 해석 및 예측을 위하여 많은 연구자들이 이론 응답 함수를 구하려는 노력들을 하고 있다. De Luca 등이 복사열에 대한 응답 함수를 제안한데 이어 Brewster 등은 QSHOD 가정과 추진제 내부로의 복사열 흡수비를 고려하여 응답 함수를 구하였다 Lee 등은 ZN 방법을 사용하여 Brewster의 응답 함수를 재계산 하였다. 이와 같은 이론 응답 함수들은 연소 불안정성에 대한 유용한 정보들을 제공하였음에도 불구하고, 현상을 너무 단순화하였다는 한계를 가지고 있다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2001.04a
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• pp.17-20
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• 2001

Combustion characteristics of KSR-III liquid rocket engine are investigated numerically in the standpoints of engine performance and acoustic instability. In the present calculation, engine performance for design and off-design conditions is estimated effectively with reasonable error. Numerical results of acoustic instability show that engine operation for the design condition has sufficient stability margin, but for a certain off-design condition, acoustic instability can be triggered by artificial pressure perturbation. The present results are in a good agreement with the available experimental results and can be adopted for the prediction of engine performance and stability, depending on the specific operating condition.

• The Journal of the Acoustical Society of Korea
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• v.39 no.1
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• pp.32-37
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• 2020

This paper proposes a new l₁-norm-Recursive Least Squares (RLS) algorithm which is numerically more robust than the conventional l₁-norm-RLS. The l₁-norm-RLS was proposed by Eksioglu and Tanc in order to estimate the sparse acoustic channel. However the algorithm has numerical instability in the inverse matrix calculation. In this paper, we propose a new algorithm which is robust against the numerical instability. We show that the proposed method improves stability under several numerically erroneous situations.

• Proceedings of the Korean Nuclear Society Conference
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• 2001.05a
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• pp.147.1-147
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• 2001