• 한국연소학회지
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• 제7권1호
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• pp.23-30
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• 2002

Instability of oblique detonation waves (ODW) at off-attaching condition was investigated through a series of numerical simulations. Two-dimensional wedge of finite length was considered in $H_2/O_2/N_2$ mixtures at superdetonative condition. Numerical simulation was carried out with a compressible fluid dynamics code and a detailed hydrogen-oxygen combustion mechanism. Present result reveals that there is a chemical kinetic limit of the ODW detachment, in addition to the theoretical limit predicted by Rankine-Hugoniot theory with equilibrium chemistry. Result also presents that ODW still attaches at a wedge as an oblique shock-induced flame showing periodically unstable motion, if the Rankine-Hugoniot limit of detachment is satisfied but the chemical kinetic limit is not. Mechanism of the periodic instability is considered as interactions of shock and reaction waves coupled with chemical kinetic effects. From the investigation of characteristic chemical time, condition of the periodic instability is identified as follows; at the detaching condition of the Rankine-Hugoniot theory, (1) flow residence time is smaller than the chemical characteristic time, behind the detached shock wave with heat addition, (2) flow residence time should be greater than the chemical characteristic time, behind an oblique shock wave without heat addition.

• 설비공학논문집
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• 제22권3호
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• pp.156-164
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• 2010

Combustion instability is a major issue in design of gas turbine combustors for efficient operation with low emissions. Combustion instability is induced by the interaction of the unsteady heat release of the combustion process and the change in the acoustic pressure in the combustion chamber. In an effort to develop a technique to predict self-excited combustion instability of gas turbine combustors, a new stability analysis method based on the transfer matrix method is developed. The method views the combustion system as a one-dimensional acoustic system with a side branch and describes the heat source as the input to the system. This approach makes it possible to use not only the advantages of the transfer matrix method but also well established classic control theories. The approach is applied to a gas turbine combustion system, which shows the validity and effectiveness of the approach.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2009년도 하계학술발표대회 논문집
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• pp.1449-1457
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• 2009

Combustion instability is a major issue in design of co-generation gas turbine combustors for efficient operation with low emissions. Combustion instability is induced by the interaction of the unsteady heat release of the combustion process and the change in the acoustic pressure in the combustion chamber. In an effort to develop a technique to predict self-excited combustion instability of co-generation gas turbine combustors, a new stability analysis method based on the transfer matrix method is developed. The method views the combustion system as a one-dimensional acoustic system with a side branch and describes the heat source as the input to the system. This approach makes it possible to use not only the advantages of the transfer matrix method but also well established classic control theories. The approach is applied to a simple co-generation gas turbine combustion system, which shows the validity and effectiveness of the approach.

• 한국항공운항학회지
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• 제23권4호
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• pp.30-35
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• 2015

In this paper, a theoretical study of low frequency non acoustic instability, the $L^*$ instability, of a solid rocket motor is investigated. The $L^*$ stability criterion is determined by analysing the $L^*$ stability curves of two very distinct propellants for five different geometrical combustors. The $L^*$ instability of two extreme fuels showed totally different behavior in terms of operating pressure of the combustor. A parametric study on the stability for different chamber volume and different throat area keeping constant $L^*$ is conducted and analyzed. It was found that one of the main parameters, the non-dimensional critical characteristic time, requires an enough margin from the critical $L^*$ stability curve.

• 천문학회보
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• 제32권2호
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• pp.52.1-52.1
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• 2007

• 천문학회보
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• 제22권
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• pp.24-24
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• 1997

• 대한기계학회논문집A
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• 제35권10호
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• pp.1205-1210
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• 2011

척추 불안정성은 요통과 관련되어 있다고 알려져 있다. 하지만 척추 불안정성과 관련된 임상적 연구가 충분하지 않아서 불안정성의 정의가 정량적으로 확립되어 있지 않다. 또한 추간판 퇴행 및 손상, 인대 결손, 협부 결손 등이 불안정성을 발생시키는 요인으로 알려져 있으나, 척추 불안정성에 미치는 영향이 명확하게 분석되어 있지 않다. 본 연구에서는 요추 단분절의 3 차원 유한요소 모델을 이용하여 인대 결손과 협부 결손이 단독 또는 복합적으로 척추 불안정성에 미치는 영향을 정량적으로 분석하였다. 여기서 굽힘 및 신전 시 척추의 시상각과 전위거리가 척추 불안정성을 나타내는 지표라고 가정하였다. 본 연구의 결과는 척추 불안정성의 원인과 기전을 이해하는데 도움이 될 수 있을 것이다.

• 한국재료학회지
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• 제32권11호
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• pp.449-457
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• 2022

Halide perovskites are emerging materials for next-generation display applications, thanks to their narrow emission linewidth and band gap tunability, capable of covering the entire range of visible light. Despite their short period of research, perovskite light emitting diodes (PeLEDs) have shown rapid progress in device external quantum efficiency (EQE) in the near-infrared (NIR), red, and green emission wavelengths, and the record EQE has exceeded over 20 %. However there has been limited progress with blue emission compared to the red and green counterparts. In this review, the current status and challenges of blue PeLEDs are introduced, and strategies to produce spectrally stable blue PeLEDs are discussed. The strategies include 1) a mixed halide system in the form of 3-dimensional (3D) perovskites, 2) colloidal perovskite nanocrystals and 3) low dimensional perovskites, known as quasi-2D perovskites. In the mixed halide system, previous reports based on the compositional engineering of 3D perovskites to reduce spectral instability (i.e., halide segregation) will be discussed. Since spectral instability issue originate from the mixed halide composition in perovskites, the two other strategies are based on enlarging the band gap with a single halide composition. Finally, the prospects for each strategy are discussed, for further improvement in spectrally stable blue PeLEDs.

• Journal of electromagnetic engineering and science
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• 제3권2호
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• pp.133-139
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• 2003

The nonuniqueness of a mathematically rigorous solution to 2-dimensional inverse scattering problems is explained in a limiting view of the numerical calculations based on the spectral-domain moment method. It is illustrated that its theoretical uniqueness cannot be assured even by performing additional measurements of the scattered fields not only along multiple lines but also with angular/frequency-diversities. In a real situation, however, computational error and measurement noise are inevitable. Those limitations render it meaningless to controvert the existence of a theoretically rigorous solution. Hence the most practical issue is how to remedy the instability of its practically approximate solution.

• 한국소음진동공학회논문집
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• 제13권5호
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• pp.400-408
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• 2003

This paper investigates the vibration characteristics of steam generator (SG) U-tubes with defect. The operating SG shell-side flow field conditions for determining the fluidelastic instability parameters such as added mass are obtained from three-dimensional SG flow calculation. Modal analyses are performed for the U-tubes either with axial or circumferential flaw with different sizes. Special emphases are on the effects of flaw orientation and size on the modal and instability characteristics of tubes, which are expressed in terms of the natural frequency, corresponding mode shape and stability ratio. Also, addressed is the effect of the internal pressure on the vibration characteristics of the tube.