• International Journal of Aerospace System Engineering
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• 제2권2호
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• pp.79-82
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• 2015

The handicap for investigating the aero-optical effect focuses on the accurate prediction on the index refraction fluctuation or density fluctuation. In recent years, with the development of CFD techniques and optical experimental techniques, the comprehension have developed on the aero-optical transmitting effect in many kinds of complex flow. This study mainly introduces the optical aberration in compressible mixing layer. And then the debates about the mechanism of aero-optical effects and assessment of image blur also present.

• 대한기계학회논문집B
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• 제20권2호
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• pp.616-623
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• 1996

This paper investigates the linear stability of reacting mixing layers with special emphasis on the existence of multiple unstable modes. The governing equations for laminar flows are from two-dimensional compressible boundary-layer equations. The chemistry is a finite rate single step irreversible reaction with Arrhenius kinetics. For the incompressible reacintg mixing layer with variable density. A necessary condition for instability has been derived. The condition requires that the angular momentum, not the vorticity, to have a maximum in the flow domain. New inflectional modes of instability are found to exist in the outer part of the mixing layer. For the compressible reacting mixing layer, supersonic unstable modes may exist in the abscence of a generalized inflection point. The outer modes at high Mach numbers in the reacting mixing layer are continuations of the inflectional modes of low Mach number flows. However, the generalized inflection point is less important at supersonic flows.

• 대한기계학회논문집B
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• 제20권2호
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• pp.605-615
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• 1996

Laminar flows in which mixing and chemical reactions take place between parallel streams of reactive species are studied numerically. The governing equations for laminar flows are from two-dimensional compressible boundary-layer equations. The chemistry is a finite rate single step irreversible reaction with Arrhenius kinetics. Ignition, premixed flame, and diffusion flame regimes are found to exist in the laminar reacting mixing layer at high activation energy. At high Mach numbers, ignition occurs earlier due to the higher temperatures in the unburnt gas. In diffusion regimes, property variations affect the laminar profiles considerably and need to be included when there are large temperature differences. The maximum temperature of a laminar reacting mixing layer is almost linear with the adiabatic flame temperature at low heat release, but only weakly at high heat release.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2005년도 창립60주년 기념 추계 학술대회
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• pp.42.2-42.2
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• 2005

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2005년도 제25회 추계학술대회논문집
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• pp.106-109
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• 2005

The Coanda effect has been used extensively in various aerodynamic applications to improve the system performance. The primary flow in Coanda ejectors is attached to the ejector wall and is expanded inducing a secondary flow. This will probably lead to the mixing of both primary and secondary flows at a down stream section. Very few works have been reported based on the optimization on such devices. The main objective of the present study is to numerically investigate the flow field on a typical Coanda ejector and validate the results with the available experimental data. Many configurations of the Coanda ejector have been analyzed. The effect of various geometric parameters of the device on the expanding mixing layer has also been obtained. The computed data agree fairly well with the experimental data available.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 1999년도 춘계 학술대회논문집
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• pp.85-89
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• 1999

Numerical Analysis has been done for the supersonic off-design jet flow due to the pressure difference between the jet and the ambient fluid. The difference of pressure generates an oblique shock or an expansion wave at the nozzle exit, The waves reflect repeatedly at the center axis and on the sonic surface in the shear layer, and the pressure difference is resolved across these waves interacted with the turbulence mixing layer. In this paper, the axi-symmetric Navier-Stokes equation has been used with two equation $k-{\varepsilon}$ turbulence closure model. The second order TVD scheme with flux limiters, based on the flux vector split by the smooth eigenvalue split, has been used to capture internal shocks and other discontinuities. The correction term for the compressible flow and the damping function are used in the turbulence model. Numerical calculations have been done to analyze the off-design jet flow due to the pressure difference. The variation of pressure along the flow axis is compared with an experimental result and other numerical result. The characteristics of the interaction between the shock cell and the turbulence mixing layer have been analyzed.

• Journal of Mechanical Science and Technology
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• 제15권5호
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• pp.646-655
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• 2001

This paper reports the effects of nozzle exit boundary layer swirl on the instability modes of underexpanded supersonic jets emerging from plane rectangular nozzles. The effects of boundary layer swirl at the nozzle exit on thrust and mixing of supersonic rectangular jets are also considered. The previous study was performed with a 30°boundary layer swirl (S=0.41) in a plane rectangular nozzle exit. At this study, a 45°boundary layer swirl (S=1.0) is applied in a plane rectangular nozzle exit. A three-dimensional unsteady compressible Reynolds-Averaged Navier-Stokes code with Baldwin-Lomax and Chiens $\kappa$-$\xi$ two-equation turbulence models was used for numerical simulation. A shock adaptive grid system was applied to enhance shock resolution. The nozzle aspect ratio used in this study was 5.0, and the fully-expanded jet Mach number was 1.526. The \"flapping\" and \"pumping\" oscillations were observed in the jets small dimension at frequencies of about 3,900Hz and 7,800Hz, respectively. In the jets large dimension, \"spanwise\" oscillations at the same frequency as the small dimensions \"flapping\" oscillations were captured. As reported before with a 30°nozzle exit boundary layer swirl, the induction of 45°swirl to the nozzle exit boundary layer also strongly enhances jet mixing with the reduction of thrust by 10%.

• 한국추진공학회지
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• 제9권2호
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• pp.97-104
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• 2005

많은 장점에도 불구하고 유동함수를 이용한 수치해석용 격자생성 좌표변환기법의 단점은 저속영역에서의 격자간격이 고속영역에 비해 상대적으로 큼에 따라 수치적 처리에 많은 오차를 내포하고 있다는 점이다. 본 연구에서는 이러한 저속영역에서의 단점을 보완하기 위하여 격자간격을 속도크기 및 영역에 따라 적절히 조절할 수 있도록 수학적으로 변형된 압축성 유동함수를 이용한 좌표변환기법을 제안하고 가스터빈엔진에 주로 적용되는 유동모델로서 동심원상 두개 이상의 난류제트혼합유동에 대해 적용하였으며 해당 실험치, 즉 축 방향 평균속도분포, 난류운동에너지, 그리고 난류전단응력분포와 비교하여 난류운동에너지가 약간 과소평가 된 대칭축을 제외한 혼합경계층 내에서 $3.5\%$ 이내의 신뢰성을 확보하였다. 본 기법은 특히 터보팬엔진에 대한 내부흐름들의 혼합유동을 규명하거나 또는 난류전단응력에 의한 제트소음발생 및 저감방법을 도모하는데 유용하게 활용될 것으로 기대된다.

• 한국항공우주학회지
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• 제40권5호
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• pp.375-384
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• 2012

본 연구에서는 초음속 비행체에 나타나는 유동 특성 해석을 위해 1988 Wilcox $\mathcal{k}-{\omega}$ 모델과 2008 Wilcox 모델의 수치 결과를 비교하였다. 충격파 - 경계층 간섭 현상과 램프 주입기 혼합 문제에 대하여 실험결과와 비교, 검토하였다. 또한, 표면 마찰 측정의 기초가 되는 평판 흐름과 전단 층 성장에 대한 상관 관계식도 비교, 분석 하였다. 램프 주입기 케이스에서 최대 주입 질량비는 1988 Wilcox 모델을 이용하였을 때 보다 신뢰성 있는 해석 결과를 예측할 수 있었다. 그러나 충격파 - 경계층 간섭 케이스에 대해서는 2008 Wilcox 모델을 적용하였을 때 더 정확한 해석 결과가 도출됨을 확인하였다.

• 한국항공우주학회지
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• 제39권3호
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• pp.232-241
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• 2011

하이브리드 로켓은 난류 산화제 유동과 고체 추진제의 기화로 인한 분사 유동 사이의 상호 작용에 의해 복잡한 형태의 혼합 전단층이 존재한다는 특별한 성질을 가지고 있다. 본 논문에서는 유동 간섭에 의해 표면에서 발생하는 진동 유동의 물리적 특성을 연구하기 위하여 압축성 효과를 고려한 질량분사가 있는 덕트 유동의 LES(Large Eddy Simulation) 해석을 수행하였다. 계산 결과에 따르면, 기화 질량이 분출됨에 따라 주유동방향 와류의 특성이 강해지고 국부적으로 발생하는 역류 현상을 근거로 벽면 근방에서 원주방향 와류가 생성됨을 확인하였다. 그리고 시간 특성을 갖고 나타나는 와류 흘림 현상은 혼합 전단층에 기인한 유동 불안정성에 의해 촉진되었으며, 분출유동에 의해 발달한 고유 진동 유동을 의미하는 압력 섭동의 특정 진동수가 $\omega$=8.8에서 검출됨을 확인하였다.