• 한국전산유체공학회지
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• 제13권2호
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• pp.8-13
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• 2008

A supersonic viscous flow over a five-degree half-angle cone is studied computationally with three-dimensional Navier-Stokes equations. Steady asymmetric solutions show that the asymmetric flow separation is caused by convective instability. The effects of angle of attacks, Reynolds numbers, and Mach numbers have been investigated and it is found that those factors affect the generation of the side force. The side force has the maximum value at ${\alpha}=22^{\circ}$, while over ${\alpha}=22^{\circ}$, asymmetric vortex becomes transient, which results in the unsteady shedding. At the angle of attack of 22 degrees, the side force increases with Reynolds number and decreases with Mach number. The increase of the side force stops over the critical Reynolds number for the present configuration.

• International Journal of Aeronautical and Space Sciences
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• 제13권4호
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• pp.428-434
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• 2012

Missiles suffer from flight instability problems at high angles of attack, since vortex flow over a fuselage cause lateral force to the body. To overcome this problem at a high angle of attack, the development of a real time vortex controller is needed. In this paper, Proper Orthogonal Decomposition (POD) and feedback controllers are developed for real time vortex control. The POD method is one of the most well known techniques for modeling low order models that represent the original full-order model. An adaptive control algorithm is used for real time control.

• 한국가시화정보학회지
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• 제16권1호
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• pp.61-69
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• 2018

Ion exchange membrane can transfer only cation or anion in electrically conductive fluids. Recent studies have revealed that such selective ion transport can initiate electroconvective instability, resulting vortical fluid motions on the membrane. This so-called electroconvective vortex (a.k.a. electroconvection (EC)) has been in the spotlight for enhancing an ion flux in electrochemical systems. However, EC under shear flow has not been investigated yet, although most related systems operate under pressure-driven flows. In this study, we present the direct visualization platform of EC under shear flow. On the transparent silicone rubber, microscale channels were fabricated between ion exchange membranes, while allowing microscopic visualization of fluid flow and ion concentration changes on the membranes. By using this platform, not only we visualize the existence of EC under shear flow, its unique characteristics are also identified: i) unidirectional vortex pattern, ii) its advection along the shear flow, and iii) shear-sheltering of EC vortices.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제23회 추계학술대회 논문집
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• pp.27-35
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• 2004

본 논문에서는 로켓 엔진의 고주파 연소불안정 현상이 연소현상과 맞물린 음향학적 현상이라는 점과 일반적으로 로켓엔진의 연소실 및 배기노즐이 원통형이라는 점을 고려하여 단면적이 변하는 원통형 관에서 음향, 엔트로피 및 와류 파동방정식의 해를 구하는 방법을 제시하였고 이를 통하여 엔트로피 및 와류파동이 음향파동에 미치는 영향을 정량적으로 해석 및 계산 할 수 있는 방법을 제시하였다.

• Wind and Structures
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• 제26권6호
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• pp.355-367
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• 2018

This paper studies the aerodynamic stability of a tensioned, geometrically nonlinear orthotropic membrane structure with hyperbolic paraboloid in sag direction. Considering flow separation, the wind field around membrane structure is simulated as the superposition of a uniform flow and a continuous vortex layer. By the potential flow theory in fluid mechanics and the thin airfoil theory in aerodynamics, aerodynamic pressure acting on membrane surface can be determined. And based on the large amplitude theory of membrane and D'Alembert's principle, interaction governing equations of wind-structure are established. Then, under the circumstance of single-mode response, the Bubnov-Galerkin approximate method is applied to transform the complicated interaction governing equations into a system of second-order nonlinear differential equation with constant coefficients. Through judging the frequency characteristic of the system characteristic equation, the critical velocity of divergence instability is determined. Different parameter analysis shows that the orthotropy, geometrical nonlinearity and scantling of structure is significant for preventing destructive aerodynamic instability in membrane structures. Compared to the model without considering flow separation, it's basically consistent about the divergence instability regularities in the flow separation model.

• 한국추진공학회지
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• 제25권2호
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• pp.24-33
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• 2021

부분예혼합 연소기 내 연소불안정 분석을 위한 수치적 연구가 수행되었다. 해석은 연료 조건에 따라 수행되었으며 대와류모사(Large eddy simulation, LES) 기법 및 PaSR 연소 모델이 솔버 내에 도입되었다. 수치해석의 타당성을 확인하기 위한 실험과의 비교 검증이 완료되었으며 정량적, 정성적인 일치도가 확인되었다. 연료 조건에 따라 변화하는 연소기 내 화염 특성이 이어서 조사되었으며 연소불안정 발생과의 연관성이 조사되었다. 해석 결과에 따르면 연료 조건에 따라 화염 길이가 크게 변화하였다. 그리고 화염 길이가 충분히 길어질 경우 화염-와류 상호작용이 벽면 주변에서 발생하였으며 이는 연소불안정 발생의 주요 원인이 되었음이 확인되었다.

• 대한기계학회논문집
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• 제6권4호
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• pp.390-396
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• 1982

The onset of longitudinal vortices in horizontal Blasius flow isothermally heated from below is studied analytically. The assumption that at the onset of thermal instability the thermal disturbances are confined within the thermal boundary layer is employed for the limiting case of large Prandtl number. Polynomial representations for the basic quantities obtained by the integral method of the boundary layer analysis have been used. Then the system of differential equations and boundary conditions for disturbance quantities is reformulated in a convenient form so that the solutions may be constructed as rapidly convergent power series. The critical buoyancy parameter G $r_{x}$ $^{*}$ /R $e^{*1.5}$ falls between 2 and 6, which is about one order of magnitude lower than the existing experimental values. It is also shown that the positions of the onset of instability can be closely predicted by the present theory.y.y.

• Wind and Structures
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• 제6권3호
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• pp.197-208
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• 2003

Galloping instability of dry inclined cables of cable-stayed bridges has been reported by Japanese researchers. A suggested stability criterion based on some experimental studies in Japan implies that many of stay cables would be expected to suffer galloping instability, which, if valid, would cause serious difficulty in the design of cable-stayed bridges. However, this is not the case in reality. Thus, it is practically urgent and necessary to confirm the validity of this criterion and possible restriction of it. In the present study, a 2D sectional cable model was tested in the wind tunnel, and effects of various physical parameters were investigated. It is found that the stability criterion suggested by Japanese researchers is more conservative than the results obtained from the current study.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2006년도 제33회 KOSCO SYMPOSIUM 논문집
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• pp.242-250
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• 2006

The impacts of equivalence ratio on the flow structure and flame dynamics in a model gas turbine combustor are investigated using large eddy simulation(LES). Dynamic k-equation model and G-equation flamelet model are employed as LES subgrid model for flow and combustion, respectively. As a result of mean flow field for each equivalence ratio, the increase of equivalence ratio brings about the decrease of swirl intensity through the modification of thermal effect and viscosity, although the same swirl intensity is imposed at inlet. The changes of vortical structure and turbulent intensity etc. near flame surface are occurred consequently. That is, the decrease of equivalence ratio can leads to the increase of heat release fluctuation by the more increased turbulent intensity and fluctuation of recirculation flow. In addition, the effect of inner vortex generated from vortex breakdown on the heat release fluctuation is increased gradually with the decrease of equivalence ratio. Finally, it can be identified that the variations of vortical structure play an important role in combustion instability, even though the small change of equivalence ratio is occurred.

• Korean Chemical Engineering Research
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• 제53권5호
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• pp.609-613
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• 2015

The onset of instability induced by impulsive spin-down of the rigid-body flow placed in the gap between two coaxial cylinders is analyzed by using the energy method. In the present stability analysis the growth rate of the kinetic energy of the base state and also that of disturbances are taken into consideration. In the present system the primary flow is a transient, laminar one. But for the Reynolds number equal or larger than a certain one, i.e. $Re{\geq}Re_G$ secondary motion sets in, starting at a certain time. For $Re{\geq}Re_G$ the dimensionless critical time to mark the onset of vortex instabilities, ${\tau}_c$, is here presented as a function of the Reynolds number Re and the radius ratio ${\eta}$. For the wide gap case of small ${\eta}$, the transient instability is possible in the range of $Re_G{\leq}Re{\leq}Re_S$. It is found that the predicted ${\tau}_c$-value is much smaller than experimental detection time of first observable secondary motion. It seems evident that small disturbances initiated at ${\tau}_c$ require some growth period until they are detected experimentally.