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임의의 종횡비를 가지는 수직축을 중심으로 회전하는 직관과 정지한 고고간 내부의 층류 유동의 유사성에 관한 수치적 연구
이공희,백제현,Lee, Gong-Hee,Baek, Je-Hyun 대한기계학회 2002 大韓機械學會論文集B Vol.26 No.6
The present study showed that a quantitative analogy of the fully developed laminar flows inorthogonally rotating rectangular ducts and stationary curved rectangular ducts of arbitrary aspect ratio could be established. In order to clarify the similarity of the two flows, the dimensionless parameters $K_{LR}$ =Re/√Ro and Rossby number Ro= $w_{m}$/$\Omega$d in a rotating strait duct were used as a set corresponding to Dean number $K_{LC}$ =Re/√λand curvature ratio λ=R/d in a stationary curved duct. Under the condition that the value of Rossby number and curvature ratio was large enough, the flow field satisfied the ‘asymptotic invariance property’: there were strong quantitative similarities between the two flows such as friction factors, flow patterns, and maximum axial velocity magnitudes for the same values of $K_{LR}$ and $K_{LC}$ .
이공희,명환주,백제현,Lee, Gong-Hee,Myung, Hwan-Joo,Baek, Je-Hyun 대한기계학회 2003 大韓機械學會論文集B Vol.27 No.7
The experiment using three-dimensional laser Dopperr velocimetery (LDV) measurements and the computation using the Reynolds stress model of the commercial code, FLUENT, were conducted to give a clear understanding on the structure of tip leakage flow in a forward-swept axial-flow fan operating at the maximum efficiency condition. The tip leakage vortex was generated near the position of the minimum wall static pressure, which was located at approximately 12% chord downstream from the leading edge of blade suction side, and developed along the centerline of the pressure trough within the blade passages. A reverse flow between the blade tip region and the casing, induced by tip leakage vortex, acted as a blockage on the through-flow. As a result, high momentum flux was observed below the tip leakage vortex. As the tip leakage vortex proceeded to the aft part of the blade passage, the strength of tip leakage vortex decreased due to the strong interaction with the through-flow and casing boundary layer, and the diffusion of tip leakage vortex caused by high turbulence. In comparison with LDV measurement data, the computed results predicted the complex viscous flow patterns inside the tip region, including the locus of tip leakage vortex center, in a reliable level.
축류형 유체기계에서 익단 누설 유동 해석을 위한 난류 모델 성능 평가
이공희,백제현,Lee, Gong-Hee,Baek, Je-Hyun 대한기계학회 2003 大韓機械學會論文集B Vol.27 No.12
It is experimentally well-known that high anisotropies of the turbulent flow field are dominant inside the tip leakage vortex, which is attributable to a substantial proportion of the total loss and constitutes one of the dominant mechanisms of the noise generation. This anisotropic nature of turbulence invalidates the use of the conventional isotropic eddy viscosity turbulence models based on the Boussinesq assumption. In this study, to check whether an anisotropic turbulence model is superior to the isotropic ones or not, the results obtained from the steady-state Reynolds averaged Navier-Stokes simulations based on the RNG k-$\varepsilon$ model and the Reynolds stress model (RSM) are compared with experimental data for two test cases: a linear compressor cascade and a forward-swept axial-flow fan. Through this comparative study of turbulence models, it is clearly shown that the RSM, which can express the production term and body-force term induced by system rotation without introducing any modeling, should be used to predict quantitatively the complex tip leakage flow, especially in the rotating environment.
회전하는 정사각 단면 U자형 곡관 내부의 유동 발달에 관한 수치적 연구(II) -난류 유동-
이공희,백제현,Lee, Gong-Hee,Baek, Je-Hyun 대한기계학회 2002 大韓機械學會論文集B Vol.26 No.6
The present study investigates in detail the combined effects of the Coriolis force and centrifugal force on the development of turbulent flows in a square-sectioned U-bend rotating about an axis parallel to the center of bend curvature. When a viscous fluid flows through a curved region of U-bend, two types of secondary flow occur. One is caused by the Coriolis force due to the rotation of U-bend and the other by the centrifugal force due to the curvature of U-bend. For positive rotation, where the rotation is in the same direction as that of the main flow, both the Coriolis force and the centrifugal force act radially outwards. Therefore, the flow structure is qualitatively similar to that observed in a stationary curved duct. On the other hand, under negative rotation, where these two forces act in opposite direction, more complex flow fields can be observed depending on the relative magnitudes of the forces. Under the condition that the value of Rossby number and curvature ratio is large, the flow field in a rotating U-bend can be represented by two dimensionless parameters : $K_{TC}$ =Re $\sfrac{1}{4}$√λand a body force ratio F=λ/Ro. Here, $K_{TC}$ has the same dynamical meaning as $K_{TC}$ =Re√λ for laminar flow.
유동 덮개 형상이 축소 APR+ 내부 유동분포에 미치는 영향에 대한 수치해석
이공희(Gong Hee Lee),방영석(Young Seok Bang),우승웅(Sweng Woong Woo),김도형(Do Hyeong Kim),강민구(Min Ku Kang) 대한설비공학회 2013 설비공학 논문집 Vol.25 No.5
In this study, in order to examine the applicability of computational fluid dynamics with the porous model to the analysis of APR+ (Advanced Power Reactor Plus) internal flow, simulation was conducted with the commercial multi-purpose computational fluid dynamics software, ANSYS CFX V.14. In addition, among the various reactor internals, the effect of flow skirt geometry on reactor internal flow was investigated. It was concluded that the porous model for some reactor internal structures could adequately predict the hydraulic characteristics inside the reactor in a qualitative manner. If sufficient computation resource is available, the predicted core inlet flow distribution is expected to be more accurate, by considering the real geometry of the internal structures, especially located in the upstream of the core inlet. Finally, depending on the shape of the flow skirt, the flow distribution was somewhat different locally. The standard deviation of the mass flow rate(σ) for the original shape of flow skirt was smaller, than that for the modified shape of flow skirt. This means that the original shape of the flow skirt may give a more uniform distribution of mass flow rate at the core inlet plane, which may be more desirable for the core cooling.
CFD해석을 통한 직관덕트와 주름덕트의 공기 유동성능에 관한 연구
이공희(Gong-Hee Lee),윤의열(Ui-Yeol Yoon),전희호(Hee-Ho Jeon),이승준(Seung-Jun Lee) 대한기계학회 2017 대한기계학회 춘추학술대회 Vol.2017 No.11
High-temperature electronic equipment dissipates heat by receiving cooled air from environmental control devices for effective heat dissipation. Due to space constraints and equipment operating environmental conditions, it is common to provide cooling air at a distance using air ducts. In this case, uniform and stable supply of cooling air is most important. But, the ducts are accompanied by flow losses and pressure variations due to changes in flow cross-sectional area, internal surface conditions, and flow velocity, and in particular, there is a difference in air flow characteristics depending on the cross-sectional shape of the duct. In this paper, the air flow performance of straight duct and flexible duct is compared by CFD(Computational Fluid Dynamics) analysis. As a result, it was confirmed that the air flow nonuniform and instability occurred due to the frictional resistance and the pressure difference of the flexible duct’s wrinkled portion.