인지 품질을 고려한 MPEG Decoder에서의 DVFS 적용 기법

백제현(Je-Hyun Baek),이재범(Jae-Beom Lee),김명진(Myoung-Jin Kim),정의영(Eui-Young Chung) 대한전자공학회 2010 대한전자공학회 학술대회 Vol.2010 No.6

축류 압축기에서의 선회실속에 관한 3 차원 수치해석

백제현(Je Hyun Baek),최민석(Minsuk Choi),오성환(Seong Hwan Oh),기덕종(Dock Jong Ki) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6

It is known that a tip leakage flow has an important role on a stall inception. However, almost of researchers have taken no interest in a role of the hub-corner-stall on the rotating stall even though it is a common feature of the flow in an axial compressor operating near stall. Using a time-accurate unsteady simulation, it is found that the hub-corner-stall may be a trigger to collapse the axisymmetric flow. An asymmetric disturbance is initially originated in the hub-corner-stall because separations are intrinsically unstable. Then this disturbance is transferred to the tip leakage flows from the hub-corner-stall and grows to be stationary stall cells, which adhere to blade passage and rotate at the same speed as rotor. When stationary stall cells reach a critical size, these cells then move along the blade row and become a rotating stall. The rotating speed of stall cells comes down to 76 percent of rotor so they rotate in the opposite direction to the rotor in the rotating frame.

두영역 모델과 직렬두요소 모델의 변수에 의한 원심 압축기 탈설계 성능의 민감도 분석

윤성호,백제현,Yoon, Sung-Ho,Baek, Je-Hyun 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.6

In this study, an off-design performance analysis procedure is developed based on Two-zone model and TEIS model. In Two-zone model, there are both primary zone and secondary zone for an isentropic core flow and an average of all non-isentropic streamtubes respectively. The level of the core flow diffusion in an impeller is calculated by using TEIS model which regards the impeller as two successive rotating elements in series. At impeller exit, the mixing process occurs with an increase in entropy, that is to say, a decrease in total pressure. In loss models including Two-zone and TEIS model, some empirical parameters have a great influence on overall performance curve. So these parameters' influences on the overall performance curve are investigated and compared with experimental data.

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.

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 $K_{LC}$ .

액적 충돌 현상에 관한 수치해석

남현우(Hyun Woo Nam),백제현(Je Hyun Baek) 한국전산유체공학회 2006 한국전산유체공학회지 Vol.11 No.3

A numerical simulation of the binary collision dynamics of water drops for size ratios of 1 and 0.75, for the Weber number range of 5 to 100, and for all impact parameter is reported. Two different types of separating collisions, namely reflexive and stretching separations, are identified. A numerical method is based on a fractional-step method with a finite volume formulation and the interface is tracked with Volume of Fluid(VOF) method, including surface tension. Numerical sresults for size ratios 1 and 0.75 are reasonablely compared with Ashgriz and Poo's experimental results.

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.

입구 경계층 두께가 축류 압축기 내부 유동에 미치는 영향 (I) - 허브 코너 실속 및 익단 누설 유동 -

최민석,박준영,백제현,Choi, Min-Suk,Park, Jun-Young,Baek, Je-Hyun 대한기계학회 2005 大韓機械學會論文集B Vol.29 No.8

A three-dimensional computation was conducted to understand effects of the inlet boundary layer thickness on the internal flow in a low-speed axial compressor operating at the design condition($\phi=85\%$) and near stall condition($\phi=65\%$). At the design condition, the flows in the axial compressor show, independent of the inlet boundary layer thickness, similar characteristics such as the pressure distribution, size of the hub comer-stall, tip leakage flow trajectory, limiting streamlines on the blade suction surface, etc. However, as the load is increased, the hub corner-stall grows to make a large separation region at the junction of the hub and suction surface for the inlet condition with thick boundary layers at the hub and casing. Moreover, the tip leakage flow is more vortical than that observed in case of the thin inlet boundary layer and has the critical point where the trajectory of the tip leakage flow is abruptly turned into the downstream. For the inlet condition with thin boundary layers, the hub corner-stall is diminished so it is indistinguishable from the wake. The tip leakage flow leans to the leading edge more than at the design condition but has no critical point. In addition to these, the severe reverse flow, induced by both boundary layer on the blade surface and the tip leakage flow, can be found to act as the blockage of flows near the casing, resulting in heavy loss.

천음속 회전익에서의 누설유동

박준영,정희택,백제현,Park, Jun-Young,Chung, Hee-Taeg,Baek, Je-Hyun 대한기계학회 2003 大韓機械學會論文集B Vol.27 No.1

It is known that tip clearance flows reduce the pressure rise, flow range and efficiency of the turbomachinery. So, the clear understanding about flow fields in the tip region is needed to efficiently design the turbomachinery. The Navier-Stokes code with the proper treatment of the boundary conditions has been developed to analyze the three-dimensional steady viscous flow fields in the transonic rotating blades and a numerical study has been conducted to investigate the detail flow physics in the tip region of transonic rotor, NASA Rotor 67. The computational results in the tip region of transonic rotors show the leakage vortices, leakage flow from pressure side to suction side and their interaction with a shock. Depen ding on the operating conditions, toad distributions and the position of shock-wave on the blade surface are very different close to the blade tip of the transonic compressor rotor. The load distribution and the shock-wave position close to the blade tip had the close relationship with the starting position of leakage vortex and the direction of leakage flow.

전향 축류형 홴에서의 익단 누설 유동 구조

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.