• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.2151-2156
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• 2003

The distinguishing features of flows at high angles of attacks are caused by the generation of free shear layers at sharp leading edges, by separation of the viscous layers from the surfaces of wings and bodies and by the flow in the wakes of the wings and bodies. In this study, systematic approach by PIV experimental method within a circulating water channel was adopted to study the fundamental characteristics of induced vortex generation, development and its breakdown appearing on a delta wing model with or without LEX in terms of four angles of attack($15^{\circ}$, $20^{\circ}$, $25^{\circ}$, $30^{\circ}$) and six measuring sections(30%, 40%, 50%, 60%, 70%, 80%) of chord length. Distributions of time-averaged velocity vectors and vorticities over the delta wing model were compared along the chord length direction. Highly swept leading edge extension(LEX) applied to delta wings has greatly improved the subsonic maneuverability of contemporary fighters. High-speed CCD camera which made it possible to acquire serial images is able to get the detailed information about the flow characteristics occurred on the delta wing. Especially quantitative comparison of the maximum vorticity featuring the induced pressure distribution were also conducted to clarity the significance of the LEX existence.

• 한국추진공학회지
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• 제19권1호
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• pp.42-49
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• 2015

3차원 Large Eddy Simulation(LES)와 Proper Orthogonal Decomposition(POD) 기법을 이용하여 고체로켓의 인히비터에서 발생하는 연소실내 압력 진동 특성을 분석하였다. 인히비터 후방에서 발생한 와류는 Flow-acoustic coupling에 의해 주기적으로 반복하여 생성, 소멸이 이루어지는 것을 확인하였고, 이 와류가 내삽 노즐 입구 도출부에 충돌하면서 유동이 불균질하게 분해되고, 후방 돔으로 유입된 유동에 의한 압력 진동은 연소실 압력 진동 가진의 원인이 된다. 또한 인히비터에서 발생하는 와흘림(vortex shedding) 주기는 연소실내 와류 발생 주기와 일치하며, 실험에서 측정된 압력 진동 주파수와 비교 분석하였다.

• Fibers and Polymers
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• 제7권2호
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• pp.146-157
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• 2006

Traditional spinning systems have reached profitability limits in developed countries due to high production costs and low system productivity. Pneumatic spinning is seen as a developing system, because productivity is much higher than conventional systems. This study evaluates one of the main problems to increase productivity in pneumatic spinning, where air mass-flow is dragged by the drafting cylinders. This flow interacts with the incoming fibres deviating them from their expected path. Via laser anemometry, airflow velocity distribution around drafting cylinders has been measured and it has been found that vorticity is created at the cylinder's inlet. Extensive CFD simulation on the air flow dragged by the cylinders has given a clear insight into the vortex created, producing valuable information on how cylinder design affects the vorticity created. Several drafting cylinder designs have been tested without giving any improvement in productivity. However, the use of a drafting cylinder with holes in it produced good results to the problem of air currents, strongly reducing them and therefore allowing a sharp increase in yarn quality, as well as an increase in productivity. An extensive study on vortex kinematics has been undertaken, bringing with it a better understanding of vortex creation, development and breakdown.

• International Journal of Fluid Machinery and Systems
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• 제4권1호
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• pp.33-46
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• 2011

Concave surface boundary-layer flows are subjected to centrifugal instability which results in the formation of streamwise counter-rotating vortices. Such boundary layer flows have been experimentally investigated on concave surfaces of 1 m and 2 m radius of curvature. In the experiments, to obtain uniform vortex wavelengths, thin perturbation wires placed upstream and perpendicular to the concave surface leading edge, were used to pre-set the wavelengths. Velocity contours were obtained from hot-wire anemometer velocity measurements. The most amplified vortex wavelengths can be pre-set by the spanwise spacing of the thin wires and the free-stream velocity. The velocity contours on the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different vortex growth regions can be identified. The occurrence of a secondary instability mode is also shown as mushroom-like structures as a consequence of the non-linear growth of the streamwise vortices. Wall shear stress measurements on concave surface of 1 m radius of curvature reveal that the spanwise-averaged wall shear stress increases well beyond the flat plate boundary layer values. By pre-setting much larger or much smaller vortex wavelength than the most amplified one, the splitting or merging of the streamwise vortices will respectively occur.

• 한국추진공학회지
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• 제18권2호
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• pp.1-9
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• 2014

환형 연소기 내에서의 난류 연소 유동을 해석하고 유동 특성을 도출하기 위해 3차원 large-eddy simulation (LES)를 수행하였다. 연소실 내 복잡한 반응 연소 유동의 화염모사를 위해 level-set flamelet 기법을 적용하였다. 계산 모델로서 GEAE사의 LM6000 환형 싱글 연소기를 이용하였으며 작동 조건은 실험결과에 근거하였다. 연소실 내에서 난류 유동의 중요한 특징인 vortex breakdown과 스월분사기에서 분사되는 연소가스의 팽창으로 인한 중심 재순환 영역, 코너 재순환 영역 등을 관찰하였고, 난류화염 구조를 분석하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.775-778
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• 2002

In order to analyse the mechanism of a flow tone around a cavity, the correlations between the flow in the cavity and the boundary layer flow in front of the cavity are studied experimentally in this paper. The instability In the boundary layer forms the vortex at the front edge of the cavity and the flow tone is occurred by the vortex breakdown at the rear edge of the cavity Therefore, the boundary layer measurement is important in the cavity flow control. We measure the velocity of the boundary layer at the entrance of the cavity using hot-wire anemometry and the flow tone around the cavity by microphone. The boundary layer characteristic is changed by the various angle of the flap on the front edge of the cavity, while it is less influenced by the ratio of length and depth of the cavity.

• Journal of Mechanical Science and Technology
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• 제18권12호
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• pp.2273-2283
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• 2004

The vortical flows over sharp-edged delta wings with and without a leading edge extension have been investigated using a computational method. Three-dimensional compressible Reynolds-averaged Navier-Stokes equations are solved to provide an understanding of the effects of the angle of attack and the angle of yaw on the development and interaction of vortices and the aerodynamic characteristics of the delta wing at a freestream velocity of 20 m/s. The present computations provide qualitatively reasonable predictions of vortical flow characteristics, compared with past wind tunnel measurements. In the presence of a leading edge extension, a significant change in the suction pressure peak in the chordwise direction is much reduced at a given angle of attack. The leading edge extension can also stabilize the wing vortex on the windward side at angles of yaw, which dominates the vortical flows over yawed delta wings.

• 대한기계학회논문집B
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• 제23권7호
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• pp.918-927
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• 1999

In the fully developed internal flow fields, there are complex transition flows caused by interaction of the cross flow and jet when jet is Injected Into the flow. These interactions are studied by means of the flow visualization methods. An instantaneous laser tomographic method is used to reveal the physical mechanism and the structure of vortices formation in the branch pipe flow. The velocity range of cross flow of the pipe is 0.7m/s and the corresponding Reynolds number $R_{cf}$, based on the duct height is $5.6{\times}10^3$, diameter/height ratios(d/H) 0.14 and velocity ratios 3.0. Oil mist with the size of $10{\mu}m$ diameter is used for the scattering particle. The instantaneous topological features of the vortex ring roll-up of the jet shear layer and characteristics of this flow are studied in detail by performing flow visualization in rectangular duct flow. It is found that the formation and roll-up of ring vortices is a periodic phenomenon. The detailed topology of the vortices in the near field of a cross -flow jet and the mechanism associated with them give enforced hints of vortex breakdown within the vortex system due to the interaction of the jet and the cross-flow.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집B
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• pp.416-421
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• 2000

The performance of gas turbine engines is affected by instabilities, like as rotating stall and/or surge. Rotating Stall is a transient intermediate stage between normal flow and complete flow breakdown leading to engine surge. Rotating Stall is associated with large amplitude nonaxisymmetric flow variations rotating around the compressor annulus. This paper presents the evolutions of stall propagation in a compressor cascade by numerical analysis. The flow phenomena due to stall cells and propagation speed are examined using 2 dimensional Navier - Stokes equations.

• 한국항공우주학회지
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• 제30권5호
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• pp.109-117
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• 2002

옆미끄럼이 있는 조건에서 LEX를 갖는 삼각날개의 와류 특성을 유동의 가시화에 의한 실험적 방법으로 연구하였다. 모델은 $65^{\circ}$ 후퇴각의 갖는 평판 날개이다. 자유 유동속도는 6.2 m/s이며, 이 속도와 날개뿌리 시위를 기준으로 한 레이놀즈 수는 $4.4\times10^5$ 이다. 받음각 범위와 옆미끄림각 범위는 각각 $16^{\circ}\sim28^{\circ}$와 $0^{\circ}\sim-15^{\circ}$ 이다. 미세수적과 레이저 평면광에 의한 가시화는 오공 프로브에 의한 측정으로부터 얻을 수 없었던 보다 자세한 와유동 구조를 관찰 할 수 있도록 하였다. 옆미끄럼각이 있는 경우, 바람쪽은 LEX 와류와 날개 와류의 상호작용과 붕괴가 촉진되는 반면, 바람반대쪽은 두 와류의 상호작용과 붕괴가 억제되었다.