• 제목/요약/키워드: Turbulent flow region

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정익과 동익의 상호작용에 의한 비정상 천이 경계층 유동의 수치해석에 관한 연구 1

  • 강동진
    • 대한기계학회논문집B
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    • 제22권6호
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    • pp.757-770
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    • 1998
  • A Navier-Stokes code with a low Reynolds number k-.epsilon. turbulence model was tested to investigate its predictability for the unsteady transitional boundary layer flow due to rotor-stator interaction. A preliminary calculation with three different numbers of time steps 300, 600, and 1000 for a rotor wake passing period was carried out to see the effects of time steps on the unsteady flow and pressure fields due to rotor-stator interaction. Numerical solutions showed that unsteady pressure was much more sensitive to the number of time steps and over 600 time steps should be used to get a numerical solution independent of the number of time steps for a rotor wake passing period. The original low Reynolds number k-.epsilon. turbulence model showed very poor prediction of the unsteady transitional boundary layer flow due to rotor-stator interaction. This was due to the excessive production of turbulent kinetic energy near the leading edge. A modification suggested by Launder was incorporated and the modified model captured well the wake induced transitional strip. Present solutions also showed improved prediction over previous Euler/boundary layer solution in terms of the onset of unsteady transition and its extent.

정익과 동익의 상호작용에 의한 비정상 천이 경계층 유동의 수치해석에 관한 연구 (II) (Numerical Prediction of Unsteady Transitional Boundary Layer Flows due to Rotor-Stator Interaction(II)-Characteristics of Unsteady Transitional Boundary Layer Flow-)

  • 강동진
    • 대한기계학회논문집B
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    • 제22권6호
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    • pp.771-787
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    • 1998
  • A Navier-Stokes code with a modified low Reynolds number k-.epsilon. turbulence model was used to study the unsteady transitional boundary layer flow due to rotor-stator interaction. The modification, proposed by Launder, to improve prediction of stagnation flows was incorporated to the low Reynolds number k-.epsilon. turbulence model by Fan-Lakshminarayana-Barnett. Numerical solution is shown to capture well the calmed laminar flow as well as the wake induced transitional strip due to rotor-stator interaction and shows improvement, in terms of onset of transition and its length, over previous Euler/boundary layer solution. The turbulent kinetic energy shows local maximum along the upstream rotor wake in the wake induced transitional strip and this characteristics is observed untill the end of transition. The wake induced strip also shown apparent even in the laminar sublayer as the upstream rotor wake penetrates inside the boundary layer.

장애물 주위의 와구조 형성과정에 관한 수치적 연구 (Numerical Study on the Vortex Evolution from a Sharp-Edged, Wall-Mounted Obstacle)

  • 황종연;양경수
    • 대한기계학회논문집B
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    • 제28권6호
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    • pp.672-681
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    • 2004
  • Direct numerical simulation was carried out to study the vortical structures of the flow around a wall-mounted cube in a channel at Re=1,000 and Re=3,500 based on cubic height and bulk mean velocity. The cubic obstacle is situated in the entrance region of the channel flow where the boundary layers are developing. Upstream of the obstacle, steady and unsteady laminar horseshoe vortex systems are observed at Re=1,000 and Re=3,500, respectively; the near-wake flow is turbulent in both cases. The flow separates at each leading sharp edge of the cube, and subsequent vortex roll-up is noticed in the corresponding free-shear layer. The vortex shedding from the upper leading edge (upper vortices) and that from the two lateral leading edges (lateral vortices) are both quasi-periodic and their frequencies are computed. The upper and lateral vortices further develop into hairpin and Λ vortices, respectively. A series of instantaneous contours of the second invariant of velocity gradient tensor helps us identify spatial and temporal behaviors of the vortices in detail. The results indicate that the length and time scales of the vortical structures at Re=3,500 are much shorter than those at Re:1,000. Correlations between the upper and lateral vortices are also reported.

진동하는 2차원 날개 단면 주위에 대한 점성 유동장 계산( Part 1. 동적실속이 없는 경우 ) (Computation of Viscous Flows around a Two-dimensional Oscillating Airfoil ( Part 1. without Dynamic Stall ))

  • 이평국;김형태
    • 대한조선학회논문집
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    • 제44권1호
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    • pp.8-15
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    • 2007
  • In this paper, numerical calculations are performed to analyze the unsteady flow of NACA airfoil sections. In order to ease the flow computation for the fluid region changing in time, improve the quality of solution and simplify the grid generation for the oscillating foil flow, the computational method adopts a moving and deforming mesh with the multi-block grid topology. The multi-block, structured-unstructured hybrid grid is generated using the commercial meshing software Gridgen V15. The MDM (Moving & Deforming Mesh) and the UDF (User Define function) function of FLUENT 6 are adopted for computing turbulent flows of the foil in pitching motion. Computed unsteady lift and drag forces are compared with experimental data. in general, the characteristics of unsteady lift and drag of the experiments are reproduced well in the numerical analysis.

충돌제트를 이용한 Pedestal 형상의 칩 냉각연구 (Jet Impingement Heat Transfer on a Cylindrical Pedestal Encountered in Chip Cooling)

  • 이대희;이준식;정영석;정승훈
    • 대한기계학회논문집B
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    • 제27권1호
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    • pp.1-8
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    • 2003
  • The heat transfer and flow measurements on a cylindrical pedestal mounted on a flat surface with a turbulent impinging jet were made. The experiments were made for the jet Reynolds number of Re = 23,000, the dimensionless nozzle-to-surface distance of L/d = 2~10, the dimensionless pedestal height of H/D = 0~1.5. Measurements of the surface temperature and the Nusselt number distributions on the plate surface were made using liquid crystal and shroud-transient technique. Flow measurements involve smoke flow visualization and the wall pressure coefficient. The results show that the wall pressure coefficient sharply decreases along the upper surface of the pedestal. However, the pressure increases when the fluid escapes from the pedestal and then collides on the plate surface. The secondary maxima in the Nusselt numbers occur in the region of 1.0 $\leq$ r/d $\leq$ 1.9. Their values for the case of H/D = 0.5 are maximum 80% higher than those for other cases. The formation of the secondary maxima may be attributed to the reattachment of flow on the plate surface which was separated at the edge of the pedestal.

KRISO 3600TEU 콘테이너 모형선 주위 유동의 난류구조에 관한 실험적 연구 (Experimental Study on Turbulent Structure of Flow around KRISO 3600TEU Container Double-deck Model)

  • 김학록;이상준
    • 대한조선학회논문집
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    • 제36권3호
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    • pp.8-14
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    • 1999
  • KRISO 3600TEU 콘테이너 이중모형선 주위 유동의 평균속도와 난류성분들을 풍동실험을 통해 실험적으로 연구하였다. 선미와 후류의 여러 단면에서의 속도분포를 x형의 열선 프로우브로 측정하여 평균속도와 레이놀즈 응력, 그리고 난류 운동에너지 등을 구하였다. 선미영역과 후류영역에서의 유동은 매우 복잡한 3차원 유동특성을 보여주고 있다. 측정 결과로부터 주유동 방향의 와 구조의 형성과 이러한 와 유동이 후류의 유동에 미치는 영향을 알 수 있었으며, 하류로 나아감에 따라 전단층 영역이 점차 확장되어짐을 확인하였다. 이와 같은 선체주위 유동의 난류 성분들의 측정은 향후 수치계산 및 정확한 난류모델의 개발에 비교자료로 사용되어질 것이다.

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국소교란에 의한 난류박리 재부착 유동의 수치해석 (Numerical Simulation of Turbulent Separated and Reattaching Flows by Local Forcing)

  • 리광훈;성형진
    • 대한기계학회논문집B
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    • 제24권3호
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    • pp.467-476
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    • 2000
  • An unsteady numerical simulation was performed for locally-forced separated and reattaching flow over a backward-facing step. The local forcing was given to the separated and reattaching flow by means of a sinusoidally oscillating jet from a separation line. A version of the $k-{\varepsilon}-f_{\mu}$ model was employed, in which the near-wall behavior without reference to distance and the nonequilibrium effect in the recirculation region were incorporated. The Reynolds number based on the step height (H) was fixed at $Re_H=33000$, and the forcing frequency was varied in the range $0{\leq}St_H{\leq}2$. The predicted results were compared and validated with the experimental data of Chun and Sung. It was shown that the unsteady locally-forced separated and reattaching flows are predicted reasonably well with the $k-{\varepsilon}-f_{\mu}$ model. To characterize the large-scale vortex evolution due to the local forcing, numerical flow visualizations were carried out.

원주상으로 배열된 다중 원형 제트의 유동 특성 (Experiments on the Flow Characteristics of Circular Multiple Jets Arrayed Circumferentially)

  • 진학수;김정수;최종욱;김성초
    • 대한기계학회논문집B
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    • 제31권3호
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    • pp.306-312
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    • 2007
  • This paper describes the flow characteristics of circular multiple jet investigated by hot-wire anemometry. The nozzle arrays were classified into two cases; 6- or 7-nozzle located circumferentially in equal interval without or with a central jet. The flow field was measured according to the number of nozzles when the Reynolds number based on the nozzle exit is about $10^4$. Mean velocity, Reynolds shear stress and turbulent kinetic energy were investigated in the downstream of jets. The Tollmien's theory holds for downstream only when a nozzle locates at the center. Jet interaction is influenced due to with or without a center nozzle. In addition, the two-dimensional numerical computation was conducted for 3-nozzle case to obtain the general flow structure near the nozzle exit, which verifies the formation of the recirculation region with captive vortices, that is, the evidence of the interaction between jets.

난류침투가 사각단면 T분기관 내 누설유동에 의해 발생한 열성층 현상에 미치는 영향 (The Effect of Turbulence Penetration on the Thermal Stratification Phenomenon Caused by Leaking Flow in a T-Branch of Square Cross-Section)

  • 홍석우;최영돈;박민수
    • 설비공학논문집
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    • 제15권3호
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    • pp.239-245
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    • 2003
  • In the nuclear power plant, emergency core coolant system (ECCS) is furnished at reactor coolant system (RCS) in order to cool down high temperature water in case of emergency. However, in this coolant system, thermal stratification phenomenon can occur due to coolant leaking in the check valve. The thermal stratification produces excessive thermal stresses at the pipe wall so as to yield thermal fatigue crack (TFC) accident. In the present study, effects of turbulence penetration on the thermal stratification into T-branches with square cross-section in the modeled ECCS are analysed numerically. $textsc{k}$-$\varepsilon$ model is employed to calculate the Reynolds stresses in momentum equations. Results show that the length and strength of thermal stratification are primarily affected by the leak flow rate of coolant and the Reynolds number of the main flow in the duct. Turbulence penetration into the T-branch of ECCS shows two counteracting effects on the thermal stratification. Heat transport by turbulence penetration from the main duct to leaking flow region may enhance thermal stratification while the turbulent diffusion may weaken it.

Flow control downstream of a circular cylinder by a permeable cylinder in deep water

  • Gozmen, Bengi;Akilli, Huseyin
    • Wind and Structures
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    • 제19권4호
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    • pp.389-404
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    • 2014
  • The flow characteristics of a circular cylinder surrounded by an outer permeable cylinder were experimentally investigated using Particle Image Velocimetry Technique in deep water flow. In order to consider the effects of diameter and porosity of the outer cylinder on flow structures of the inner cylinder, five different outer cylinder diameters (D=37.5, 52.5, 60, 75 and 90 mm) and eight different porosities (${\beta}$=0.4, 0.5, 0.6, 0.65, 0.7, 0.75, 0.8 and 0.85) were selected. During the experiments, the diameter of inner cylinder was kept constant as d=30 mm. The depth-averaged free-stream velocity was adjusted as U=0.156 m/s, which corresponds to the Reynolds number of Re=5000 based on the inner cylinder diameter. It has been concluded that both the outer permeable cylinder diameter and the porosity have important influences on the attenuation of vortex shedding in the wake region. The presence of outer permeable cylinder decreases the magnitude of Reynolds shear stress and turbulent kinetic energy compared to the bare cylinder case. Moreover, the spectral analysis of vortex shedding frequency has revealed that the dominant frequency of vortex shedding downstream of the cylinder arrangement also reduces substantially due to the weakened Karman shear layer instability.