• Title/Summary/Keyword: 와열

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Application of an Opensource OpenFoam for Unsteady Aerodynamics Analysis of an Oscillating Cylinder (진동하는 원통 비정상 공력해석을 위한 OpenFoam OverSet격자 활용)

  • Kim, GunHong;Han, Cheolhuei
    • Journal of Institute of Convergence Technology
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    • v.11 no.1
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    • pp.39-42
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    • 2021
  • 2차원 유동장내 수직 진동하는 원통의 Von Karman 와열 유동 현상에 대한 진동 주파수 계산 문제는 진동하는 물체의 비정상 공력 해석 연구 검증에 많이 사용하는 고전적 방법이다. 본 연구에서는 오픈소스 OpenFoam 기반의 중첩격자 기법을 사용하여 층류 유동장내의 수직방향 진동하는 원통 주변의 비정상 유동 현상과 원통 벽면에서의 공력 특성 변화를 해석하기 위한 일련의 해석 단계들과 결과를 타 연구그룹과 비교하였다. 원통 형상과 진동에 의한 와열 유동의 주기적 유동 특성과 복잡성 해석의 건전성을 확보하기 위하여 격자와 시간제어에 대한 해의 정확도에 미치는 영향을 평가하였다. 본 연구에서 수행한 해석 방법은 일관성과 신뢰성 있는 해석 결과들 보여주었으며, 향후 보다 실제적인 진동하는 에어 포일의 비정상 공력 해석 연구에 적용 가능함을 확인하였다.

Fluid Flow Behaviors around Wedge-shaped Body using Lattice Boltzmann Method (LBM을 이용한 쇄기형 물체 주위의 유동특성)

  • Taher, M.A.;Jung, H.Y.;Lee, Y.W.
    • Journal of Power System Engineering
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    • v.13 no.4
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    • pp.24-30
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    • 2009
  • 본 연구에서는 기존에 널리 사용되어져 온 Wavier-Stokes 방정식을 풀이하는 전통적인 CFD 해석에서 벗어나 최근에 그 응용 분야를 넓혀가고 있는 LBM의 해석코드를 개발하고, 이를 이용하여 이차원 채널속에 놓여진 쇄기형 물체 주위의 유동특성을 조사하였다. D2Q9 격자계 및 Bhatnagar-Gross-Krook (LBGK) 모델을 채택하였으며, 수치해석 결과는 기존의 실험결과의 잘 일치하였다. 쇄기형 물체에서 와의 형성 및 방출 Reynolds 수 범위는 $32{\leq}Re{\leq}620$ 이며, 원형실린더에서 알려진 Karman 와열을 형성하는 주기적인 와방출은 대칭적인 와가 형성된 후 $Re{\geq}85$부터 시작되며 Reynolds 수의 증가에 따라 와 방출 주파수는 증가되었다.

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Towards Prediction of Unsteady Turbulent Flow over a Square Cylinder using Two-Equation Turbulence Models (2-방정식 난류모델을 이용한 정사각주 주위의 비정상 난류유동의 예측)

  • Lee Sangsan
    • Journal of computational fluids engineering
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    • v.1 no.1
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    • pp.47-54
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    • 1996
  • 비유선형 물체 주위의 유동은 정체유동, 경계층 박리 및 재부착, 주기적 와열의 생성등의 복잡한 유동현상이 공존한다. 이와 같은 유동의 2-방정식 난류모델을 이용한 정확한 예측은 일반적으로 불가능 하다고 인식되어 왔으나, 본 연구에서는 기존의 비교적 단순한 난류모델을 활용한 정사각주 주위의 비정상 난류유동의 예측 가능성을 체계적으로 규명하였다. 적절한 난류모델의 선정과 더불어 시간 정확도, 공간 정확도 및 대류항 처리법 등이 해석의 결과에 미치는 영향을 살펴 보았다. 기존의 표준 κ-ε모델은 정체점 주위에서 난류생성항의 과도한 예측으로 말미암아 재부착 및 와열생성의 정확한 예측이 불가능 하였으나, RNG κ-ε 모델을 사용한 경우 이와 같은 현상을 제거 할 수 있었다. 그러나 이 경우에도 예측의 정확도가 시간 증분, 격자의 크기 및 대류항 처리법 등에 영향을 받았으며, 특별히 대류항 처리법에 따라 상당히 민감하게 변하는 것을 알 수 있었다.

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Numerical Study about Heat Transfer Enhancement of Water-Microparticles Suspension (물-미립자 현탁액의 난류 열전달 향상에 관한 수치해석적 연구)

  • 정세훈;손창현
    • Journal of Advanced Marine Engineering and Technology
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    • v.24 no.3
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    • pp.29-35
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    • 2000
  • The present numerical study investigates heat transfer enhancement mechanism for suspensions of polystyrene particles in water. Numerical simulations were done for turbulent hydrodynamic fully developed flows in a circular duct with constant wall heat flux. The experimental result of microparticle suspensions show 25∼45% heat transfer enhancement over those of water. The present numerical results show the main parameter for the heat transfer enhancement of microparticle suspension in a circular duct is the change of velocity profile by the non-Newtonian fluid behavior.

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Unsteady Fluid Forces Acting on a Pitching Foil (피칭 운동익에 작용하는 비정상 유체력)

  • Yang, C.J.
    • The KSFM Journal of Fluid Machinery
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    • v.8 no.6 s.33
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    • pp.47-54
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    • 2005
  • An oscillating foil can produce a driving force through the generation of a reversed $K\'{a}rm\'{a}n$ vortex street, and it can be expected to be a new highly effective propulsion system. A simple pitching foil model was made and it was operated within a water channel. The wake formation behind a pitching foil was visualized and unsteady fluid forces were measured using a 6-axis force sensor based on force and moment detectors. We have been examined various conditions such as reduced frequency, amplitude and pivot point in NACA 0010. The results showed that thrust coefficients increased with a reduced frequency. We also presented the experimental results on the characteristics of a pitching foil at various parameters.

Noise-Vibration Phenomenon inducing Propeller on the 55ft Class Sailing Yacht (55피트급 세일링 요트의 프로펠러 유기 소음·진동 현상)

  • Lee, Donchool;Kim, Hobin;Eam, Gitak
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • 2013.10a
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    • pp.753-756
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    • 2013
  • The demand for sailing yacht is increasing in consonance the improvement of people's live. These yachts can be dually propelled by wind and by diesel engine power. A singing (humming, whistling) phenomenon induced on the propeller was discovered on a 55-foot catamaran sailing yacht. As a result, an increase in the structural vibration of the stern tube room and propulsion system with abnormal noise was detected due to this flow. In this study, the cause of the phenomenon is investigated and its possible countermeasures proposed.

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Development and Application of the Computer Program for the Performance and Noise Prediction of Axial Flow Fan (축류형 송풍기의 성능 및 소음 예측을 위한 전산 프로그램의 개발 및 적용)

  • Chung, Dong-Gyu;Hong, Soon-Seong;Lee, Chan
    • The KSFM Journal of Fluid Machinery
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    • v.3 no.3 s.8
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    • pp.31-40
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    • 2000
  • A computer program is developed for the prediction of the aerodynamic performance and the noise characteristics in the basic design step of axial flow fan. The flow field and the performance of fan are analyzed by using the streamline curvature computing scheme with total pressure loss and flow deviation models. Fan noise is assumed to be generated due to the pressure fluctuations induced by wake vortices of fan blades and to radiate via dipole distribution. The vortex-induced fluctuating pressure on blade surface is calculated by combining thin airfoil theory and the predicted flow field data. The predicted performances, sound pressure level and noise directivity patterns of fan by the present method are favorably compared with the test data of actual fan. Furthermore, the present method is shown to be very useful in optimizing design variables of fan with high efficiency and low noise level.

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PIV Measurements of Non-cavitating and Cavitating Flow in Wake of Two-dimensional Wedge-shaped Submerged Body (PIV를 이용한 2차원 쐐기형 몰수체 후류의 비공동 및 공동 유동장 계측)

  • Hong, Ji-Woo;Jeong, So-Won;Ahn, Byong-Kwon
    • Journal of Ocean Engineering and Technology
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    • v.33 no.1
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    • pp.26-32
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    • 2019
  • The vortex flow behind a bluff body has been a subject of interest for a very long time because of its engineering applicability such as to vortex induced vibration. In the near wake of a bluff body, vortices are periodically shed in two shear layers, which originate in the trailing edges. The far wake is made up of the classical Karman vortices, which are connected together by streamwise and spanwise vortices. These vortex formations have been studied in many experimental and numerical ways. However, most of the studies considered non-cavitating flow. In this study, we investigated cavitating flow in the wake of a two-dimensional wedge. Experiments were conducted in a cavitation tunnel of Chungnam National University. Using a particle image velocimetry (PIV), we measured the velocity fields under two different flow conditions: non-cavitating and cavitating regimes. We also investigated the vortex shedding frequencies using an absolute pressure transducer mounted on the top of the test window. Throughout the experiments, it was found that the shedding frequency of the vortex was strongly affected by cavitation, and the Strouhal number could exceed its value in the non-cavitating regime.

On the Flow Characteristics around a Circular Cylinder according as the Water Depth from the Free Surface (자유수면에 인접한 원형실린더형 몰수체 주위의 유동특성에 관한 연구)

  • Gim, Ok-Sok;Shon, Chang-Bae;Lee, Gyoung-Woo
    • Journal of Navigation and Port Research
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    • v.34 no.5
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    • pp.331-336
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    • 2010
  • The free surface influenced the wake behind a circular cylinder and its effects were investigated experimentally in a circulating water channel with the variation of water depth. Instantaneous velocity fields were measured in this paper. The measured results has been compared with each other to investigate the flow characteristics of the circular cylinder's 2-dimensional section at $Re=1.0{\times}10^3$ using 2-frame grey level cross correlation PIV method. The flow around the circular cylinder with free surface affected the wake structure. Especially, in case of d=1.0D, the boundary layer was measured in the whole area. The separation point and boundary layer of the circular cylinder could be controlled by the water depth.

Analysis on Vortex Streets Behind a Square Cylinder at High Reynolds Number Using a Large-Eddy Simulation Model: Effects of Wind Direction, Speed, and Cylinder Width (큰에디모의 모형을 이용한 높은 레이놀즈 수에서의 사각 기둥 후면의 와열 분석: 풍향과 풍속, 기둥 너비의 영향)

  • Han, Beom-Soon;Kwak, Kyung-Hwan;Baik, Jong-Jin
    • Atmosphere
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    • v.27 no.4
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    • pp.445-453
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    • 2017
  • This study investigates turbulent flow around a square cylinder mounted on a flat surface at high Reynolds number using a large-eddy simulation (LES) model, particularly focusing on vortex streets behind the square cylinder. Total 9 simulation cases with different inflow wind directions, inflow wind speeds, and cylinder widths in the x- and y-directions are considered to examine the effects of inflow wind direction, speed, and cylinder widths on turbulent flow and vortex streets. In the control case, the inflow wind parallel to the x-direction has a maximum speed of $5m\;s^{-1}$ and the width and height of the cylinder are 50 m and 200 m, respectively. In all cases, down-drafts in front of the cylinder and updrafts, wakes, and vortex streets behind the cylinder appear. Low-speed flow below the cylinder height and high-speed flow above it are mixed behind the cylinder, resulting in strong negative vertical turbulent momentum flux at the boundary. Accordingly, the magnitude of the vertical turbulent momentum flux is the largest near the cylinder top. In the case of an inflow wind direction of $45^{\circ}$, the height of the boundary is lower than in other cases. As the inflow wind speed increases, the magnitude of the peak in the vertical profile of mean turbulent momentum flux increases due to the increase in speed difference between the low-speed and high-speed flows. As the cylinder width in the y-direction increases, the height of the boundary increases due to the enhanced updrafts near the top of the cylinder. In addition, the magnitude of the peak of the mean turbulent momentum flux increases because the low-speed flow region expands. Spectral analysis shows that the non-dimensional vortex generation frequency in the control case is 0.2 and that the cylinder width in the y-direction and the inflow wind direction affect the non-dimensional vortex generation frequency. The non-dimensional vortex generation frequency increases as the projected width of the cylinder normal to the inflow direction increases.