• 제목/요약/키워드: Turbulence structure

검색결과 485건 처리시간 0.02초

축대칭 하향단흐름에서 자유흐름 난류강도의 영향 (Effects of the free Stream Turbulence Intensity on the Flow Over an Axisymmetric Backward-Facing Step)

  • 양종필;김경천;부정숙
    • 대한기계학회논문집
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    • 제19권9호
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    • pp.2328-2341
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    • 1995
  • An experimental study on the flow over the axisymmetric backward-facing step was carried out. The purposes of the present study are to investigate the effect of the free stream turbulence intensity on the reattachment length and to understand the turbulence structure of the recirculating flows. Local mean and fluctuating velocity components were measured in the separated and reattaching axisymmetric turbulent boundary layer over the wall of convex cylinder placed in a water tunnel by using 2-color 4-beam fiber optics laser Doppler velocimetry. As the free stream turbulence intensity increased, the reattachment length became shorter due to the enhanced mixing in the separated shear layer. It was also observed that the reverse flow velocity and turbulent kinetic energy increase with increasing free stream turbulence intensity. Spectral data and flow visualization showed that low-frequency motions occur in the separated flow behind a backward-facing step. These motions have a significant effect on the time-averaged turbulence data.

Full-scale investigation of wind-induced vibrations of a mast-arm traffic signal structure

  • Riedman, Michelle;Sinh, Hung Nguyen;Letchford, Christopher;O'Rourke, Michael
    • Wind and Structures
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    • 제20권3호
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    • pp.405-422
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    • 2015
  • In previous model- and full-scale studies, high-amplitude vertical vibrations of mast-arm traffic signal structures have been shown to be due to vortex shedding, a phenomenon in which alternatingly shed, low-pressure vortices induce oscillating forces onto the mast-arm causing a cross-wind response. When the frequency of vortices being shed from the mast-arm corresponds to the natural frequency of the structure, a resonant condition is created causing long-lasting, high-amplitude vibrations which may lead to the fatigue failure of these structures. Turbulence in the approach flow is known to affect the cohesiveness of vortex shedding. Results from this full-scale investigation indicate that the surrounding terrain conditions, which affect the turbulence intensity of the wind, greatly influence the likelihood of occurrence of long-lasting, high-amplitude vibrations and also impact whether reduced service life due to fatigue is likely to be of concern.

점성 수치파랑수조 기술을 이용한 평판간 난류유동의 LES 해석 (Large-Eddy Simulation of Turbulent Channel Flow Using a Viscous Numerical Wave Tank Simulation Technique)

  • 박종천;강대환;윤현식;전호환
    • 한국해양공학회지
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    • 제18권2호
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    • pp.1-9
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    • 2004
  • As the first step to investigate the nonlinear interactions between turbulence and marine structures inside a viscous NWT, a LES technique was applied to solve the turbulent channel flow for =150. The employed turbulence models included 4 types: the Smagorinsky model, the Dynamic SGS model, the Structure Function model, and the Generalized Normal Stress model. The simulated data in time-series for the LESs were averaged in both time and space, and statistical analyses were performed. The results of the LESs were compared with those of a DNS, developed in the present study and two spectral methods by Yoon et al.(2003) and Kim et a1.(1987). Based on this research, the accuracy of LESs has been found to be still related to the number of grids for fine grid size).

축류압축기 회전차유동에 대한 난류모델의 성능평가 (Evaluation of Turbulence Models for A Compressor Rotor)

  • 이용갑;김광용
    • 유체기계공업학회:학술대회논문집
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    • 유체기계공업학회 1999년도 유체기계 연구개발 발표회 논문집
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    • pp.179-186
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    • 1999
  • Three-dimensional flow analysis is implemented to investigate the flow through transonic axial-flow compressor rotor(NASA R67), and to evaluate the performances of k-$\epsilon$ and Baldwin-Lomax turbulence models. A finite volume method is used for spatial discretization. And, the equations are solved implicitly in time with the use of approximate factorization. Upwind difference scheme is used for inviscid terms, but viscous terms are centrally differenced. The flux-difference-splitting of Roe is used to obtain fluxes at the cell faces. Numerical analysis is performed near peak efficiency and near stall. And, the results are compared with the experimental data for NASA R67 rotor. Blade-to-Blade Mach number distributions are compared to confirm the accuracy of the code. From the results, we conclude that k-$\epsilon$ model is better for the calculation of flow rate and efficiency than Baldwin-Lomax model. But, the predictions for Mach number and shock structure are almost same.

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Wakes of two inline cylinders at a low Reynolds number

  • Zafar, Farhan;Alam, Md. Mahbub;Muhammad, Zaka;Islam, Md.
    • Wind and Structures
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    • 제29권1호
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    • pp.55-64
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    • 2019
  • The effect of vortex impingement on the fluid dynamics around a cylinder submerged in the wake of another of different diameters is numerically investigated at a Reynolds number Re = 200. While the diameter (D) of the downstream cylinder is fixed, impinging vortices are produced from the upstream cylinder diameter (d) varied as d/D = 0.24, 0.4, 0.6, 0.8 and 1.0, with a spacing ratio L=5.5d, where L is the distance between the center of the upstream cylinder to the front stagnation point of the downstream cylinder. Two-dimensional simulations are carried out using the finite volume method. Fluid forces acting on the two cylinders are correlated with impinging vortices, vortex shedding, and wake structure. Different facets of wake formation, wake structure, and flow separation and their connections to fluid forces are discussed.

난류가 플랑크톤의 거동에 미치는 영향: 역학적 관점에서 본 얇은 층의 형성과정 (Effect of Turbulence on the Plankton Behavior: Mechanical Perspective of a Process for Developing Thin Layers)

  • 황진환
    • 한국해양학회지:바다
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    • 제17권4호
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    • pp.283-291
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    • 2012
  • 본 연구에서는 최근 물리적 관측 방법으로 그 존재가 확인된 얇은 층(thin layer)의 형성 메커니즘에 대하여 검토하였다. 기존의 연구들에서는 성층과 전단응력의 상대적 크기인 Richardson수가 얇은 층의 중요한 결정요인인 것으로 밝혀졌다. 성층이 없는 조건하에서의 물리적 수치 실험은 미세 난류(micro-structure turbulence)가 플랑크톤의 거동에 변화를 주어 성장과 재생산 등에 영향줄 수 있음을 설명하였다. 기존의 플랑크톤 거동에 성층과 전단응력의 효과를 고려하여 최근 Gyrotaxis의 메커니즘으로 얇은 층이 형성되는 과정을 설명하였다. 이러한 생태학적 문제에 물리학적 연구방법론의 적용이라는 최근의 연구경향은 현재 및 향후에 해양생태학에서 중요한 접근방법이라고 할 수 있다.

마이크로 스케일 난류에 의한 화염안정성 및 NOx 생성 (Flame Stability and NOx Formation by Micro scale Turbulence)

  • 김인수;서정무;이근선;이충원
    • 한국연소학회:학술대회논문집
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    • 한국연소학회 2001년도 제23회 KOSCO SYMPOSIUM 논문집
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    • pp.57-62
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    • 2001
  • The effect of micro scale turbulence on flame structure and stability were experimentally investigated by changing the area of micro turbulence generator(MTG) and air velocity in terms of low NOx and high efficiency combustion. NOx and CO concentration were also measured for different MTG areas to investigate whether a vane swirler having MTG has a possibility of using as part for low NOx combustor. From the obtained results, it is shown that flame stability region increases and flame size becomes small as MTG area increases since MTG in itself makes small scale recirculation flow and swirler does large scale recirculation one. It is also shown that low NOx concentration(about 20${\sim}$30ppm@$O_2$ 11%) is achieved for all MTG areas without any increase in CO concentration regardless of air velocity range tested in this study when the equivalence ratio is 0.7. The results obtained in this study can give basic guideline for the design of compact low NOx high efficiency combustor using a vane swirler having MTG.

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Effect of trunk length on the flow around a fir tree

  • Lee, Jin-Pyung;Lee, Eui-Jae;Lee, Sang-Joon
    • Wind and Structures
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    • 제18권1호
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    • pp.69-82
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    • 2014
  • Flow around a small white fir tree was investigated with varying the length of the bottom trunk (hereafter referred to as bottom gap). The velocity fields around the tree, which was placed in a closed-type wind tunnel test section, were quantitatively measured using particle image velocimetry (PIV) technique. Three different flow regions are observed behind the tree due to the bottom gap effect. Each flow region exhibits a different flow structure as a function of the bottom gap ratio. Depending on the gap ratio, the aerodynamic porosity of the tree changes and the different turbulence structure is induced. As the gap ratio increases, the maximum turbulence intensity is increased as well. However, the location of the local maximum turbulence intensity is nearly invariant. These changes in the flow and turbulence structures around a tree due to the bottom gap variation significantly affect the shelter effect of the tree. The wind-speed reduction is increased and the height of the maximum wind-speed reduction is decreased, as the gap ratio decreases.

Flow-induced vibrations of dual-cylinders in axial flow via LES simulations

  • Kangfei Shi;Yu Cao;Zhanying Zheng;Shun Lu;Menglong Liu
    • Nuclear Engineering and Technology
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    • 제56권9호
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    • pp.3812-3825
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    • 2024
  • The axial-flow-induced vibration of fuel rods in the nuclear power plant is closely related to nuclear safety. In this article, a numerical study is performed on vibration of two elastic cylinders arranged side-by-side in axial flow. Large eddy simulation is employed to predict the turbulent flow. The numerical method has been verified using the experimental root-mean-square vibration amplitude of a single cylinder. A wide range of inflow velocities u*, incident turbulence intensity Tu and space ratio P/D have been examined, where D and P are the diameter and centre-to-centre distance of the cylinders, respectively. The results show that the vibration amplitudes increase with an increasing u*, comparable to the case of a single cylinder in axial flow. However, the two cylinders could bend outwards during a relatively high u* and low Tu. Although Tu significantly affects the amplitudes of the cylinders, it does not change the vibration frequency and the critical velocity at which buckling instability occurs. As the gap between the two cylinders is sufficiently small, the vibration amplitude enhances significantly due to the pronounced hydrodynamic interaction between the two elastic cylinders and surrounding fluid. The direction of buckling is no longer random but fixed.

선박용 SCR 시스템에서 혼합기 구조에 따른 난류유동과 우레아 수용액의 혼합특성 (Effect of Mixer Structure on Turbulence and Mixing with Urea-water Solution in Marine SCR System)

  • 김태경;성연모;한승한;하상준;최경민;김덕줄
    • Journal of Advanced Marine Engineering and Technology
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    • 제36권6호
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    • pp.814-822
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    • 2012
  • 선박용 SCR 시스템에서 유동 및 혼합특성을 개선하기 위해 상하유도 및 스월 형상의 혼합기가 고려되었다. 본 연구의 목적은 혼합기 구조에 따른 난류강도 및 균일지수(Uniformity Index)를 상세히 분석하여 SCR 성능을 개선하는 것이다. 그 결과, 촉매부 전단에서의 농도균일도는 혼합기를 사용하였을 경우 약 5% 개선됨을 알 수 있었다. 상하유도형 혼합기의 사용에 따라 주위에서 높은 RMS 수치 및 상대난류강도를 보이지만, 하류로 진행하면서 두 값은 감소하는 경향을 보였다. 스월형 혼합기의 경우 유동의 진행에 대해 RMS 수치와 상대난류강도의 감소가 비교적 적고 상대난류강도의 경우 상대적으로 균일한 분포를 보였다. 두 혼합기에서 발생하는 유동특성에 의해 혼합효과 및 혼합거리가 달랐음을 알 수 있었다.