• Title/Summary/Keyword: Turbulence Separation

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Wake Turbulence RE-CAT 적용과 효과성에 관한 연구 (The Study on the Application of RE-CAT and Effectiveness)

  • 최상일;최지호;유수정;임민성;오민하;이수정;김현미;김휘양
    • 한국항공운항학회지
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    • 제29권3호
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    • pp.34-43
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    • 2021
  • Wake turbulence generated by the lead aircraft has a significant impact on the following aircraft and it is has been considered a key factor to consider whenin determining the longitudinal separation between the aircraft. ICAO classifies aircraft into four wake turbulence categories based on the maximum takeoff weight and provides the longitudinal separation minima for each category. Due to richer measured data and better understanding of physical processes, it is raised that classifying aircraft with only four wake turbulence grades is imprecise and leads to over-separation in many instances. In this regards, much research on a new method of classifying Wake Turbulence Category(Re-CAT) has been done by EURO-CONTROL, FAA, and ICAO. The main purpose of this study is to conduct a comparative analysis of the existing wake turbulence separation standards with Re-CAT in terms of departure capacity and the resulting benefits of Re-CAT using the data from the Incheon International Airport. The results show that EUROCONTROL and new ICAO standards have the greater effect on reducing wake turbulence separation, compared to the FAA RE-CAT standards. It is also concluded that Re-CAT presents different results of wake turbulence separation depending on the flight characteristics of each airport.

자유류 난류와 표면 트립 와이어가 구 주위 유동에 미치는 영향 (Effects of the Free-Stream Turbulence and Surface Trip Wire on the Flow past a Sphere)

  • 손광민;최진;전우평;최해천
    • 유체기계공업학회:학술대회논문집
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    • 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
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    • pp.187-190
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    • 2006
  • In the present study, effects of tree-stream turbulence and surface trip wire on the flow past a sphere at $Re\;=\;0.4\;{\times}\;10^5\;{\sim}\;2.8\;{\times}\;10^5$ are investigated through wind tunnel experiments. Various types of grids are installed upstream of the sphere in order to change the tree-stream turbulence intensity. In the case of surface trip wire, 0.5mm and 2mm trip wires are attached from $20^{\circ}\;{\sim}\;90^{\circ}$ at $10^{\circ}$ interval along the streamwise direction. To investigate the flow around a sphere, drag measurement using a load cell, surface-pressure measurement, surface visualization using oil-flow pattern and near-wall velocity measurement using an I-type hot-wire probe are conducted. In the variation of free-stream turbulence, the critical Reynolds number decreases and drag crisis occurs earlier with increasing turbulence intensity. With increasing Reynolds number, the laminar separation point moves downstream, but the reattachment point after laminar separation and the main separation point are fixed, resulting in constant drag coefficient at each free-stream turbulence intensity. At the supercritical regime, as Reynolds number is further increased, the separation bubble is regressed but the reattachment and the main separation points are fixed. In the case of surface trip wire directly disturbing the boundary layer flow, the critical Reynolds number decreases further with trip wire located more downstream. However, the drag coefficient after drag crisis remains constant irrespective of the trip location.

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다양한 난류 모델을 이용한 천음속 항공기에서의 흐름 박리 예측 (FLOW SEPARATION PREDICTION ON TRANSONIC AIRCRAFT USING VARIOUS TURBULENCE MODELS)

  • 이남훈;곽인근;이승수
    • 한국전산유체공학회:학술대회논문집
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    • 한국전산유체공학회 2011년 춘계학술대회논문집
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    • pp.420-427
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    • 2011
  • In this study, numerical simulations of transonic aircraft configurations are performed with various turbulence models and the effect of turbulence models on flow separation are examined. A three-dimensional RANS code and three turbulence models are used for the study. The turbulence models incorporated to the code include Menter's ${\kappa}-{\omega}$ model, Coakley's $q-{\omega}$, and Huang and Coakley's ${\kappa}-{\omega}$, model. Using the code, numerical simulations of DLR-F6 configurations obtained from AIAA CFD Drag Prediction Workshop are conducted. Flow separations on the wing-body juncture and the wing lower surface near pylon are observed. and flow features of the regions are compared with experimental data and other numerical results.

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Effects of inflow turbulence and slope on turbulent boundary layer over two-dimensional hills

  • Wang, Tong;Cao, Shuyang;Ge, Yaojun
    • Wind and Structures
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    • 제19권2호
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    • pp.219-232
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    • 2014
  • The characteristics of turbulent boundary layers over hilly terrain depend strongly on the hill slope and upstream condition, especially inflow turbulence. Numerical simulations are carried out to investigate the neutrally stratified turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, two kinds of inflow condition, laminar turbulent, are considered. An auxiliary simulation, based on the local differential quadrature method and recycling technique, is performed to simulate the inflow turbulence be imposed at inlet boundary of the turbulent inflow, which preserves very well in the computational domain. A large separation bubble is established on the leeside of the steep hill with laminar inflow, while reattachment point moves upstream under turbulent inflow condition. There is stable separation on the side of low hill with laminar inflow, whilw not turbulent inflow. Besides increase of turbulence intensity, inflow can efficiently enhance the speedup around hills. So in practice, it is unreasonable to study wind flow over hilly terrain without considering inflow turbulence.

NACA 0021 익형 유동장의 수치해석적 연구 (A Numerical Study on the Flowfield around a NACA 0021 Airfoil at Angles of Attack)

  • 김상덕
    • 한국항공운항학회지
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    • 제24권4호
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    • pp.20-25
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    • 2016
  • A primary benefit of flight at high angle-of-attack conditions is to be able to reduce the speed of flight and maneuvers, which can enhance the capability of sensing and obstacle avoidance for a small UAV. The flight at high angle-of-attack conditions, however, is easy to be beyond stall which is characterized by substantial flow separation over an airfoil. Current numerical analysis was conducted on the capabilities of three representative turbulence models to predict the aerodynamic characteristics of a typical airfoil at angle-of-attack conditions. The investigation shows that these turbulence models provide good comparison with experimental data for attached flow at moderate angle-of-attack conditions. Calculation by current turbulence models are, however, not appropriate at high angle-of-attack conditions with flow separation.

기저부 유동 및 난류가 다단 로켓의 단 분리 운동에 미치는 영향 (EFFECT OF BASE FLOW AND TURBULENCE ON THE SEPARATION MOTION OF STRAP-ON ROCKET BOOSTERS)

  • 고순흠;김재관;한상호;김진호;김종암
    • 한국전산유체공학회:학술대회논문집
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    • 한국전산유체공학회 2007년도 춘계 학술대회논문집
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    • pp.83-86
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    • 2007
  • Turbulent flow analysis is conducted around the multi-stage launch vehicle including base region and detachment motion of strap-on boosters due to resultant aerodynamic forces and gravity is simulated. Aerodynamic solution procedure is coupled with rigid body dynamics for the prediction of separation behavior. An overset mesh technique is adopted to achieve maximum efficiency in simulating relative motion of bodies and various turbulence models are implemented on the flow solver to predict the aerodynamic forces accurately. At first, some preliminary studies are conducted to show the importance of base flow for the exact prediction of detachment motion and to find the most suitable turbulence model for the simulation of launch vehicle configurations. And then, developed solver is applied to the simulation of KSR-III, a three-stage sounding rocket researched in Korea. From the analyses, after-body flow field strongly affects the separation motions of strap-on boosters. Negative pitching moment at initial stage is gradually recovered and a strap-on finally results in a safe separation, while fore-body analysis shows collision phenomena between core rocket and booster. And a slight variation of motion is observed from the comparison between inviscid and turbulent analyses. Change of separation trajectory based on viscous effects is just a few percent and therefore, inviscid analysis is sufficient for the simulation of separation motion if the study is focused only on the movement of strap-ons.

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RNG k-$\varepsilon$ 난류모델을 이용한 유동박리 및 선회를 가지는 난류유동의 예측 (Prediction of Turbulent Flows with Separation and Swirl Using the RNG K-$\varepsilon$ Turbulence Model)

  • 김성구;오군섭;김용모;이창식
    • 한국자동차공학회논문집
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    • 제4권5호
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    • pp.119-129
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    • 1996
  • This study is concerned with the critical evaluation of predicative capability of a k-$\varepsilon$ turbulence model using the Renormalization Group(RNG) theory. The present numerical model for solution of the Navier-Stokes System is based on the modified PISO algorithms. Computations have been performed with the RNG-based K-$\varepsilon$ model for the two-dimensional flow over a backward-facing step, a confined coaxial jet, and a swirling flow in a swirl combustor. Numerical results are compared with experimental data in terms of mean flow velocities, turbulent kinetic energy, and turbulent stresses. Numerical results clearly indicate that the RNG-based K-$\varepsilon$ turbulence model shows a significant improvement over a standard K-$\varepsilon$ model in predicting the turbulent flows with flow separation and swirl.

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구에 설치한 딤플과 표면 거칠기에 의한 항력 감소 메커니즘 (Mechanism of Drag Reduction by Dimples and Roughness on a Sphere)

  • 최진;전우평;최해천
    • 유체기계공업학회:학술대회논문집
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    • 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
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    • pp.191-194
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    • 2006
  • In this paper, we present a detailed mechanism of drag reduction by dimples and roughness on a sphere by measuring the streamwise velocity above the dimpled and roughened surfaces, respectively. Dimples cause local flow separation and trigger the shear layer instability along the separating shear layer, resulting in generation of large turbulence intensity. With this increased turbulence, the flow reattaches to the sphere surface with high momentum near the wall and overcomes strong adverse pressure gradient formed in the rear sphere surface. As a result, dimples delay main separation and reduce drag significantly. The present study suggests that generation of a separation bubble, i.e. a closed-loop streamline consisting of separation and reattachment, on a body surface is an important flow-control strategy for drag reduction on a bluff body such as the sphere and cylinder. In the case of roughened sphere, the boundary layer flow is directly triggered by roughness and changes to a turbulent flow. Due to this change, the drag significantly decreases. As the Reynolds number further increases, transition to turbulence occurs earlier on the sphere surface. Because of faster growth of turbulent boundary layer by roughness, earlier transition thickens the boundary layer, resulting in earlier separation and drag increase with increasing Reynolds number

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충격파 경계층 상호작용에서 난류모델 및 난류점성의 효과 (EFFECTS OF TURBULENCE MODEL AND EDDY VISCOSITY IN SHOCK-WAVE / BOUNDARY LAYER INTERACTION)

  • 전상언;박수형;변영환
    • 한국전산유체공학회지
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    • 제18권2호
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    • pp.56-65
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    • 2013
  • Two compression ramp problems and an impinging shock problem are computed to investigate influence of turbulence models and eddy viscosity on the shock-wave / boundary layer interaction. A Navier-Stokes boundary layer generation code was applied to the generation of inflow boundary conditions. Computational results are validated well with the experimental data and effects of turbulence models are investigated. It is shown that the behavior of turbulence (eddy) viscosity directly affects both the extent of the separation and shock-wave positions over the separation.

Effects of Combustor-Level High Free-Stream Turbulence on Blade-Surface Heat/Mass Transfer in the Three-Dimensional Flow Region near the Endwall of a High-Turning Turbine Rotor Cascade

  • Lee Sang Woo;Kwon Hyun Goo;Park Byung-Kyu
    • Journal of Mechanical Science and Technology
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    • 제19권6호
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    • pp.1347-1357
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    • 2005
  • Effects of combustor-level high free-stream turbulence on the blade-surface heat/mass transfer have been investigated in the three-dimensional flow region near the endwall within a high-turning turbine rotor cascade passage. Free-stream turbulence intensity and integral length scale in the high turbulence case are 14.7 percents and 80 mm, respectively. The result shows that there is no considerable discrepancy in the blade heat/mass transfer near the endwall between the low and high turbulence cases. As departing from the endwall, however, the deviation between the two cases becomes larger, particularly in the region where flow separation and re-attachment occur. Under the high turbulence, flow disturbances such as boundary-layer separation and re-attachment seem to be suppressed, which makes the blade heat/mass transfer more uniform. Moreover, there are some evidences that endwall vortices tend to be weakened under the high turbulence.