• Title/Summary/Keyword: 항력

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A Study of Wake Behind a Truck using Roof-fairing (Roof-fairing설치에 따른 화물차의 후류특성 연구)

  • Kim, Jin-U;Lee, Seok-Am;Yun, Jeong-Uk;Jeong, Geum-Chae;Lee, Do-Hyeong
    • Proceeding of EDISON Challenge
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    • pp.85-88
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    • 2012
  • 루프 페어링은 화물차의 주행저항 및 유도항력을 줄이기 위해 설치되는 장치 중 하나이다. 이 장치는 화물차의 지붕 윗부분에 설치하여 화물차가 받는 항력을 줄이는데 사용이 된다. 이번 과제에서는 루프 페어링의 효과에 대해 유체역학적인 증명 및 확인을 위해 루프 페어링의 설치 전-후 트럭의 형상에 따른 유동의 변화와 항력계수의 값을 EDISON CFD를 사용하여 구한 후 각각 비교 하였다. 이를 위해 EDISON CFD의 압력 contour와 stream line, 정압계수(Cp)그래프를 통해 트럭의 주행 중 발생하는 박리현상을 가시적으로 확인하였고, 항력계수(Cd)값을 비교하는 것을 통해 루프 페어링을 설치한 경우가 미설치된 경우보다 주행 시 더욱 유리하다는 것으로 결론을 내렸다.

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Velocity Loss Due to Atmospheric Drag and Orbit Lifetime Estimation (항력에 의한 속도 손실 및 궤도 수명 예측)

  • Park, Chang-Su;Jo, Sang-Beom;No, Ung-Rae
    • Aerospace Engineering and Technology
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    • v.5 no.2
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    • pp.205-212
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    • 2006
  • Atmospheric drag is the most significant factor effecting the low Earth satellites under the altitude of 800 km Although the atmospheric density of the low Earth orbit is very low compared to that of the sea level, the accumulated effect of the atmospheric drag slowly lowers the satellite velocity at the perigee. Decrease in velocity at perigee directly causes decrease in altitude at apogee which changes the eccentricity of the orbit. The orbit finally reaches a circular orbit before reentering the Earth. This paper states the methods of calculating the atmospheric drag and the lifetime of the satellite. The lifetime of the kick motor and the satellites which will be used on KSLV-L are calculated by Satellite Tool Kit.

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Fundamental Experiments of Drag Reduction for a High Speed Vehicle Using Plasma Counterflow Jets (플라즈마 분사장치를 활용한 초고속 비행체의 항력감소 기초 실험)

  • Kang, Seungwon;Choi, Jongin;Lee, Jaecheong;Huh, Hwanil
    • Journal of the Korean Society of Propulsion Engineers
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    • v.21 no.6
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    • pp.57-63
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    • 2017
  • In this study, the fundamental test for drag reduction is carried out by using a plasma generator. Fundamental test setup is constructed of visualization and drag measurement system. The schlieren technique is used to visualize the plasma counterflow in supersonic flow. The penetration of the plasma jet and the change of the flow structure are observed through visualization results. Load cell is used to confirm possibility of drag reduction. Results show that drag was reduced by 6.2% using plasma jet in supersonic flow.

Requirement Analysis and Drag Prediction for the Aerodynamic Configuration of a Bearingless Rotor Hub (무베어링 로터 허브 형상에 대한 요구도 분석 및 항력 예측)

  • Kang, Hee-Jung
    • Aerospace Engineering and Technology
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    • v.11 no.1
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    • pp.19-26
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    • 2012
  • The requirement for the aerodynamic hub drag, allocated from the system requirement of development of a bearingless rotor hub, was analyzed and embodied to be substantiated by the methodology assigned from the requirement. Drag prediction for the initial hub configuration was carried out by hand calculation using aerodynamic drag coefficients and the design change about the sectional shape of torque tube was suggested to satisfy the requirement. Finally, drag prediction was performed for the changed hub configuration by using unstructured overset mesh technique and parallel computation and the calculated result satisfied the requirement of the aerodynamic hub drag. It was found that the drag of final hub configuration was also within the range of drag inferred from the trendline of developed helicopter.

Impact of Drag-Related Weighting Coefficients in Vegetated Open-Channel Flows (식생된 개수로에서 항력가중계수가 흐름에 미치는 영향 분석)

  • Kang, Hyeongsik;Choi, Sung-Uk
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.26 no.5B
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    • pp.529-537
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    • 2006
  • This paper investigates the impacts of the drag-related weighting coefficients on mean velocity and turbulence structures. The transport equations for the Reynolds stress of vegetated open-channel flows are derived by using the temporal- and horizontal-averaging scheme. It is found that the total Reynolds stress of vegetated open channel flows consists of the Reynolds stress due to temporally fluctuating velocities and the Reynolds stress due to spatially fluctuating velocities. The drag-related weighting coefficient $C_{fk}$ for the total Reynolds stress component is found to be unit, while the coefficient for the Reynolds stress due to temporally fluctuating velocities can be negligible. This is the reason why very small weighting coefficients in previous studies yield very good agreements with measured data. In other words, the Reynolds stress due to spatially fluctuating velocities remains still unknown, especially due to the large number of measuring locations. Through a developed Reynolds stress model, vegetated open-channel flows are simulated and compared with measured data from the literature. Comparisons reveal that the computed mean flow and Reynolds stress structures are hardly affected by the drag-related weighting coefficients. However, the computed turbulence intensity profiles are significant different with the drag-related weighting coefficients. A budget analysis of the transport equations for the Reynolds stress component is carried to investigate why turbulence intensity is affected by the drag-related weighting coefficients.

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Effects of Drag Models on the Hydrodynamics and Heat Transfer in a Conical Fluidized Bed Combustor (원추형 유동층 연소기의 수력학적 특성 및 열전달에 항력 모델이 미치는 영향에 대한 연구)

  • Kang, Seung Mo;Abdelmotalib, Hamada;Ko, Dong Guk;Park, Woe-Chul;Im, Ik-Tae
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.39 no.11
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    • pp.861-869
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    • 2015
  • In this study, wall to bed heat transfer and hydrodynamic characteristics in a conical fluidized bed combustor was investigated using computational fluid dynamics method. A two-fluid Eulerian-Eulerian model was used with applying the kinetic theory for granular flow(KTGF). The effects of the two drag models, Gidaspow and the Syamlal-O'Brien model, different inlet velocities($1.4U_{mf}{\sim}4U_{mf}$) and different particle sizes on the hydrodynamics and heat transfer were studied. The results showed that the hydrodynamic characteristics such as bed expansion ratio and pressure drop were not affected significantly by the drag models. But the heat transfer coefficient was different for the two drag models, especially at lower gas inlet velocities and small particle sizes.

Experimental Study about Two-phase Damping Ratio on a Tube Bundle Subjected to Homogeneous Two-phase Flow (균질 2상 유동에 놓인 관군에 작용하는 감쇠비에 대한 실험적 연구)

  • Sim, Woo Gun;Dagdan, Banzragch
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.41 no.3
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    • pp.171-181
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    • 2017
  • Two-phase cross flow exists in many shell-and-tube heat exchangers such as condensers, evaporators, and nuclear steam generators. The drag force acting on a tube bundle subjected to air/water flow is evaluated experimentally. The cylinders subjected to two-phase flow are arranged in a normal square array. The ratio of pitch to diameter is 1.35, and the diameter of the cylinder is 18 mm. The drag force along the flow direction on the tube bundles is measured to calculate the drag coefficient and the two-phase damping ratio. The two-phase damping ratios, given by the analytical model for a homogeneous two-phase flow, are compared with experimental results. The correlation factor between the frictional pressure drop and the hydraulic drag coefficient is determined from the experimental results. The factor is used to calculate the drag force analytically. It is found that with an increase in the mass flux, the drag force, and the drag coefficients are close to the results given by the homogeneous model. The result shows that the damping ratio can be calculated using the homogeneous model for bubbly flow of sufficiently large mass flux.

Numerical Investigation of Drag and Lift Characteristics of Cavitator of Supercavitating Underwater Vehicle (초공동 수중운동체 캐비테이터의 항력과 양력특성에 관한 수치해석적 연구)

  • Kang, Byung Yun;Jang, Seyeon;Kang, Shin-Hyoung
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.38 no.10
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    • pp.797-805
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    • 2014
  • The purpose of this study was to investigate the drag and lift characteristics of the cavitator of a supercavitating underwater vehicle and the pressure loss due to water intake. These investigations were performed by changing the diameter, velocity, radius of curvature of the intake, and angle of attack of the cavitator. With increasing ratio of the intake diameter to the cavitator diameter ratio($d/D_1$), the drag coefficient and the pressure loss coefficient of the water intake decreased. The greater the increase in the ratio of the intake velocity-to-free stream velocity ratio(S), the smaller was the decrease in the drag coefficient and the lift coefficient. When the intake had a radius of curvature(c), the pressure loss coefficient decreased. On the contrary, the effect of the radius of curvature on the drag coefficient was imperceptible. For angles of attack (${\alpha}$) of the caviatator in the range of $0^{\circ}$ to $10^{\circ}$, the drag coefficient and the pressure loss coefficient changed slightly, whereas the lift coefficient increased linearly with increasing angle of attack.

Study on the Midwater Trawl Available in the Korean Waters - IV (한국 근해에 있어서의 중층트로올의 연구 - IV)

  • 이병기
    • Journal of the Korean Society of Fisheries and Ocean Technology
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    • v.23 no.1
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    • pp.6-10
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    • 1987
  • The authors carried out an experiment to measure the hydrodynamic resistance of the gear and the efficiency of otter board of the midwater trawl, which is the same used in the former experiment in this series of studies. The whole resistance of the gear was measured by a 10 ton scale recording tension meter, and they were analyzed into the shearing force and the drag with relevant factors determined in the former experiments. The results obtained can be summarized as follows: 1. The whole drag of gear T(kg) and the drag of net R sub(N) (kg) can be expressed as T=2.15 v super(1.12). R sub(N)=1.96 v super(1.01) (v: towing speed in m/sec) 2. The formula of estimating the drag of net deduced by Koyama's method can be expressed as R sub(N)=4.3$\times$d/l$\times$abv 3. The shearing force and the drag of otter board is about 19 to 22% and 5 to 7% of the drag of net, respectively. Whereas, the shearing coefficient and the drag coefficient calculated by the resistance and the opening of gear are 1.5 and 0.42, respectively.

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Application of 3-D Numerical Wave Tank for Dynamic Analysis of Nonlinear Interaction between Tsunami and Vegetation (쓰나미-식생 비선형 상호작용의 동적해석을 위한 3차원 수치파동수조의 적용)

  • Lee, Woo-Dong;Hur, Dong-Soo
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.36 no.5
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    • pp.831-838
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    • 2016
  • The disaster preventing system using vegetation has been growing in the field of coastal engineering in recent years. To analyze wave and flow fields under nonlinear interactions between tsunami and vegetation, the purpose of this study is to evaluate newly-developed 3-D numerical wave tank including energy dissipation by tsunami-vegetation interaction based on existing N-S solver with porous body model. Comparing numerical results using mean drag coefficient and dynamic drag coefficient due to Reynolds number to existing experimental results it is revealed that computed results considering the dynamic drag coefficient are in good agreement with the laboratory test results for time-domain waveform. In addition, the calculated transmission coefficients of solitary waves in various vegetation densities and incident wave heights are also in good agreement with the experimental values. This confirms the validity and effectiveness of the developed 3-D numerical wave tank with the fluid resistance by vegetation.