• 산업기술연구
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• 제22권B호
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• pp.27-34
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• 2002

3-D numerical studies are performed to investigate the effect of the air dam height and approaching air velocities on the pressure distribution of notchback road vehicle. For this purpose, the models of test vehicle with four different air dam heights are introduced and PHOENICS, a commercial CFD code, is used to simulate the flow phenomena and to estimate the values of pressure coefficients along the surface of vehicle. The standard $k-{\varepsilon}$ model is adopted for the simulation of turbulence. The numerical results show that the height variation of air dam makes almost no influence on the distribution of the value of pressure coefficient along upper and rear surface but makes strong effects on the bottom surface. That is, the value of pressure coefficient becomes smaller as the height is increased along the bottom surface. Approaching air velocity makes no differences on pressure coefficients. Through the analysis of pressure coefficient on the vehicle surface, one tries to assess aerodynamic drag and lift of vehicle. The pressure distribution on the bottom surface affects more on lift than the pressure distribution on the upper surface of the vehicle does. The increase of air dam height makes positive effects on the lift decrease but no effects on drag reduction.

• Fisheries and Aquatic Sciences
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• 제1권2호
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• pp.168-179
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• 1998

Circulation in the Jinhae Bay in the southern sea of Korea is examined using the Princeton Ocean Model (POM) with a free surface in a sigma coordinate, governed by primitive equations. The model well corresponds to the time series of the observed velocities at several layers obtained from a long-term mooring observation. In the residual velocity field of the model, persistent downward flow fields are formed along the central deep regions in the bay, and they are caused by bottom topographic effect. In addition, a comparison between a depth-averaged (2D) model and the POM is given, and a dependance of the results on bottom drag coefficient is also examined.

• 한국자동차공학회논문집
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• 제10권6호
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• pp.178-186
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• 2002

3-D numerical studies are performed to investigate the effect of the trunk height and approaching air velocities on the pressure distribution of notchback road vehicle. For this purpose, the models of test vehicle with four different trunk heights are introduced and PHOENICS, a commercial CFD code, is used to simulate the flow phenomena and to estimate the values of pressure coefficients along the surface of vehicle. The standard k-$\xi$ model is adopted for the simulation of turbulence. The numerical results say that the height variation of trunk makes almost no influence on the distribution of the value of pressure coefficient along upper surface but makes very strong effects on the rear surface. That is, the value of pressure coefficient becomes smaller as the height is increased along the rear surface and the bottom surface. Approaching air velocity make no differences on pressure coefficients. Through the analysis of pressure coefficient on the vehicle surfaces one tried to assess aerodynamic drag and lift of vehicle. The pressure distribution on the rear surface affected more on drag and lift than pressure distribution on the front surface of the vehicle does. The increase of trunk height makes positive effects on the lift decrease but negative effects on drag reduction.

• 한국해양공학회지
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• 제26권6호
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• pp.74-79
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• 2012

The present study aims at suggesting the systematic approach to analyze the local drag components as the resistance performance characterized by the flow of the ship. In order to identify the local areas, the hull surface is decomposed into SVM (Station-Vertical Section Map) which consists of 20 stations along the longitudinal direction and 20 sections along the vertical direction (from the bottom to the waterline). Successively, on the SVM, the friction and pressure drag coefficients as the components of total drag coefficient have been analyzed for two different hull forms of Wigley and KVLCC by using CFD.

• 한국해안·해양공학회논문집
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• 제20권6호
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• pp.532-539
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• 2008

권 등(2008)은 수면보다 높은 사각기둥 저항체를 등간격으로 배치하고 항력계수가 일정한 값을 갖는 흐름조건$(Re>10^4)$에서 수리실험을 수행하여 바닥마찰과 형상저항에 의한 흐름저항을 나타내는 상당저항계수로서의 Manning계수 값을 산정하였다. 그리고 힘의 평형방정식으로부터 유도된 이론식에 실험결과를 사용하여 얻어진 경험적 항력상호작용계수를 도입하여 얻은 상당저항계수 식을 제안하였다. 본 연구에서는 이 수리실험을 3차원 전산유체해석 프로그램인 FLOW-3D를 사용하여 수치모의하고 그 결과를 실험결과 및 상당저항계수 식과 비교하여, 그값들이 매우 잘 일치하는 것을 보였다. 이로부터 이론식에 나타나 있는 것처럼 수면보다 높은 저항체가 존재할때 Manning계수가 수심의 2/3승으로 증가한다는 것과 항력상호작용계수가 이격거리의 지배적 영향을 받고 있음을 확인할 수 있었다.

• 대한기계학회논문집
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• 제13권2호
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• pp.277-286
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• 1989

본 논문에서는 유정열 등에서 수행된 단순화된 자동차형 2차원 물체 주위의 유동에 관한 실험적 연구와 병행하여 이루어진 수치해석적 연구의 일부와 계속된 일련의 연구를 다루었다. 수치해석은 ｋ-.epsilon. 난류모델과 body-fitted 좌표계를 채택하여 수행하였으며, 우선 수치계산의 합리성 및 문제점을 파악하였다. 이어서 지면효과와 물체 후미의 경사각의 영향에 대하여 수치해석적으로 연구하였다.

• 한국전산유체공학회지
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• 제20권2호
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• pp.37-45
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• 2015

A parametric study on the shapes of bobsleigh bumpers has been performed to reduce the aerodynamic drag. Effects of geometric parameters, such as leading angle of leading bumper, the ratio of minimum width to maximum width of leading bumper, the ratio of leading bumper length to trailing bumper length, trailing angle of trailing bumper, and the ratio of bumper height to installation location of bumper from the bottom of bobsleigh, on the aerodynamic performance of the bobsleigh were estimated using 3-D Reynolds-averaged Navier-Stokes equations. The turbulence was analyzed using the shear stress turbulence model. Reynolds number based on the hydraulic diameter of the external flow channel was in the range of 150,000~1,000,000. Numerical results for drag coefficient were validated compared to experimental data. Ranges of the five geometric parameters were determined according to the rule of Federation Internationale de Bobsleigh et de Tobaganning. The aerodynamic performance of the bobsleigh sled was most sensitive to the leading angle of leading bumper and the ratio of minimum width to maximum width of leading bumper.

• 대한토목학회논문집
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• 제34권1호
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• pp.107-116
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• 2014

본 연구는 조도보정 블록수로에서의 체적밀집도를 정의하고, 평균유속(V)과 수리반경(R)의 곱인 VR, 블록 Reynolds수($Re^*$), 항력계수($\acute{C}_D$) 및 바닥전단특성의 바닥조도계수($n_b$)를 분석하여 조도계수(n)를 산정하였다. VR과 블록Reynolds수가 증가함에 따라 조도계수가 감소하여 일정함에 수렴하는 경험적인 양상을 확인하였다. 블록Reynolds수의 증가에 따라 항력계수는 감소하여 일정한 값에 수렴하는 것으로 나타났다. 블록Reynolds수가 큰 난류구간에서는 항력계수는 밀집도로 정의한 조도블록의 형상에 지배적임을 볼 수 있다. 정확한 조도계수의 산정을 위해서는 블록Reynolds수와 체적밀집도에 의한 상관식의 개발이 요구된다. n-VR, $\acute{C}_D-Re^*$, $n_b-\acute{C}_D$상관에 대한 관계곡선식을 제시하였다. 조도계수를 산정할 수 있는 블록Reynolds수와 체적밀집도와의 상관관계식을 제시하였다. 실험결과를 토대로 블록Reynolds수와 체적밀집도에 의한 조도 계수 산정식을 이용한 HEC-RAS의 수리특성 분석결과는 실험결과와 잘 일치함을 보여주어 산정한 조도계수 추정식의 적용성을 확인하였다.

• Wind and Structures
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• 제20권2호
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• pp.327-347
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• 2015

This paper investigates the effects of Reynolds number (Re) on the aerodynamic characteristics of a twin-deck bridge. A 1:36 scale sectional model of a twin girder bridge was tested using the Wall of Wind (WOW) open jet wind tunnel facility at Florida International University (FIU). Static tests were performed on the model, instrumented with pressure taps and load cells, at high wind speeds with Re ranging from $1.3{\times}10^6$ to $6.1{\times}10^6$ based on the section width. Results show that the section was almost insensitive to Re when pitched to negative angles of attack. However, mean and fluctuating pressure distributions changed noticeably for zero and positive wind angles of attack while testing at different Re regimes. The pressure results suggested that with the Re increase, a larger separation bubble formed on the bottom surface of the upstream girder accompanied with a narrower wake region. As a result, drag coefficient decreased mildly and negative lift coefficient increased. Flow modification due to the Re increase also helped in distributing forces more equally between the two girders. The bare deck section was found to be prone to vortex shedding with limited dependence on the Re. Based on the observations, vortex mitigation devices attached to the bottom surface were effective in inhibiting vortex shedding, particularly at lower Re regime.

• 대한토목학회논문집
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• 제28권6B호
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• pp.635-642
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• 2008

개수로 난류흐름내 잠기지 않는 저항체들이 에너지 손실에 미치는 영향을 수리실험을 통해 연구하였다. 개수로의 잠기지 않는 구조물이나 식생을 연구하기 위하여 정방형 다열기둥을 실험에 사용하였다. 실험수로에 흐름방향 다열기둥을 횡방향으로 1열 및 2열로 배치하고 평균적 등류수위를 유지하여 일련의 유량과 수로경사를 조건으로 실험을 수행하고 측정하였다. 실험결과를 Manning공식에 적용하여 바닥마찰과 기둥의 형상저항에 의한 흐름저항 및 에너지손실을 표현하는 상당저항계수 값을 등류수심과 기둥의 이격거리를 변화시키며 산정하였다. 그리고 실험을 통해 산정된 값과 항력상호작용계수를 포함하는 운동량방정식으로부터 유도된 상당저항계수의 이론식과 비교하였다. 이론식과 실험치의 비교로부터 기둥에 의한 흐름저항의 상호작용이 수로전체의 에너지손실에 미치는 영향이 매우 중요함을 알 수 있었다. 그리고 기둥폭에 대한 기둥이격거리가 약 2.2배 이하 일 때 값이 감소하며, 항력상호작용계수가 기둥이격거리에 지배적으로 의존한다는 사실로 부터 이론식에 나타난 값이 등류수심에 2/3승으로 증가함이 유효함을 알 수 있었다.