• 대한기계학회논문집B
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• 제27권3호
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• pp.311-318
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• 2003

The present work evaluates the effects of mixing vane shape on the flow structure and heat transfer downstream of mixing vane in a subchannel of fuel assembly. by obtaining velocity and pressure fields. turbulent intensity. flow-mixing factors. heat transfer coefficient and friction factor using three-dimensional RANS analysis. Four different shapes of mixing vane. which were designed by the authors were tested to evaluate the performances in enhancing the heat transfer. Standard k-$\varepsilon$ model is used as a turbulence closure model. and. periodic and symmetry conditions are set as boundary conditions. The flow blockage ratio is kept constant. but the twist angle of mixing vane is changed. The results with three turbulence models were compared with experimental data.

• 대한기계학회논문집B
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• 제32권3호
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• pp.198-206
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• 2008

A numerical study has been carried out to investigate heat transfer enhancement in channel flow using large-scale vortices. A square cylinder, inclined with respect to the main flow direction, is located at the center of the channel flow, generating a separation region and Karman vortices. Two cases are considered; one with a fixed blockage ratio and the other one with a fixed cylinder size. In both cases, the flow characteristics downstream of the cylinder significantly change depending on the inclination angle. As a result, heat transfer from channel wall is significantly enhanced due to increased vertical-velocity fluctuations induced by the large-scale vortices shed from the cylinder. Quantitative results as well as qualitative physical explanation are presented to justify the effectiveness of the inclined square cylinder as a vortex generator to enhance heat transfer from channel wall.

• Wind and Structures
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• 제17권5호
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• pp.465-477
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• 2013

In this paper, the effects of parapets on the mean and fluctuating wind pressures which are acting on a flat top of a finite cylinder vertically placed on a flat plate have experimentally been investigated. The aspect ratio (AR) of cylinder is 1 and the Reynolds number (Re) based on cylinder diameter and free stream velocity is 150000. The pressure distributions on the flat top and the side wall of the finite cylinder immersed in a simulated atmospheric boundary layer have been obtained for different parapet heights. The large magnitudes of mean and minimum suction pressures occurring near the leading edge were measured for the cases with and without parapet. They shift to the further downstream on the circular top with increasing parapet height. It is seen that the parapets reduce the local high suction on the top up to 24%.

• 한국안전학회지
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• 제35권6호
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• pp.32-45
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• 2020

The problem of determining the discharge rates of gases from pressurized vessels through pressure relief devices was dealt with comprehensively. First, starting from basic fluid flow equations, detailed modeling procedures were presented for isentropic nozzle flows and frictional flows in a pipe, respectively. Meanwhile, physical explanations were given to choking phenomena in terms of the acoustic velocity, elucidating the widespread use of Mach numbers in gas flow models. Frictional flows in a pipe were classified into adiabatic, isothermal, and general flows according to the heat transfer situation around the pipe, but the adiabatic flow model was recommended suitable for gas discharge through pressure relief devices. Next, for the isentropic nozzle flow followed by adiabatic frictional flow in the pipe, two equations were established for two unknowns that consist of the Mach numbers at the inlet and outlet of the pipe, respectively. The relationship among the ratio of downstream reservoir pressure to upstream pressure, mass flux, and total frictional loss coefficient was shown in various forms of MATLAB 2-D plot, 3-D surface plot and contour plot. Then, the profiles of gas properties and velocity in the pipe section were traced. A method to quantify the relationship among the pressure head, velocity head, and total friction loss was presented, and was used in inferring that the rapid increase in gas velocity in the region approaching the choked flow at the pipe outlet is attributed to the conversion of internal energy to kinetic energy. Finally, the Levenspiel chart reproduced in this work was compared with the Lapple chart used in API 521 Standatd.

• 한국수자원학회논문집
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• 제49권7호
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• pp.635-644
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• 2016

본 연구에서는 2차원 수치모형을 이용하여 개수로 분류부에서 분류수로 폭과 유량비 변화에 따른 흐름특성을 파악하였다. 2차류 영향을 고려한 분류부 수치모의시 흐름분포를 실험결과에 더 정확하고 안정하게 모의가능하다. 분류수로내 통수능을 감소시키는 흐름분리구역과 2차류의 상호 작용에 의한 흐름정체 효과는 분류유량비를 감소시킨다. 분류부 상류 유입유량과 유속이 감소할수록 수로폭 변화에 따른 분류유량비 변화가 더 크다. 동일 하류단 경계조건에서 분류수로 폭을 감소시킬 때, 본류 하류부 프루우드 수-분류유량비 관계식의 변화율은 -2.4843~-2.6675로 유사하게 나타난다. 동일 분류유량비 조건에서 분류수로 폭이 감소할수록 수축계수는 증가하고, 흐름분리구역의 폭은 감소한다. 분류수로 폭을 증가시킬 경우 분류부 상류 유입유량이 적을수록, 그리고 분류부 상류 유입량을 증가시킬 경우 분류수로 폭이 좁을수록 흐름분리구역 폭 감소율이 더 크다. 동일 상류 유입유량 조건에서 분류수로 폭이 감소할수록 분류유량비, 흐름분리구역의 길이와 폭은 감소한다.

• 대한토목학회논문집
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• 제31권6B호
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• pp.523-529
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• 2011

감조하천은 조석의 영향을 받는 하천으로, 하루에 두 번 수위를 상승 및 하강시킨다. 감조하천에서는 홍수시 하천유속 보다 비홍수기 조석에 의한 유속이 구조물 설계에 보다 지배적인 인자가 될 수 있다. 본 연구는 한강 하류부 감조구간에서 비홍수기에 발생된 유속 및 수위 관측을 실시하고 수치해석의 검증자료로 활용하였다. 흐름해석을 위해 부정류 해석이 가능한 UNET모형을 이용하였다. 신곡수중보 아래 감조구간의 조도계수 추정을 위해 통계적인 방법이 사용되었다. 통계적 방법으로 실측수위와 계산수위 간의 불일치율을 이용하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.465-471
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• 2000

The flow characteristics around an elliptic cylinder with axis ratio of AR=2 located near a flat plate were investigated experimentally to study the interaction between the cylinder wake and the turbulent boundary layer. The pressure distributions on the cylinder surface and on the flat plate were measured with varying the angle of attack of the cylinder. In addition, the velocity profiles of wake behind the cylinder were measured using a hot-wire anemometry As the angle of attack increases, the location of peak pressure on the windward and leeward surfaces of the cylinder moves toward the rear and front of the cylinder, respectively. At positive angles of attack, the position of the minimum pressure on the flat plate surface is moved downstream, but it is moved upstream at negative angles of attack. With increasing the angle of attack, the vortex shedding frequency is gradually decreased and the critical angel of attack exists in terms of the gap ratio. By installing the elliptic cylinder at negative angle of attack, the turbulent boundary layer over the flat plate is disturbed more than that at positive incidence. This may be attributed to the shift of separation point on the lower surface of the cylinder according to the direction of the angle of attack.

• 한국가시화정보학회지
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• 제7권2호
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• pp.64-71
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• 2010

The flow and noise characteristics of wake behind wind-turbine blades have been investigated experimentally using a two-frame particle image velocimetry (PIV) technique. Experiments were carried out in a POSTECH subsonic large wind-tunnel ($1.8^W{\times}1.5^H{\times}4.3^L\;m^3$) with KBP-750D (3-blade type) wind-turbine model at a freestream velocity of $U_o\;=\;15\;m/s$ and a tip speed ratio $\lambda\;=\;6.14$ (2933 rpm). The wind-turbine blades are connected to an AC servo motor, brake, encoder and torque meter to control the rotational speed and to extract a synchronization signal for PIV measurements. The wake flow was measured at four azimuth angles ($\phi\;=\;0^{\circ}$, $30^{\circ}$, $60^{\circ}$ and $90^{\circ}$) of the wind-turbine blade. The dominant flow structure of the wake is large-scale tip vortices. The turbulent statistics such as turbulent intensity are weakened as the flow goes downstream due to turbulent dissipation. The dominant peak frequency of the noise signal is identical to the rotation frequency of blades. The noise seems to be mainly induced by the tip vortices.

• 대한기계학회논문집B
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• 제24권7호
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• pp.976-983
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• 2000

The flow characteristics around an elliptic cylinder with axis ratio of AR=2 located near a flat plate were investigated experimentally to study the interaction between the cylinder wake and the turbulent boundary layer. The pressure distributions on the cylinder surface and on the flat plate were measured with varying the angle of attack of the cylinder. In addition, the velocity profiles of wake behind the cylinder were measured using a hot-wire anemometry As the angle of attack increases, the location of peak pressure on the windward and leeward surfaces of the cylinder moves toward the rear and front of the cylinder, respectively. At positive angles of attack, the position of the minimum pressure on the flat plate surface is moved downstream, but it is moved upstream at negative angles of attack. With increasing the angle of attack, the vortex shedding frequency is gradually decreased and the critical angle of attack exists in terms of the gap ratio. By installing the elliptic cylinder at negative angle of attack, the turbulent boundary layer over the flat plate is disturbed more than that at positive incidence. This may be attributed to the shift of separation point on the lower surface of the cylinder due to the presence of a ground plate nearby.

• Wind and Structures
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• 제29권1호
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• pp.43-53
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• 2019

Vortex-induced vibration of three circular cylinders (each of diameter D) in an equilateral triangular arrangement is investigated using the immersed boundary method. The cylinders, with one placed upstream and the other two side-by-side downstream, are free to vibrate in the cross-flow direction. The cylinder center-to-center spacing L is adopted as L/D = 2.0. Other parameters include the Reynolds number Re = 100, mass ratio $m^*=2.0$, reduced velocity $U_r=2{\sim}15$ and damping ratio ${\zeta}=0$. Cylinder vibration responses are dependent on $U_r$ and classified into five regimes, i.e. Regime I ($U_r{\leq}3.2$), Regime II ($3.2<U_r{\leq}5.0$), Regime III ($5.0<U_r{\leq}6.4$), Regime IV ($6.4<U_r{\leq}9.2$) and Regime V ($U_r>9.2$). Different facets of vibration amplitude, hydrodynamic forces, wake patterns and displacement spectra are extracted and presented in detail for each regime.