• 한국유체기계학회 논문집
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• 제12권2호
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• pp.8-16
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• 2009

The objective of this study is to investigate the three-dimensional turbulence flow characteristics of a rotor passage of an one-stage axial flow gas turbine and to investigate the effects of a boundary layer fence installed on the hub endwall of the rotor passage. Secondary flows occurring within the rotor passage (e.g. horseshoe vortex, passage vortex, and cross flow) cause secondary loss and reduce turbine efficiency. To control these secondary flows, a boundary layer fence measuring half the height of the thickness of the inlet boundary layer was installed on the hub endwall of the rotor passage. This study was performed numerically. The results show that the wake and secondary flows generated by the stator reduced the rotor load to constrain the development of cross flow and secondary flow reinforced by the rotor passage. In addition, the secondary vortices occurring within the rotor passage were reduced by the rotation of the rotor. Although, the boundary layer fence induced additional vortices, giving rise to an additional loss of turbine, its presence was shown to reduce the total pressure loss when compared to effects of the case without fence regardless of the relative position of blades by enervating secondary vortices occurred within the rotor passage.

• Wind and Structures
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• 제35권3호
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• pp.177-191
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• 2022

Trains emerging on a streamlined bridge-girder may have salient interference effects on the aerodynamic properties of the bridge. The present paper aims at investigating these interferences by wind tunnel measurements, covering surface pressure distributions, near wake profiles, and flow visualizations. Experimental results show that the above interferences can be categorized into two primary effects, i.e., an additional angle of attack (AoA) and an enhancement in flow separation. The additional AoA effect is demonstrated by the upward-moved stagnation point of the oncoming flow, the up-shifted global symmetrical axis of flow around the bridge-girder, and the clockwise-deflected orientation of flow approaching the bridge-girder. Due to this additional AoA effect, the two critical AoAs, where flow around the bridge-girder transits from trailing-edge vortex shedding (TEVS) to impinging leading-edge vortices (ILEV) and from ILEV to leading-edge vortex shedding (LEVS) of the bridge-girder are increased by 4° with respect to the same bridge-girder without trains. On the other hand, the underlying flow physics of the enhancement in flow separation is the large-scale vortices shedding from trains instead of TEVS, ILEV, and LEVS governed the upper half bridge-girder without trains in different ranges of AoA. Because of this enhancement, the mean lift and moment force coefficients, all the three fluctuating force coefficients (drag, lift, and moment), and the aerodynamic span-wise correlation of the bridge-girder are more significant than those without trains.

• 대한기계학회논문집B
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• 제40권6호
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• pp.373-381
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• 2016

본 연구는 고 레이놀즈 영역에서 후류측에 분리된 분할판(Detached splitter plate)을 가진 원주의 유동장 특성을 양 항력측정 실험과 PIV를 이용한 가시화 실험으로 파악한 것이다. 실험파라메터는 원주 한변의 길이에 대한 분할판의 폭비(H/B=0.5~1.5) 및 원주 후면에서부터 분할판의 앞전까지의 거리(G/B=0~2)로 했다. 분할판의 폭비를 고정시킨 경우 원주의 항력감소율은 간격비가 증가할수록 증가한 후 감소하는 특성을 보였다. 또한 같은 간격비에서는 분할판의 폭비가 클수록 원주의 항력감소율이 컸다. 분리된 분할판의 설치에 의해 분할판의 상, 하부에는 서로 반대방향의 볼테스가 발생되었고, 이 볼텍스가 원주 후류측에 역류를 발생시켜, 원주에 작용하는 항력을 감소시켰다.

• 대한조선학회논문집
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• 제43권3호
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• pp.322-330
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• 2006

Numerical calculations are carried out for flow past a circular cylinder forced oscillating normal to the free-stream flow at a fixed Reynolds number equal to 185. The cylinder oscillation frequency ranged from 0.8 to 1.2 of the natural vortex-shedding frequency, and the oscillation amplitude extended up to 20% of the cylinder diameter. IBM (Immersed Boundary Method) with direct momentum forcing was adopted to handle both of a stationary and an oscillating cylinder Present results such as time histories of drag and lift coefficients for both stationary and oscillating cases are in good agreement with previous numerical and experimental results. The instantaneous wake patterns of oscillating cylinder with different oscillating frequency ratios showed the synchronized wakes pattern in the lock-in region and vortex switching phenomenon at higher frequency ratio than the critical frequency ratio.

• 한국전산유체공학회지
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• 제21권2호
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• pp.81-89
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• 2016

Numerical simulations are carried out for the fluid flow and particle transport around two nearby circular cylinders in a side-by-side arrangement. The present study aims to understand the effects of the particle Stokes number and the spacing between two cylinders on particle dispersion and deposition characteristics. Simulations are based on an Eulerian-Lagrangian approach where the motion of particles is calculated by a Lagrangian approach based on one-way coupling. Results show that the flow structure is very different depending on the cylinder spacing, eventually affecting the overall pattern of particle dispersion significantly. It is also found that particles with smaller Stokes number tend to be distributed more uniformly in the wake of two cylinders, being located even inside the vortex cores. Meanwhile, particle deposition is analyzed in terms of the deposition efficiency and deposition location. The deposition efficiency of particles strongly depends on the Stokes number, whereas it is slightly affected by the cylinder spacing. The deposition location gets wider as the Stokes number increases, and it becomes asymmetric about the center of each cylinder as the cylinders get close.

• 한국전산유체공학회지
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• 제17권2호
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• pp.11-20
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• 2012

In this paper, the authors test various open codes and commercial codes based on CFD technology on the wind field around the complex terrain, which is a very important transport physics in the event of forrest fire. To study the physical mechanism inside the transition from surface fire to crown fire, the wake flow behind a parallel array of trees is studied numerically to show the flow separation in the turbulent boundary layer. Two sites near to Kunsan National University are chosen for the measurement of real wind field, and obtained data are compared with those from various computational codes such as Wind-Ninja, NIST-FDS, ANSYS-CFX, and ANSYS-FlUENT, etc. Through this research, feasibility and accuracy of the present CFD codes are investigated quantitatively, compared with the measured data with AWS.

• 한국연소학회지
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• 제3권1호
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• pp.71-78
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• 1998

The flame stabilization and the combustion characteristics of diffusion flame formed in the wake of a cylindrical bluff body with fuel injection are studied. With the turbulence generator, the flame stability limits and ion currents were measured and analyzed. The results from this experimental study are summarized as follows. The region with highest average value of ion currents in the middle of flame is moved to the upstream side by the turbulent components of main stream. The flame mass with partially active reaction is moved fast for uniform flow and turbulence generator G3, but the flame mass with relatively slow reaction is moved slowly for turbulence generator G1. If the turbulence generator with strong turbulent component is installed, the turbulent time scale is increased with movement from main stream side to recirculation zone as well as the flame stability limits is deteriorated. Though the special dominant frequency is not appeared in the eddy which exists in flame, high frequency characteristics are appeared in uniform flow and turbulence generator G3, and low frequency characteristics are appeared in uniform flow, turbulence generator G3 and G1.

• 한국항해항만학회지
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• 제27권5호
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• pp.473-478
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• 2003

평행한 벽 사이에 원형배관(circular pipeline)을 놓고 그 주위의 유동특성에 대한 수치연구를 수행하였다. 비압축성 유체를 가지고, Navier-Stokes 방정식을 풀었고 3차 풍상(upwind) 차분의 수치해법을 이용하였다. 한쪽 벽과의 거리가 매우 작아질 때, 볼텍스 떨어짐이 상당히 억압되는데 이것은 벽 경계와의 상호 박리 작용 때문으로 간수된다. 본 연구는 레이놀드 수의 변함과 물체가 벽에 접근함에 따른 볼텍스 떨어짐의 특성을 규명하는데 있다. 원형배관 후류와 평행벽내 유기된 박리의 상호작용을 집중적으로 다루며 서로 다른 조건에서 박리와 와역한(vorticity dynamics)의 특성을 해석하였다.

• 대한조선학회논문집
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• 제41권6호
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• pp.24-32
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• 2004

The effects of guide vanes and tip clearances on the characteristics nf axial flow fans are investigated both computationally and experimentally. Performance test of fans carried out in full scale shows considerable effects of tip clearance between rotor tip and duct on the characteristics of fans. The tested results are compared with the computation based on the finite volume method to solve the Navier-Stoke equations with $textsc{k}$-$\varepsilon$ turbulence model. The comparison shows good agreements between experimental and computational results. In addition, the effects of shape of guide vanes are numerically studied. The results show that increased volume of separated region around the guide vane reduces the recovery of tangential component of kinetic energy in the wake, resulting in loss of efficiency

• 대한기계학회논문집
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• 제19권5호
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• pp.1300-1307
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• 1995

The effect of thermal stratification on the turbulent dispersion from a fine cylindrical heat source was experimentally examined in a wind tunnel with and without a strong temperature gradient. A 0.5 mm dia. nichrome wire was used as a line heat source. Turbulent intensities, r.m.s. value of temperature and convective heat fluxes were measured by using a hot-wire and cold-wire combination probe. The results show that the peack value and the spread of the vertical turbulent intensity for the stratified case are far lower than those in the neutral case, which indicates that the stable temperature gradient suppresses the vertical velocity component. All of the third order moments including heat fluxes measured in the stable condition have very small values than those of the neutral case. This nature suggests that the decrease of scalar fluctuations in the stably stratified flow is mainly due to the suppression ofthe turbulent diffusion processes by the stable stratification. A simple gradient model with a composite timescale which has a simple weighted algebraic mean between dynamic and thermal time scale yields reasonably good numerical values in comparison with the experimental data.