• 동력기계공학회지
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• 제20권6호
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• pp.64-70
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• 2016

The unsteady-state, incompressible and three-dimensional large-eddy simulation(LES) was carried out to evaluate the vorticity distribution of a small-size axial fan(SSAF). The X-component vorticity profiles developed around blade tips turn from axial to radial, and diminish the density of distribution according to the increase of static pressure. Otherwise, the Z-component vorticity profiles evenly develop at the region larger than the half radial distance of blade at the operating points of A and B, partly at the trailing-edge region of blade and radially over bellmouth according to the increase of static pressure.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2009년 추계학술대회논문집
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• pp.44-48
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• 2009

In this study, a vortex visualization method based on the vorticity magnitude is developed. One of the simplest models for a vortex is a vortex filament with the maximum vorticity on its center. The proposed method is based on the observation of this ideal distribution of vorticity magnitude. Laplacian and Hessian matrix of vorticity magnitude are tested for detecting the local maximum of vorticity magnitude. These ideas were applied to wake flow past a sphere. It was found that the Laplacian method is not able to distinguish vortices from the underlying shear layer clearly, while the Hessian matrix method does not suffer from this problem.

• 한국가스학회지
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• 제23권3호
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• pp.27-32
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• 2019

본 연구에서는 단일 원관에서 입구의 유속이 일정한 경우와 주기적인 변동이 있는 경우에 대하여 시간에 따라 원관 주위에서 와류 분포와 온도 분포 변화를 비교 분석하였다. 또한, 양력과 항력의 시간 변화와 PSD(power spectral density)를 분석하여 유동의 주파수 특성을 규명하였다. 일정 입구 유속의 경우는 잘 알려진 칼만 와류 분포를 보여 주었으며 원관의 상하에서 교대로 와류가 발생하는 것을 관찰할 수 있었다. 주기적인 입구 유속 변동의 경우는 원관의 상하에서 동시에 와류가 발생하는 것을 관찰할 수 있었다. 두 가지 모두 시간에 따른 온도 분포 변화는 와류 분포 변화와 거의 유사하게 거동하는 것을 확인할 수 있었다. 일정 입구 유속의 경우의 와류 유동 주파수는 31.15 Hz 이며 주기적인 입구 유속의 경우는 와류 유동 주파수는 입구 유속의 주파수와 동일하게 15.57 Hz으로 나타났다. 원관 표면 평균 누셀수는 일정 입구 유속의 경우는 99.6이며 주기적인 입구 유속 변동의 경우는 110.7로 나타나서 주기적인 입구 속도 변동의 경우가 열전달이 11.1% 증가하는 것을 알 수 있었다.

• 해양환경안전학회:학술대회논문집
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• 해양환경안전학회 2004년도 춘계학술발표회
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• pp.89-92
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• 2004

포텐셜와도는 해류의 순환을 설명하는데 유용하게 활용된다. 동해에서 포텐셜와도의 분포 특성을 이용하여 쓰시마난류를 포함한 상층부의 해류순환을 설명하였다. 상층부를 표층부분과 쓰시마난류의 분포 층 그리고 수온약층 분포 수층으로 구분하였다. 극전선남쪽은 쓰시마난류가 해류순환의 중심을 형성하며, 포텐셜와도는 쓰시마난류와 극전선의 분포 특성을 잘 나타내어준다. 포텐셜와도의 분포로부터, 극전선 북쪽은 일본 분지와 그 서쪽해역의 순환 세포로 구분된다.

• 대한조선학회논문집
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• 제28권2호
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• pp.69-76
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• 1991

포텐셜 유동에서 규정된 속도 분포에 대한 2차원 날개 단면을 설계하는 방법을 제시하며 날개 표면에는 보오텍스를 분포시켰다. 보오텍스 분포를 결정하는데 사용한 경계조건은 날개 표면에서 유체유동이 유선이라는 것이다. 최종 2차원 요구 단면을 얻기위하여 반복 계산법을 채택하였고 개발된 program으로 설계한 몇가지 수치해를 나타내었다.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2004년도 Proceedings of 2004 Korea-Japan Joint Seminar on Particle Image Velocimetry
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• pp.1-13
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• 2004

A scanning stereo-PIV system was developed to measure the three-dimensional distribution of three-component velocity in a turbulent round jet. A laser light beam produced by a high repetition rate YLF pulse laser was expanded vertically by a cylindrical lens to form a laser light sheet. The light sheet is scanned in a direction normal to the sheet by a flat mirror mounted on an optical scanner, which is controlled by a programmable scanner controller. Two high-speed mega-pixel resolution C-MOS cameras captured the particle images illuminated by the light sheet, and stereoscopic PIV method was adopted to acquire the 3D-3C-velocity distribution of turbulent round jet in an octagonal tank filled with water. The jet Reynolds number was set at Re=1000 and the streamwise location of the measurement was fixed at approximately x = 40D. Time evolution of three-dimensional vortical structure, which is identified by vorticity, is visualized. It revealed that the existence of a group of hairpin-like vortex structures was quite evident around the rim of the shear layer of the jet. Turbulence statistics shows good agreement with the previous data, and divergence of a filtered (unfiltered) velocity vector field was $7\%\;(22\%)$ of root-me an-squared vorticity value.

• Journal of Advanced Marine Engineering and Technology
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• 제26권5호
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• pp.573-580
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• 2002

Animation and time-resolved analysis of the wake characteristics of 2-D bluff body flows were examinated by applying the multi-vision PIV to square cylinders(three angles of attack: $0^{circ}, 30^{circ} and 45^{\circ}$) and circular cylinders(three rotating speeds: 0rpm, 76rpm, 153rpm) submerged within a circulating water channel $(Re=10^4)$, The macroscopic shedding patterns and their dominant frequencies were discussed in terms of instantaneous velocity, vorticity and turbulent quantities such as turbulent intensity, turbulent kinetic energy and three Reynolds stresses. Particularly the time-averaged distribution of turbulent intensity 'islands' where their peak magnitudes were focused always small regions behind the bodies without noticeable spatial migration were particularly discovered in all cases. And the dominant frequencies of the turbulent quantities in the wake regions were two times larger than those of the velocity and vorticity.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2004년도 Proceedings of 2004 Korea-Japan Joint Seminar on Particle Image Velocimetry
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• pp.41-45
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• 2004

The dynamic particle image velocimetry (PIV) is consisted of a high frequency pulse laser, high speed cameras and a timing controller. The three velocity components of flow downstream of an axial flow fan for PC cooling system are measured using the dynamic PIV system. An Axial flow fan has seven blades of 72 mm in diameter. The rotating speed is 1800 rpm. The downstream flow is visualized by smoke particles of about $0.3-1\;{\mu}m$ in diameter. The three-dimensional instantaneous velocity fields are measured at three downstream planes. The swirl velocity component was diffused downstream and the change in time-mean vorticity distribution downstream was also discussed. The spatio-temporal change in axial velocity component with the blades passing is recognized by the instantaneous vector maps. And the dynamic behavior of vorticity moving with the rotating blades is discussed using the unsteady vorticity maps.

• 한국산업융합학회 논문집
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• 제25권4_2호
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• pp.637-644
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• 2022

In this study, the flow characteristics according to the shape of the vortex nozzle was studied by numerical analysis and the amount of microbubble generation was measured experimentally. The shape of the vortex nozzle is cylindrical, diffuser, and conical type. The axial fluid velocity in the induced tube gradually increased from the inlet to the outlet. In particular, the fluid velocity in the nozzle part increased rapidly. The velocity distribution of the fluid at the inlet of the induced tube showed that the flow rotates counterclockwise in the outer region and the inner center of the induced tube. At the outlet of the induced tube, the cylindrical and conical type showed rotational flow, and the diffuser type showed irregular turbulent flow. The dimensionless pressure ratio 𝜂 of the inner region of the induced tube was lower than that of the outer region. Also, 𝜂 near the outlet of the induced tube in cylindrical and conical type showed a similar tendency to the inlet area. At the outer region of inlet of induced tube, intense vorticity was observed on the wall and in lower region. At the inner region of inlet of induced tube, intense vorticity was observed on the inner wall of the induced tube and in the central region of the inlet of the induced tube. At the outlet of induced tube, in the case of the cylindrical and conical type, intense vorticity was observed near the inner wall, the diffuser type showed irregular strong vorticity inside the tube. The total number of bubbles measured was the most in the cylindrical type, and the microbubbles less than 50mm occurred the most in the conical type.

• 한국해양공학회지
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• 제36권5호
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• pp.326-339
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• 2022

Recently, not only Korea but also the world has been suffering from problems related to coastal erosion. The hard defense method has been primarily used as a countermeasure against erosion. However, this method is expensive and has environmental implications. Hence, interest in other alternative methods, such as the eco-friendly vegetation method, is increasing. In this study, we aim to analyze the hydraulic characteristic of submerged rigid vegetation according to the cross-sectional change through a hydraulic experiment and numerical simulation. From the hydraulic experiment, the reflection coefficient, transmission coefficient, and energy dissipation coefficient were analyzed according to the density, width, and multi-row arrangement of the vegetation zone. From numerical simulations, the flow field, vorticity distribution, turbulence distribution, and wave distribution around the vegetation zone were analyzed according to the crest depth, width, density, and multi-row arrangement distance of the vegetation zone. The hydraulic experiment results suggest that the transmission coefficient decreased as the density and width of the vegetation zone increased, and the multi-row arrangement condition did not affect the hydraulic characteristics significantly. Moreover, the numerical simulations showed that as the crest depth decreased, the width and density of vegetation increased along with vorticity and turbulence intensity, resulting in increased wave height attenuation performance. Additionally, there was no significant difference in vorticity, turbulence intensity, and wave height attenuation performance based on the multi-row arrangement distance. Overall, in the case of submerged rigid vegetation, the wave energy attenuation performance increased as the density and width of the vegetation zone increased and crest depth decreased. However, the multi-row arrangement condition did not affect the wave energy attenuation performance significantly.