• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.7
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• pp.595-601
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• 2001

We applied PIV method to obtain instantaneous and ensemble averaged velocity fields from the first row to the fifth row of a staggered tube bundle. The Reynolds number based on the tube diameter and the maximum velocity was set to be 4,000. Remarkably different natures are observed in the developing bundle flow. Such differences are depicted in the mean recirculating bubble length and the vorticity distributions. The jet-like flow seems to be a dominant feature after the second row and usually skew. However, the ensemble averaged fields show symmetric profiles and the flow characteristics between the third and fourth measuring planes are not so different. comparison between the PIV data and the RANS simulation yields severe disagreement in spite of the same Reynolds number. It can be explained that the distinct jet-like unsteady motions are not to be accounted in th steady numerical analysis.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2010.04a
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• pp.389-391
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• 2010

The Free surface influenced the wake behind a circular cylinder and its effects were investigated experimentally in a circulating water channe with the variation of water depth Instantaneous velocity fields were measured in this paper. the measured results has been compared with each other to investigate the flow characteristics of the circular cylinder's 2-dimensional section at $Re=1.0\times10^3$ using 2-frame grey level cross correlation PIV method. The flow around the circular cylinder with free surface affected the wake structure. especially, in case of d=1.0D, the boundary layer was measured in the whole area. The separation point and boundary layer of the circular cylinder could be controlled by the water depth.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.11a
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• pp.206-212
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• 2011

Hysteresis phenomena in fluid flow systems are frequently encountered in many industrial and engineering applications and mainly appear during the transient processes of change of the pressure ratio. Shock-containing flow field in supersonic nozzles is typically subject to such hysteresis phenomena, but associated flow physics is not yet understood well. In the present study, experimental work has been carried out to investigate supersonic nozzle flows during the transient processes of change in the nozzle pressure ratio. Time-dependent surface wall pressures were measured by a multiple of pressure transducers and the flow field was visualized using a nano-spark Schlieren optical method. The results obtained show that the hysteresis phenomenon is strongly dependent on the nozzle geometry as well as the time scale of the change of pressure ratio.

• Proceedings of the Korea Water Resources Association Conference
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• 2021.06a
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• pp.24-24
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• 2021

자연하천 바닥 경계층 내에서는 복잡한 난류 구조가 형성되며 이들은 하상에 강한 모멘텀을 전달한다. 바닥 부근에 분포하는 유사 입자들은 경계층 내에서 발생한 난류 흐름으로부터 모멘텀을 전달받아 소류사 혹은 부유사 형태로 이송되게 되며, 이러한 유사 이송 과정을 역학적으로 설명하기 위해서는 경계층 내 유체 흐름에 대한 이해가 선행되어야한다. 경계층 내 난류 흐름 특성이 유사 입자의 움직임에 미치는 영향에 대해 분석하기 위해서는 바닥 경계층 내 고해상도 유속 자료와 유사 움직임을 동시에 포착할 수 있는 기술이 요구된다. 하지만 현재까지 수행된 대부분의 선행 연구들은 점 유속을 측정할 수 있는 음파 도플러 유속계 (Acoustic Doppler Velocimetry) 혹은 2차원 입자 영상 유속계를 활용하였으며, 이들은 복잡한 3차원 난류 흐름 특성을 분석하기에는 한계가 있다. 본 연구의 목적은 실험실 실험을 통해 바닥 경계층 내 3차원 난류 흐름이 유사 이송에 미치는 영향에 대해 조사하는 것이다. 본 연구에서는 유사 주변에서의 고해상도 3차원 흐름 유동장 및 순간적인 유사 움직임에 대해서는 합성 개구 (synthetic aperture) 기반의 3차원 입자 영상 유속계 및 입자 추적 유속계를 활용하여 취득하였다. 취득된 흐름 유동장을 기반으로 레이놀즈 전단응력을 산정하였으며 이를 통해 유체가 하상에 미치는 모멘텀의 크기를 파악하였다. 복잡한 난류 흐름 구조에 대해서는 팔분원 분석 (octant analysis)을 통해 구분했으며, 유사가 움직이는 순간의 유속장을 기반으로 유사 이송을 발생시키는 지배적인 난류 흐름 특성에 대해 규명하였다. 본 연구는 바닥 경계층 내 복잡한 3차원 난류 흐름과 유사 입자의 움직임을 동시에 분석함으로써 기존에 수행되어왔던 선행 연구들의 한계점을 극복하고 보다 명확한 유사 이송의 발생 원인에 대해 분석했다는 점에 의의가 있다.

• Journal of the Society of Naval Architects of Korea
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• v.37 no.3
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• pp.37-44
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• 2000

An experimental study has been carried out at circulating water channel to investigate the viscous flow around breaking waves generated by a submerged hydrofoil(NACA0012). Detailed flow measurements were made at several critical points including an incipient wave-breaking point and a fully-developed wave breaker. Particle Image Velocimetry(PIV) was employed to visualize the flow field very close to the breaker as well as at the near- and far-wake of the breaker. Generation, development and decay of the wave breaker have been investigated. It is found that PIV technique could be well applied to the complex flow field, including the vortical structures near the free surface as well as the wake of the hydrofoil.

• Solar Energy
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• v.18 no.2
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• pp.115-121
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• 1998

In this study a channel cavity flow was performed. The channel has small size in the upper cavity region. At the gap is supplied by driven flow for Reynolds number. The experimental study was carried out in a channel cavity with square heat surface by visualization equipment with Mach-Zehnder interferometer and laser apparatus. Heat source was uniform heat flux($0.4W/cm^2,\;0.8W/cm^2,\;1.2W/cm^2$). When the bottom wall is heated, the tendency of natural convection flows are vigorous increasing heat flux.

• Journal of Advanced Marine Engineering and Technology
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• v.34 no.1
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• pp.76-83
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• 2010

The Characteristics of the flow field through cylinders with in-line and staggered arrangements were calculated by vortex method. Vortex distributions and velocity profiles around the cylinders with in-line and staggered arrangements were simulated at the pitch ratio of Pt/D=1.25~2.0 and Reynolds number of Re=$4.0{\times}10^1{\sim}4.0{\times}10^4$. As the results the vortices of clockwise at the upper separation point cylinder and the vortices of anticlockwise at the lower separation point of each cylinder were generated at both in-line and staggered arrangements. The generation of the reverse flow in the rear region of the cylinders was caused by the pitch ratio and Reynolds number, the boundary region was at the pitch ratio of Pt/D=1.5 and Reynolds number of Re=$4.0{\times}10^2{\sim}4.0{\times}10^3$ in case of in-line arrangement and was at the pitch ratio of Pt/D=1.4 and Reynolds number of Re=$4.0{\times}10^1{\sim}4.0{\times}10^2$ in case of staggered arrangement.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.8
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• pp.829-837
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• 2012

Direct numerical simulations were carried out for turbulent channel flows with $Re_{\tau}$ = 180, 395 and 590 to investigate the turbulent flow structure related to the Reynolds shear stress. By examining the probability density function, the second quadrant (Q2) events with the largest contribution to the mean Reynolds shear stress were identified. The change in the inclination angle of Q2 events varies with wall units in $y^+<50$ and with the channel half height in y/h > 0.5. Conditionally averaged flow fields for the Q2 event show that the flow structures associated with Reynolds shear stress are a quasi-streamwise vortex in the buffer layer and a hairpin-shaped vortex in the outer layer. Three-dimensional visualization of the distribution of high Reynolds shear stress reveals that the organization of hairpin vortices in the outer layer having a size of 1.5~3 h is associated with large-scale motions with high Reynolds shear stress in the outer layer.

• Proceedings of the Korean Society for Rock Mechanics Conference
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• 1994.03a
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• pp.38-51
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• 1994

지하원유비축 기지 저장공동의 원유 유출이나 기화누설을 방지하기 위하여 지하수압을 조절하는 수벽공의 운영이나 공동주위 암반의 그라우팅 공법 설계에 있어서는 공동의 굴착으로 인한 주위 암반의 변형에 따른 투수계수의 변화와 지하수위의 변화에 대한 해석이 대단히 중요하다. 본 연구에서는 순간증압법을 이용한 삼축압축하의 암석의 투수계수 측정을 통하여 변형율과 투수계수와의 관계함수식을 구하였다. (중략)

• Korean Journal of Food Science and Technology
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• v.32 no.6
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• pp.1313-1318
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• 2000

총고형분 함량$(TS:\;50{\sim}57%)$을 달리하여 숙성시킨 고추장의 유동특성, 정적 및 동적점탄특성을 관찰하였다. 고추장의 TS가 증가함에 따라서 점조도 지수(K)와 겉보기 점도$({\eta}_{a.5})$는 증가하였으며 유동성 지수(n)는 1보다 훨씬 낮은 0.24-0.31의 범위를 나타내어 고추장은 shear-thinning 성질을 보여주었다. 고추장의 항복응력 측정에서는 고추장의 TS가 증가함에 따라 Casson 항복응력$({\sigma}_{oc})$은 증가하는 경향을 나타내었으며, 또한 고추장의 액상 매개체인 serum 점도$({\eta}_{se})$는 고추장의 TS와 매우 좋은 상관관계$(R^2=0.97)$를 나타내었다. 고추장의 동적점탄성 실험에서는 주파수$({\omega})$가 증가함에 따라 저장탄성률(G#)와 손실탄성률(G@)은 증가하였으며 양(+) 기울기로 G#수치가 G@수치보다 높게 나타났다. 이는 고추장이 약한 겔과 같은 구조적 특성을 갖고 있으며 또한 shear-thinning 거동을 가지고 있음을 나타낸다. 정적점탄성 실험에서 고추장의 총고형분 함량에 따라서 순간탄성 변형$(J_0)$과 지연탄성변형$(J_1)$은 총고형분 함량이 증가함에 따라서 감소하였으며 점도$({\eta}_N)$와 지연시간 $({\tau}_1)$은 증가하였다. 따라서 고추장은 농도가 증가함에 따라 보다 탄력적이며 안정된 구조를 형성하고 있음을 알 수 있었다.