• Proceedings of KOSOMES biannual meeting
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• 2007.11a
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• pp.177-183
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• 2007

Flow characteristics of the cylinder wake controlled with a small control cylinder were experimentally investigated by the PN (Particle Image Velocimetry) technique. Flow visualization of the flow around a circular cylinder was conducted in the Circulating Water Channel. The control cylinder having diameter of d=5mm, 10mm and 20mm was installed behind a circular cylinder of D=50mm. And the Reynolds number were $Re=4.9{\times}10^3,\;Re=9.9{times}10^3$ and $Re=1.9{\times}10^4$. In this study, the frequency characteristics of the controlled wake were analyzed by using spectral analysis of the measured wake velocity signals. As a result, the controlled wake had smaller vortex shedding frequency than that of circular cylinder wake by the effect of the control cylinder. Governing parameters of the flow control were d/D, and Reynolds number and they largely influenced the frequency characteristics of the cylinder wake. And vortex shedding frequency appeared most lowly at d=0.2D

• Journal of Advanced Marine Engineering and Technology
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• v.35 no.7
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• pp.957-965
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• 2011

The wakes of a cylindrical body have been investigated. The cylindrical body was attached with a thin film. The film is made of silicon with configurations of 50mm(W) ${\times}$ 150mm(L) ${\times}$ 0.3mm(T). The cylinder wakes have been measured with PIV experiments under the conditions with and without the thin film. The diameter of the installed cylinder body is 30mm and the Reynolds numbers are 2730, 6160 and 9750 with the diameter. The measurement system consists of an Ar-ion laser(6W), a high speed camera(1024 ${\times}$ 992 pixel, 500fps) and a host computer. FFT analyses have been carried out using the velocity vectors obtained by PIV measurements at the point X/D=1.52 and Z/D=0.52. For understanding the three-dimensional flow structures, a new Volumetric PTV(particle tracking velocimetry) has been constructed, in which the same four high-resolution cameras have been used. It has been verified that the flexible film suppresses or damps the vortices separated from the cylinder body, which makes the cylinder's wakes stable. With increase of Re numbers the intensity of the dominant frequency of the wakes become smaller.

• Wind and Structures
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• v.26 no.6
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• pp.397-413
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• 2018

Excitation mechanism of interference effect between two tall buildings is investigated with wind tunnel experiments. Synchronized building surface pressure and flow field measurements by particle image velocimetry (PIV) are conducted to explore the relationship between the disturbed wind flow field and the consequent wind load modification for twin buildings in tandem. This reveals evident excitation mechanisms for the fluctuating across-wind loads on the buildings. For small distance (X/D < 3) between two buildings, the disturbed flow pattern of impaired vortex shedding is observed and the fluctuating across-wind load on the downstream building decreases. For larger distance ($X/D{\geq}3$), strong correlation between the across-wind load of the downstream building and the oscillation of the wake of the upstream building is found. By further analysis with conditional sampling and phase-averaged techniques, the coherent flow structures in the building gap are clearly observed and the wake oscillation of the upstream building is confirmed to be the reason of the magnified across-wind force on the downstream building. For efficient PIV measurement, the experiments use a square-section high-rise building model with geometry scale smaller than the usual value. Interference factors for all three components of wind loads on the building models being surrounded by another identical building with various configurations are measured and compared with those from previous studies made at large geometry scale. The results support that for interference effect between buildings with sharp corners, the length scale effect plays a minor role provided that the minimum Reynolds number requirement is met.

• Journal of the Korean Society for Marine Environment & Energy
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• v.9 no.2
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• pp.79-84
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• 2006

The present study deals with the flow over a flat plate with repeated roughness elements of 2-dimensional rectangular shape, which can be applied into the study on the natural geographical roughness and the turbulent flow on roughened solid surface. Experiment was performed using PIV technique in the circulating water channel. Results showed that the flow over roughness elements was characterized by the high shear flow emanating from top of roughness element and the recirculating region formed at the trough of two roughness elements. In general, the ratio between the spacing and the height of roughness elements plays a crucial role in developing the flow pattern near wall surface.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.10 s.241
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• pp.1092-1100
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• 2005

In the present study, characteristics of steady state laminar flows of a straight duct connected to a 180$^{o}$ curved duct were examined in the entrance region through experimental and numerical analyses. For the analysis, the governing equations of laminar flows in the Cartesian coordinate system were applied. Flow characteristics such as velocity profiles and secondary flows were investigated numerically and experimentally in a square cross-sectional straight duct by the PIV system and a CFD code(STAR CD). For the PIV measurement, smoke particles produced from mosquito coils. The experimental data were obtained at 9 points dividing the test sections by 400 3m. Experimental and numerical results can be summarized as follows. 1) Reynolds number, Re was increased, dimensionless velocity profiles at the outer wall were increased due to the effect of the centrifugal force and secondary flows. 2) The intensity of a secondary flow became stronger at the inner wall rather than the outer wall regardless of Reynolds number. Especially, fluid dynamic phenomenon called conner impact were observed at dimensionless axial position, x/D$_{h}$=50.

• Journal of Navigation and Port Research
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• v.34 no.5
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• pp.331-336
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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 channel 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 $Re=1.0{\times}10^3$ 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 Korea Water Resources Association Conference
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• 2022.05a
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• pp.281-281
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• 2022

바닥에서 생성되는 난류는 순간적으로 강한 모멘텀을 바닥에 전달함과 동시에 바닥에 있는 입자를 움직이게 한다. 경계층 내 난류 운동에 대한 분석은 다양한 유사 이송 문제를 이해하기 위해 필수적이며 이에 따라 많은 선행 연구들은 실험실 실험을 통해 해당 연구를 수행하였다. 본 연구에서는 선행 연구에서 사용하지 못했던 진보된 실험 방법을 활용하여 바닥 경계층 내의 난류 운동에 대해 확인하고 해당 운동에 의해 관성 입자의 움직임이 어떻게 발생하는지에 대하여 물리적으로 설명하고자 한다. 다양한 흐름 조건에서 3가지의 입경 크기를 가지는 모래 입자를 가지고 실험을 수행하였으며, 실험 조건별 고해상도 유속장 및 관성 입자의 움직임은 3차원 입자 영상 유속계 (Particle Image Velocimetry; 이하 PIV)와 입자 추적 유속계 (Particle Tracking Velocimetry; 이하 PTV)를 동시에 적용하여 파악하였다. 취득된 3차원 유속장과 입자 궤적을 기반으로 실험 조건별 흐름 및 입자 거동 특성에 대해 분석하였으며, 관성 입자의 움직임을 발생시키는 3차원 난류 운동은 측정된 유속장에서 산정한 Q-criterion 값을 기반으로 도식화하였다. 측정값 내에는 난류 운동에 대한 정보와 더불어 잡음이 포함되어 있으므로 이를 제거하고자 적합 직교 분해 (Proper Orthogonal Decomposition; 이하 POD) 방법을 적용하였다. 그리고 POD로 추출한 유속장을 통해 바닥면 부근에 존재하는 헤어핀 와류 운동 혹은 와류 묶음과 같은 난류 고유 구조를 파악하였다. 해당 와류 운동들의 3차원 난류 특성을 확인하고자 비등방성 불변 지도(anisotropy invariant map)를 활용하였으며 경계층 내부에서 난류의 형태가 흐름 방향으로 늘어진 럭비공 형태임을 확인하였다. 마지막으로, 입자의 움직임을 발생시키는 난류 이벤트를 결정하고자 사방구 분석 (Quadrant analysis) 기법을 적용하였으며 흐름 조건별로 입자를 움직이게 하는 난류 이벤트는 달라짐을 확인하였다.

• Journal of Power System Engineering
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• v.4 no.2
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• pp.11-16
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• 2000

Simultaneous quantitative measurements are made of the velocity fields for two-dimensional natural convection in a rectangular enclosure using PIV(Particle Image Velocimetry). The experiments are performed at a Prandtl number of 6.62, an aspect ratio of 1.0, Rayleigh numbers from $1.294{\times}10^6\;to\;3.8841{\times}10^6$, and angles of inclination of $0^{\circ},\;30^{\circ}\;and\;60^{\circ}$ inside a $30mm{\times}30mm{\times}8mm$ cavity made of an acrylic glass 10mm, with two isothermal copper walls kept at a prescribed temperature. The experimental results agreed very well with the numerical results. It was found that the flow consisted of a large double convection cell at angle of inclination of $60^{\circ}$.

• Journal of Advanced Marine Engineering and Technology
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• v.28 no.8
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• pp.1185-1194
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• 2004

Virtual images for PIV are produced for the construction of a benchmark test tool of PTV systems, Camera parameters obtained by an actual experiment are used to construct the virtual images, LES(Large Eddy Simulation) data sets of a channel flow are used for generation of the virtual images, Using the virtual images and the camera's parameters. three-dimensional velocity vectors are obtained for a channel flow. The capabilities of a 3D-PTV algorithm are investigated by comparing the results obtained by the virtual images and those by an actual measurement for the channel flow.

• 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.