• International Journal of Aeronautical and Space Sciences
• /
• 제16권3호
• /
• pp.339-346
• /
• 2015

Aerodynamic flow control phenomena were investigated with a low-current DC surface discharge plasma actuator. The plasma actuator was found to operate in three different discharge modes with similar discharge currents of about 1 mA or less. Stable continuous DC discharge without audible noise was obtained at higher ballast resistances and lower discharge currents. However, even with continuous DC power input, a low-frequency self-pulsed discharge was obtained at lower ballast resistances, and a high-frequency self-pulsed discharge was obtained at higher set-point currents and higher ballast resistances, both with audible noise. The Schlieren image reveals that the low-frequency self-pulsed mode produces a synthetic jet-like flow implying that a gas heating effect plays a role, even though the discharge current is small. The high-frequency self-pulsed mode produces pulsed jets in a tangent direction, and the continuous DC mode produces a steady straight pressure wave. Particle image velocimetry (PIV) images reveal that the induced flow field by the low-frequency self-pulsed mode has flow propagating in the radial direction and centered between the electrodes. The high-frequency self-pulsed mode and continuous DC mode produce flow from the anode to the cathode. The perturbed region downstream of the cathode is larger in the high-frequency self-pulsed mode with similar maximum speeds.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• 제29권11호
• /
• pp.1182-1188
• /
• 2005

An x-ray PIV (Particle Image Velocimetry) technique was developed to measure quantitative information on flows inside opaque conduits and on opaque-fluid flows. At first, the developed x-ray PIV technique was applied to flow in an opaque Teflon tube. To acquire x-ray images suitable for PIV velocity field measurements, refraction-based edge enhancement mechanism was employed using detectable tracer particles. The optimal distance between with the sample and detector was experimentally determined. The resulting amassed velocity field data were in reasonable agreement with the theoretical prediction. The x-ray PIV technique was also applied to blood flow in a microchannel. The flow pattern of blood was visualifed by enhancing the diffraction/interference -bas ed characteristic s of blood cells on synchrotron x-rays without any contrast agent or tracer particles. That is, the flow-pattern image of blood was achieved by optimizing the sample (blood) to detector distance and the sample thickness. Quantitative velocity field information was obtained by applying PIV algorithm to the enhanced x-ray flow images. The measured velocity field data show a typical flow structure of flow in a macro-scale channel.

• Proceedings of the KOSOMBE Conference
• /
• 대한의용생체공학회 1993년도 추계학술대회
• /
• pp.81-84
• /
• 1993

Artificial Heart Valve is the one of the most important artificial organ which has been implanted to many patients. The most important problems related to the artificial heart valve prosthesis are thrombosis and hemolysis. Usual method to test against this problem in vivo experiment, which is complex and hard work. Nowadays the request for In vitro Artificial Heart Valve testing system is increasing. Several papers has announced us flow pattern of Artificial Heart Valve is highly correlated with thrombosis and hemolysis. They usually gel flow pattern by LDA, it is also hard work and has narrow measuring region. In this reason we have determined to develop PTV(Particle Tracking Velocimetry). By using High-speed camera and image processing technique, flow pattern could be relatively easily obtained. Parachute and Bileaflet Artificial Heart Valve designed by SNU were testified.

• Journal of the Korean Society of Visualization
• /
• 제17권2호
• /
• pp.48-57
• /
• 2019

A straightforward strategy for particle image velocimetry (PIV) interrogation and post-processing has been proposed, aiming at reducing errors and clarifying vortex structures. The interrogation window size should be kept small to reduce bias error and improve spatial resolution. A spatial filter is then applied to the velocity field to reduce random error and clarify flow structure. The performance of three popular spatial filters were assessed: box filter, median filter, and local quadratic polynomial regression filter. In order to quantify random uncertainty, the image matching (IM) method is applied to an experimental dataset of homogeneous and isotropic turbulence (HIT) obtained by 2D-PIV. We statistically analyze the uncertainty propagation through the spatial filters, and verify the reduction in random uncertainty. Moreover, we illustrate that the spatial filters help clarify vortex structures using vortex identification criteria. As a result, PIV random uncertainty was reduced and the vortex structures became clearer by spatial filtering.

• Korean Chemical Engineering Research
• /
• 제47권5호
• /
• pp.572-579
• /
• 2009

This study is to investigate experimentally the flame structure and propagation mechanism in dust explosions and to provide the fundamental knowledge. Upward propagating laminar dust flames in a vertical duct of 1.8 m height and 0.15 m square cross-section are observed and flame front is visualized using by a high-speed video camera. Also, the thicknesses of preheated and reaction zone have been determined by a schlieren, electrostatic probe and thermocouple. The thickness of preheated zone in lycopodium dust flame is observed to be 4~13 mm, about several orders of magnitude higher than that of premixed gaseous flames. From the experimental results by a PIV(Particle Image Velocimetry) system, a certain residence time of the unburned particle in preheated zone is needed to generate combustible gas from the particle. The residence time will depend on preheated zone thickness, particle velocity and flame propagation velocity.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• 제29권3호
• /
• pp.139-146
• /
• 2017

In order to understand the efficacy of submerged breakwater constructed for the beach protection, laboratory experiments were carried out by observing the characteristics of flow around a single shore-parallel submerged breakwater. The velocity field near the shoreline was measured by utilizing the LSPIV (Large-Scale Particle Image Velocimetry) technique, and mean surface and wave height distributions were observed around the submerged breakwater, according to various combinations of incident waves and submerged breakwaters. In this experiment, it was found that the mean flow pattern behind the submerged breakwater was determined by the balance among the gradients of mean water surface and excess wave-momentum flux (i.e., radiation stress tensors) which interact with the wave-induced current developed by the gradients on the rear and the side of the submerged breakwater. The divergent and convergent flow patterns behind the submerged breakwater (i.e., accretion and erosion response) of the numerical study of Ranasinghe et al.(2010) were observed in the measured velocity distributions, and their empirical formula mostly agreed with the experimental results. However, for some cases in this experiment, it was difficult to say that the flow pattern was one of them and was agreed with the empirical formula.

• Journal of Korea Water Resources Association
• /
• 제41권7호
• /
• pp.653-667
• /
• 2008

The hydraulic model test for the L-type Groynes with "ㄱ" shape were conducted to analyze flow characteristics around these groynes. The results of model tests should be used for the fundamental information to design the L-type Groyne constructed in the field. Main hydraulic factors such as the velocity and thalweg line changes in main channel and separation area were analyzed in this study. The thalweg line is stream line where the maximum velocity occurs, and the separation area is a boundary of main flow and recirculation zone. Model tests with 5 different arm-lengths of the L-type Groynes were conducted changing the velocity. The LSPIV(Large Scale Particle Image Velocimetry) technique was used to measure and analyze the flow variation around the L-type Groynes. The velocity in main channel was increased 1.5 times and there was no effects of different groyne arm-length on the velocity changes. The width of thalweg lines $(T_{CL})$ was changed to $55{\sim}58%$ of chanel width, and the Froude number did not affect on the thalweg line $(T_{CL})$ and separation line $(S_h)$ changes.

• Transactions of the Korean Society of Automotive Engineers
• /
• 제24권2호
• /
• pp.169-182
• /
• 2016

This paper is the third investigation on the evaluation methods of flow characteristics in a steady flow bench. In the previous works, several assumptions used in the steady flow bench were examined and the flow characteristics were estimated both by the conventional impulse swirl meter and a particle image velocimetry at 1.75B position. From these works, it was concluded that the assumption of the solid rotation might cause serious problems and both of the eccentricity and the velocity profile distort the flow characteristics when using the ISM at 1.75B plane. Therefore, the understanding of the detail velocity profiles is very important to keep discussing the issues about the steady flow evaluation method. For this purpose, the planar velocity profiles were measure at 1.75B position by particle image velocimetry and the characteristics were examined according to the valve angles and lifts. The results show that the planar velocity profiles of 11, 16, $21^{\circ}$ valve angle heads according to the lift are similar to each other, however, that of $26^{\circ}$ angle is an exceptional case in the all aspects. In addition, the swirl behaviors are not apparent up to 6~8 mm lift under the $21^{\circ}$ angle and somewhat arranged motions are observed over the whole plane near the highest lift. At this point, the narrower the angle, the lower the lift at which the swirl motions become clear. On the other hands, when the angle is $26^{\circ}$, the center of swirl is always farthest from the cylinder center and only the indistinct swirl is observed even if at the highest lift. Also, all the swirl centers are quite apart from the cylinder center so that the effect of eccentricity may not be negligible at 1.75B regardless the valve angle. Related to the tangential velocity along with the radial direction, the bands of the velocity distribution are very wide and the mean velocities of cylinder center basis are lower than the velocity which is assumed in the ISM evaluation. Lastly, the mean tangential velocity profiles of swirl center basis are sometimes higher than that of ISM-assumed up to 0.6 non-dimensional distance less than 6mm lift, however, as the lift increases the profiles are different according to the angles and profile $11^{\circ}$ is the most closed to the ideal profile. Consequently, the real velocity profile is far from the assumption of ISM evaluation.

• Proceedings of the Korea Water Resources Association Conference
• /
• 한국수자원학회 2008년도 학술발표회 논문집
• /
• pp.1774-1778
• /
• 2008

유량 환산에 이용되는 수위-유량 관계곡선식은 하천의 흐름을 정상 등류상태로 가정하고 유속계를 하천에 투입하여 년간 정해진 횟수의 유량측정을 실시하여 이로부터 갱신하여 작성하고 있다. 평수기에는 이렇게 기기를 이용하여 유량측정이 가능하지만 홍수기나 갈수기에는 접촉식 유속계를 이용한 하천유량 측정이 불가능한 실정이다. 홍수기에는 기기 손상과 관측자의 안전이 위협받는 실정이고, 갈수기에는 유속이 너무 느려서 (0.1 m/s 이하) 프로펠러 유속계의 경우 유속의 정확한 관측이 힘들다. 또한 전지구적 빈번한 이상기후의 현실정에서 가장 중요한 기초 수문자료인 홍수량의 정확한 측정 자료는 많지 않다. 홍수유량을 측정하기 위해서 현재에도 기존의 봉부자를 이용하거나 유비쿼터스 센서를 장착한 봉부자를 이용하는 유량측정 기법이 향해지고 계속적으로 소개되고 있는 실정이지만 봉부자의 특성상 정확한 유량을 계산하기에는 어려움이 많다. 현재 선진국에서는 흐름과 비접촉식 방법을 이용한 하천유량측정 방법이 지난 10 여년간 꾸준이 연구되어 왔다. 그중 대표전인 것이 전자파를 이용한 방법과 영상해석에 의한 방법이다. 전자의 경우 국내에서는 수자원공사에서 10년 이상 연구 개발하여 상품화 시킨바 현업에서 이를 이용하여 홍수유량측정을 실시하고 있다. 후자의 방법은 유체역학 분야에서 흐름해석에 주로 이용되어지던 PIV(particle image velocimetry) 기법을 하천과 같이 대규모의 흐름영역에 적용가능하도록 개발된 기술로 LSPIV (large-casle particle image velocimetry)라 불리우는 기술이다. 본 연구에서는 미국 Iowa 대학에서 개발한 LSPIV를 이용하여 홍수파의 진행시 수위와 유량의 두 변수 사이에 나타나는 Loop rating curve의 이론적인 관계를 하천현장에서 일정시간 간격으로 실측을 통하여 파악하고자 하였다. 현장실험을 위한 대상지점으로 미국 Iowa주 Coralville 시내 Clear Creek의 USGS (US Geologival Survey) 수위관측소 지점을 선택하여 본 연구에서 실시한 유량측정 결과의 비교가 가능토록 하였다. LSPIV는 그 특성상 야간에는 적용하는데 어려움이 있어 아침시간부터 해가 지기 직전까지의 자연채광 조건의 영상취득이 가능한 시간대에서 표면유속을 측정하였고 이에 수심평균유속환산계수를 적용하여 유량을 계산하였다. 강우의 발생으로 인한 홍수파의 진행시 총 43회의 유량을 측정하였는바 이를 이용하여 이 지점의 수위-유량 관계식과 비교한 결과 거의 일치하는 결과를 나타냈다. 특히 홍수파의 진행시 고수위 영역에서의 측정한 결과는 수위의 상승기에는 최고로 7.5% 까지 측정유량이 수위-유량관계식에서 계산한 유량보다 컸으며, 수위의 하강기에는 반대로 최고 5.4% 정도까지 측정유량이 수위-유량관계식에서 계산한 유량보다 작게 나타났다. 또한 최대유량의 발생시기는 최고수위 발생직전의 수위라는 것이 파악되었다. 이러한 경향은 수위-유량 관계곡선의 이론과 잘 일치하는 것이다.

• The Korean Journal of Malacology
• /
• 제30권1호
• /
• pp.9-15
• /
• 2014

This study is aimed at figuring out the reasons of the mass mortality of abalone and the increase in its mortality rate in the sea cage. The study suggests that lack seawater circulation in an abalone aquaculture cage is an important culprit for it. We analyzed the current distribution around a 1/20 scaled-down abalone unit cage of 4 rows and 10 columns by fluid flow visualization technique (PIV : Particle Image Velocimetry). The speed of current in the model cage definitely slowed down in the first column of a unit cage. We also observed currents going down to the bottom of a water tank from the unit cages placed in the middle. The speed of wakes behind inside the row in the middle was slower than that outside the row. Water velocity inside and outside a real abalone cage at Nowha Island adjacent to Wan Island was measured to verify results from the tank test. The speed of current in front of the cage by 2 m was 0.11 m/sec while it was only 0.0009 m/sec inside the cage. It had similar findings with those of a tank test.