• 대한기계학회논문집B
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• 제32권7호
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• pp.518-528
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• 2008

The effects of surface roughness on a turbulent boundary layer (TBL) were investigated using particle image velocimetry (PIV). The roughness elements used were periodically arranged two-dimensional spanwise rods, and the roughness height was ${\kappa}/{\delta}$. Introduction of the roughness elements increased the wake strength and the turbulent stress not only in the roughness sublayer but also in the outer layer. This indicates the existence of interaction between inner and outer layers for 2D rod-roughened wall. Roughness effects on a turbulence structure near the wall were obtained by PIV measurements. Iso-contours of mean velocities and Reynolds stresses in the roughness sublayer showed a very good agreement with previous DNS results.

• International Journal of Air-Conditioning and Refrigeration
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• 제13권1호
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• pp.44-50
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• 2005

In the present study, the characteristics of upward bubble flow were experimentally investigated in a liquid bath. An electro-conductivity probe was used to measure local volume fraction and bubble frequency. Since the gas was concentrated at the near the nozzle, the flow parameters were high near the nozzle. In general their axial and radial values tended to decrease with increasing distance. For visualization of flow characteristics, a Particle Image Velocimetry (PIV) and a thermo-vision camera were used in the present study. The experimental results showed that heat transfer from bubble surface to water was largely completed within z = 10 mm from the nozzle, and then the temperature of bubble surface reached that of water rapidly. Due to the centrifugal force, the flow was more developed near the wall than at bubble-water plume. Vortex flow in the bottom region was relatively weaker than that in the upper region.

• 한국가시화정보학회지
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• 제17권2호
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• pp.39-47
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• 2019

Impingement cooling utilizing synthetic jets is emerging as a popular cooling technique because of its high local cooling efficiency. The interaction between the vortex structure of the synthetic jet and the surface is crucial in understanding the mechanism of this technique. In this study, the impinging vortex structure and its advection are investigated by experiments with jet-to-surface spacing $2{\leq}H/D{\leq}7$, and synthetic jet Reynolds number $5120{\leq}Re{\leq}9050$. Using phase-locked particle image velocimetry, ensemble averaged (phase averaged) flow fields are obtained, and vortex identification and quantification techniques are applied. The shape, trajectory, and intensity change of the vortex are assessed. A sharp decline in the vortex intensity and the occurrence of a counter-rotating vortex at the impingement point are observed.

• 설비공학논문집
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• 제9권3호
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• pp.268-275
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• 1997

Using the film-based particle image velocimetry, natural convective flows have been measured quantitatively in a rectangular enclosure with a heater located on the bottom surface. The success rate of the present interrogation method has been obtained as a function of the number of particle pairs and the distance between the particle pairs. The influence of the diffraction halo at the center have been effectively eliminated by rotating-subtracting the original Fourier-transformed image. By utilizing the coded multiple pulsed illumination with two different time intervals, the minimum measurable velocity have been improved. The results of the velocity distributions and the heat transfer correlations have been obtained for different locations of heater in the enclosure.

• 한국가시화정보학회지
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• 제10권3호
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• pp.21-24
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• 2012

Although the motion of the three-phase contact line on a solid substrate has been extensively studied thus far, the understanding of the dynamics of the contact line of liquid/liquid/gas phases is far from complete. Here we deposit a drop of isopropyl alcohol (IPA) on water and HFE-7100 whose free surfaces are exposed to air to observe the flow field around the contact line. By combining the shadowgraph and high-speed imaging techniques, we find that vortices are spontaneously generated at the contact line, which grow in size with time. The flow is attributed to the Marangoni stress that pulls a liquid of lower-surface tension toward a liquid surface having a higher surface tension. However, it is not still clear why the entrained lower-surface-tension liquid should whirl rapidly beneath the contact line. We also visualize the flow by the particle image velocimetry (PIV) to find out that the rotational velocity reaches the order of 1 mm/s near the free surface.

• 한국해양공학회지
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• 제24권1호
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• pp.1-9
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• 2010

A series of experiments employing particle image velocimetry (PIV) technique was conducted to produce benchmark wave kinematics data for regular waves having four different wave slopes in 2-D wave tank. Water velocities and accelerations near the free surface of regular waves were computed from image pair obtained by PIV systems. With the measured wave velocity field, the wave accelerations were computed using a centered finite difference scheme. Both local and convective components of the total accelerations are obtained from experimental data. With increasing the wave slope, the horizontal velocity and the vertical accelerations near the wave crest obtained by PIV technique became larger than theoretical results, which are well-known phenomena of the wave nonlinearity. It is noted that the relative magnitude of convective acceleration to the local acceleration became larger with increasing wave slope.

• 한국환경과학회지
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• 제24권4호
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• pp.403-414
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• 2015

This study is daytime and nighttime runoff image data caused by heavy rain on May 27, 2013 at Oedo Water Treatment Plant of Oedo-Stream, Jeju to compute runoff by applying Surface image velocimeter (SIV) and analyzing correlation according to current. At the same time, current was comparatively analyzed using ADCP observation data and fixed electromagnetic surface current meter (Kalesto) observed at the runoff site. As a result of comparison on resolutions of daytime and nighttime runoff images collected, correlation coefficient corresponding to the range of 0.6~0.7 was 6.8% higher for nighttime runoff image compared to daytime runoff image. On the contrary, correlation coefficient corresponding to the range of 0.9~1.0 was 17% lower. This result implies that nighttime runoff image has lower image quality than daytime runoff image. In the process of computing current using SIV, a rational filtering process for correlation coefficient is needed according to images obtained.

• 한국가시화정보학회지
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• 제12권2호
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• pp.18-22
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• 2014

Caenorhabditis elegans (C. elegans) is an undulatory nematode which exhibits two distinct locomotion types of swimming and crawling. Although in its natural habitat C. elegans lives in a non-Newtonian fluidic environment, our current understanding has been limited to the behavior of C. elegans in a simple Newtonian fluid. Here, we present some experimental results on the penetrating behavior of C. elegans at the interface from liquid to solid environment. Once C. elegans, which otherwise swims freely in a liquid, makes a contact to the solid gel boundary, it begins to penetrate vertically to the surface by changing its stroke motion characterized by a stiffer body shape and a slow stroke frequency. The particle image velocimetry (PIV) analysis reveals the flow streamlines produced by the stroke of worm. For the worm that crawls on a solid surface, we utilize a technique of traction force microscopy (TFM) to find that the crawling nematode forms localized force islands along the body where makes direct contacts to the gel surface.

• 한국수자원학회논문집
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• 제41권7호
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• pp.701-709
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• 2008

표면영상유속계(SIV)는 영상 처리 기술을 이용하여 수표면의 유속을 측정하는 장비이다. 표면영상유속계는 하천의 유속을 매우 간편하게 측정할 수 있도록 한다. 그러나, 표면영상유속계를 이용하여 유량을 산정하고자 할 경우, 하천 표면의 평면 측량 자료와 하천의 단면 측량 자료가 반드시 필요하다. 이 때문에 표면영상유속계의 간편성과 유용성에도 불구하고, 이용자들이 쉽게 이용하기 어렵다는 그릇된 인식을 줄 수 있다. 만일 효율적이고 간편하게 하천의 평면을 추정할 수 있다면, 표면영상유속계를 마치 일반적인 프로펠러 유속계처럼 쉽게 이용할 수 있을 것이며, 그 적용성도 크게 증진될 것이다. 이 연구는 보정된 카메라를 이용하여 실제의 평면 좌표(물리 좌표)를 추정하는 방법에 대한 것이다. 이 방법을 이용하면 유속장을 추정하는 과정을 반자동화할 수 있다. 보정된 카메라에서 평면 좌표를 산정하기 위해 사진 측량학적 기법을 채택하였다. 이 기법들은 컴퓨터 시각 분야에서 오랜 동안 연구되어 온 것이다. 이 기법을 표면영상유속계에 적용하여 사영 변환을 위한 참조점들의 좌표를 구할 수 있다. 이를 통해 참조점 측량에 대한 번거로운 과정을 생략할 수 있다. 개발된 방법을 실제 적용해 본 결과는 오차를 무시할 수 있을 정도임을 입증하였다.

• Ecology and Resilient Infrastructure
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• 제8권2호
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• pp.88-96
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• 2021

표면영상유속계를 이용한 유속 측정 시 하천의 넓은 영역을 경사지게 촬영하기 때문에 필연적으로 영상 왜곡이 발생하게 된다. 이와 같이 경사영상을 정사영상으로 변환하는 방법으로 수표면과 동일한 평면상의 참조점 좌표를 이용하는 2차원 투영 좌표 변환법을 사용할 경우 홍수 시 수위가 변할 경우 대응이 어렵다는 한계가 있다. 이에 본 연구에서는 수위가 변하더라도 참조점을 재설정할 필요가 없는 경사영상 왜곡 보정 방법을 개발하였다. 본 연구에서 개발한 기법은 높은 위치에 설정한 참조점의 좌표와 카메라의 좌표, 그리고 카메라의 수표면 사이의 연직거리 간의 기하학적인 관계를 이용해 수위 변화에도 대응할 수 있는 경사영상 보정 기법이다. 본 연구에서 개발한 영상 왜곡 보정 방법의 검증을 위해 실규모 하천 실험을 수행하였으며, 참조점 변환식에 대한 검증과 표면유속 측정 결과에 대한 검증을 수행하였다. 검증 결과 개발 기술의 경사영상 보정 정확도는 97% 이상을 나타냈고, 유속 검증 결과 개발 기술을 적용하여 산정한 유속을 비교한 결과 약 4% 이내의 차이를 보이는 것으로 나타나 높은 정확도 확보가 가능한 것으로 나타났다. 따라서 개발 기술을 영상 기반의 고정형 자동유량계측 시스템에 적용한다면 수위가 급변하는 홍수 시 유량측정의 정확도를 개선할 수 있을 것으로 기대한다.