• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.9
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• pp.1335-1341
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• 2003

This paper describes the process of nanoparticle synthesis by laser ablation of microparticles and consolidated sample. We have generated nanoparticles by high-power pulsed laser ablation of AI, Cu and Ag microparticles using a Q-switched Nd:YAG laser (wavelength 355nm, FWHM 6ns, fluence $0.8{\sim}2.0J/cm^2$). Microparticles of mean diameter $18{\sim}80{\mu}m$ are ablated in the ambient air. The generated nanoparticles are collected on a glass substrate and the size distribution and morphology are examined using a scanning electron microscope and a transmission electron microscope. The effect of laser fluence, collector position and compacting pressure on the distribution of particle size is investigated. To better understand the process of laser ablation of microparticle(LAM), we investigated the Nd: YAG laser-induced breakdown of Cu microparticle using time-resolved optical shadow images. Nanosecond time-resolved images of the ablation process are also obtained by laser flash shadowgraphy. Based on the experimental results, discussions are made on the dynamics of ablation plume.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.6 s.144
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• pp.559-565
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• 2005

As the wave impinges on and overtops the structure, a large highly aerated region is created in front of the structure and water splashs on top of the structure. The broken wave in front of the structure and associated green water on top of the structure are highly aerated containing not only a large number of bubbles but also very large sizes of bubbles. In this paper, the velocity field of the highly aerated region and the splashing water on the top is measured using a modified PIV method incorporating the traditional PIV method with the shadowgraphy technigue by correlating the ' texture ' of the bubble images. The velocity fields of a plunging wave impacting on a structure in a two-dimensional wave flume is measured. It is found that the maximum fluid particle velocity in flout of the structure during the impinging process is about 1.5 times the phase speed of the wave, while the maximum horizontal velocity above the top is less than the phase speed, It is also found that the dam breaking solution does not work well in predicting the green water velocity.

• Ecology and Resilient Infrastructure
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• v.6 no.3
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• pp.137-144
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• 2019

This study carried out experimental investigation through measurements of void fraction of aerated flows generated near hydraulic structures. The imaging technique applying the shadowgraphy method was employed to quantify aeration by measuring void fractions. Aerated flows in the vicinity of a sloped weir and a stepped weir were measured using the imaging technique. Spatially distributed void fraction that was possible from gray images gives aeration rate of the flows over hydraulic structures with eco-environmental function. Aeration over the stepped weir takes place more strongly and widely in its front slope and downstream compared to the sloped weir.

• Journal of the Korean Society of Visualization
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• v.19 no.1
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• pp.28-35
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• 2021

In this study, an image processing method for the measurement of two-phase bubbly flow is developed. Shadowgraphy images obtained by high-speed camera are used for analysis. Some bubbles are generated as single unit and others are overlapped or clustered. Single bubbles can be easily analyzed using parameters such as bubble shape, centroid, and area. But overlapped bubbles are difficult to transform clustered bubbles into segmented bubbles. Several approaches were proposed for the bubble segmentation such as Hough transform, connection point method and watershed. These methods are not enough for bubble segmentation. In order to obtain the size distribution of bubbles, we present a method of splitting overlapping bubbles using watershed and approximating them to ellipse. There is only 5% error difference between manual and automatic analysis. Furthermore, the error can be reduced down to 1.2% when a correction factor is used. The ellipse fitting algorithm developed in this study can be used to measure bubble parameters accurately by reflecting the shape of the bubbles.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.03a
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• pp.837-841
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• 2008

Propulsion of gene coated micro-particles is desired for non-intrusive drug delivery inside biological tissue. This has been achieved by the development of a device that uses high power laser pulses. The present paper looks at the mechanisms of micro-particle acceleration. Initially, a high power laser pulse is focused onto the front side of a thin aluminium foil leading to its ablation. The ablation front drives a compression wave inside the foil, thus leading to the formation of a shock wave, which will later reflect from the rear side of the foil, due to acoustic impedance mismatch. The reflected wave will induce an opposite motion of the foil, characterized by a very high speed, of the order of several millimeters per microsecond. Micro-particles, which are deposited on the rear side of the foil, thus get accelerated and ejected as micro-projectiles and are able to penetrate several hundreds of micrometers inside tissue-like material. These processes have been observed experimentally by using high-speed shadowgraphy and considered analytically.

• Journal of the Korean Society of Visualization
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• v.21 no.3
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• pp.75-84
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• 2023

In this study, we conducted analysis of bubble dynamics and flow of liquid phase change material(PCM) using shadowgraphy and particle image velocimetry(PIV). Characteristics of internal flow varied depending on locations of injection when solid PCM was liquefied from heated vertical wall. When bubbles rose immediately, they exhibited elliptical shape and zigzag trajectory. In contrast, when bubbles rose after merging at the bottom of solid PCM, with equivalent diameter for the inter-wall distance of 0.64 or greater, they showed a jellyfish shape and strong rocking behavior. It was observed by the PIV that the small ellipse bubbles made most strong flow inside the liquid PCM. Furthermore, the flow velocity was highest in the case of front injection, as the directions of temperature gradients and bubble-driven flow were aligned. The results underscore the significant influence of injection location on various characteristics, including bubble size, shape, rising path of bubbles, and internal flow.

• Journal of Korea Water Resources Association
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• v.48 no.7
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• pp.535-543
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• 2015

Measurements of multiphase flows containing bubbles have been limited because most existing methods target one phase flows. Especially, multiphase flows with a high void ratio have been rarely successful in measurements due to the sudden change of density and thick interfaces between air and water. This study introduces two methods that are capable of measuring flow fields regardless of bubble void ratio, named bubble image velocimetry and bundle fiber optic flow meter. The calculation of the depth of field is suggested to reduce and estimate errors by perspective image velocimetry. The bundle fiber optic flow meter is designed to increase a measurement rate using many optical fibers with a thin diameter. The two methods measured bubble plumes to test reliability and the velocity measurements show good agreement. In addition a hydraulic jump, one of the multiple flows in rivers was measured to test applicability of the methods.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.7
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• pp.585-592
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• 2015

Effects of opening distance on liquid-gas spray of pintle injector were experimentally investigated under atmospheric condition by using water and air as simulants of propellants. Discharge coefficients($C_d$) and mass flow rates were calculated with various injection conditions; 0.1 bar - 1.0 bar for water pressures and 0.2 mm - 1.0 mm for the pintle opening distances. Spray angles were measured from the spray images that were obtained by a shadowgraphy method. When opening distance is 0.2 mm, liquid sheet is not formed properly and it show non-uniform spray. than it can result in combustion instability. it has a weak correlation between the momentum flux ratio and the spray angle, while it has a strong correlation between the momentum ratio and spray angle. Finally the spray angles reduced exponentially when the momentum ratio increased and the spray angles converged to about 40 degrees.

• Transactions of the Korean Society of Automotive Engineers
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• v.14 no.3
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• pp.103-111
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• 2006

The spray behavior of direct-injection spark-ignition(DISI) engines is crucial for obtaining the required mixture distribution for optimal engine combustion. The spray characteristics of DISI engines are affected by many factors such as piston bowl shape, air flow, ambient temperature, injection pressure and fuel temperature. In this study, the effect of fuel temperature on the spray and combustion characteristics was partially investigated for the wall-guided system. The effect of fuel temperature on the fan spray characteristics was investigated in a steady flow rig embodied in a wind tunnel. The shadowgraphy and direct imaging methods were employed to visualize the spray development at different fuel temperatures. The microscopic characteristics of spray were investigated by the particle size measurements using a phase Doppler anemometry(PDA). The effect of injector temperature on the engine combustion characteristics during cold start and warming-up operating conditions was also investigated. Optical single cylinder DISI engine was used for the test, and the successive flame images captured by high speed camera, engine-out emissions and performance data have been analyzed. This could give the way of forming the stable mixture near the spark plug to achieve the stable combustion of DISI engine.

• Journal of the Optical Society of Korea
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• v.13 no.1
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• pp.37-41
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• 2009

We observe the proton signals produced by laser interaction with thin-foil targets of polyimide and of copper. We change the thickness of the polyimide target to $7.5{\mu}m$, $12.5{\mu}m$, and $50{\mu}m$. High-energy protons with the maximum energy of ${\sim}2.3\;MeV$ from $7.5{\mu}m$ thick polyimide are observed. This proton beam with the maximum energy of multi-MeV has various applications such as a proton shadowgraphy.