• International Journal of Naval Architecture and Ocean Engineering
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• v.13 no.1
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• pp.566-574
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• 2021

The process of cavitation involves generation, growth, coalescence, and collapse of small bubbles and is tremendously influenced by bubble-bubble interactions. To understand these interactions, a new cavitation model based on the transport equation is proposed herein. The modified Rayleigh-Plesset equation is analyzed to determine the bubble growth rate by assuming equal-sized spherical bubble clouds. The source term in the transport equation is then derived according to the bubble growth rate with the bubble-bubble interaction. The proposed model is validated by various test simulations, including microscopic bubble cloud evolution as well as macroscopical two- and three-dimensional cavitating flows. Compared with previous models, namely the Kunz and Zwart cavitation models, the newly proposed model does not require adjustable parameters and generally results in better predictions both microscopic and macroscopical cases. This model is more physical.

• Journal of Ship and Ocean Technology
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• v.8 no.1
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• pp.42-50
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• 2004

In these ten years, the cavitation and erosion phenomena in the rudder have been increased for high-speed container ships. The cavitation in the rudder blades which is injurious to rudder efficiency is mainly caused by the main flow with a large angle of attack induced by propellers, and the erosion which occurs as a result of repeated blows by shock wave that cavitation collapse may produce was observed in the gap legion of the rudder. However, gap cavitation is not prone to occur in model experiments because of low Reynolds number. So, the viscous effect should be considered for solving the flow of the narrow gap. In order to predict the cavitation phenomena and to improve the performance of the rudder, the analysis of the viscous flow in the rudder gap is positively necessary. In this study, numerical calculation for the solution of the RANS equation is applied to the two-dimensional flow around the rudder gap including horn part and pintle part. The velocity and pressure field are numerically acquired according to Reynolds number and the case that the round bar is installed in the gap is analyzed. For reduced the acceleration that pressure drop can be highly restrained numerically and in model experiment, the cavitation bubbles can be reduced.

• Journal of the Korean Society of Visualization
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• v.21 no.1
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• pp.94-102
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• 2023

Three design parameters were considered in this study: outlet nozzle angle (30°, 60°, 80°), neck length (1 mm, 3 mm), and flow rate (0.5, 0.6, 0.7, 0.8 lpm). A neck diameter of 0.5 mm induced cavitation flow at a venture nozzle. A secondary transparent chamber was connected after ejection to increase bubble duration and shape visibility. The bubble size was estimated using a Gaussian kernel function to identify bubbles in the acquired images. Data on bubble size were used to obtain Sauter's mean diameter and probability density function to obtain specific bubble state conditions. The degree of bubble generation according to the bubble size was compared for each design variable. The bubble diameter increased as the flow rate increased. The frequency of bubble generation was highest around 20 ㎛. With the same neck length, the smaller the CV number, the larger the average bubble diameter. It is possible to increase the generation frequency of smaller bubbles by the cavitation method by changing the magnification angle and length of the neck. However, if the flow rate is too large, the average bubble diameter tends to increase, so an appropriate flow rate should be selected.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.5
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• pp.612-619
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• 2003

This work experimentally studies the fundamental mechanisms by which the ultrasonic vibration enhances convection and pool boiling heat transfer. A thin platinum wire is used as both a heat source and a temperature sensor. A high speed video imaging system is employed to observe the behavior of cavitation and thermal bubbles. It is found that when the liquid temperature is below its boiling point, cavitation takes place due to ultrasonic vibration while cavitation disappears when the liquid reaches the boiling point. Moreover, when the gas dissolved in liquid is removed by pre-degassing, the cavitation arises only locally. Depending on the liquid temperature, heat transfer rates in convection, subcooled boiling and saturated boiling regimes are examined. In convection heat transfer regime, fully agitated cavitation is the most efficient heat transfer enhancement mechanism. Subcooled boiling is most enhanced when tile local cavitation is induced after degassing. In saturated boiling regime, acoustic pressure is shown to be a dominant heat transfer enhancement mechanism.

• Journal of Advanced Marine Engineering and Technology
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• v.12 no.4
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• pp.46-52
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• 1988

The cavitation inception in oil hydraulic pipeline was investigated experimentally and numerically. In the experiment, negative pressures below-1 MPa(absolute pressure) were measured, associated with the transient flows in oil hydraulic pipeline. These experimental results show that the common hydraulic oil in the experimental pipeline withstands large tensions. In order to interpret the experimental results on cavitation inception, the growth of a spherical bubble in viscous compressible fluid due to a stepwise pressure drop was investigated by numerical analysis, and the critical bubble radius was obtained. The calculated value of the critical bubble radius corresponding to the negative pressure measured in the experiment is so small that the premised conditions about the bubble shape in the analysis is unsatisfactory. The physical significance of this calculated result implies the fact that there hardly exist free bubbles which can act as cavitation nuclei in the experimental pipeline.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.6
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• pp.1971-1982
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• 1996

The breakup of liquid jet is the result of competing, unstable hydrodynamic forces acting on the liquid jet as it exit the nozzle. The nozzle geometry and up-stream injection conditions affect the characteristics of flow inside the nozzle, such as turbulence and cavitation bubbles. A set of calculation of the internal flow in a single hole type nozzle were performed using a two dimensional flow simulation under different nozzle geometry and up-stream flow conditions. The calculation showed that the turbulent intensity and discharge coefficient are related to needle position. The diesel nozzle with sharp inlet under actual engine condition has possibility of cavitation, but round inlet nozzle has no possibility of cavitation.

• Food Science and Industry
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• v.51 no.2
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• pp.114-126
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• 2018

The production of high quality oil to meet new standard needs a 'next generation' innovative oil refining tool in paradigm shift. 'Nanoneutralization' using controlled hydrodynamic cavitation-assisted Nanoreactor is successfully being introduced and commercialized into edible oil industry and it plays a key driver for sustainable development of food processing. This emerging technology using bubble dynamics as a consequence of Bernoulli's principle by hydrodynamic cavitation in Venturi-designed multi-flow through cell is radically changing the conventionally chemical-oriented neutralization. Nanoneutralization derived by the creation of nanometer-sized bubbles formed through scientifically structured geometric channels under high pressure has been proven to improve mass transfer and reaction rate so substantially reduce the chemicals required for refined vegetable oil and to increase oil yield while even improving oil quality. More researches on science behind this revolutionary technology will help usto better understand the principle and process hence makes its potential applications expandable in extraction, refining and modification of fats and oils processing.

• Journal of Environmental Science International
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• v.16 no.6
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• pp.699-706
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• 2007

Many lakes or irrigative reservoirs in Korea are rapidly contaminated due to the ever increasing pollutants. Although lots of treatment processes have been recommended and practiced, economical and technical improvement is currently needed. In this study, contaminated irrigation reservoir was treated using the proposed process which is consisted of fine air bubbles, coagulation and flotation. Fine bubbles, approximate diameter of 3 to $10{\mu}m$, were generated using cavitation in the pressurized tank and polyaluminum chloride was used as coagulants. This fine bubbles, coagulation and flotation effectively controlled the low density algae, for example, Chlorophyll-a was removed more than 97 %. Removal efficiency of COD, SS, T-N and T-P were 80.7%, 94.3%, 64.1 % and 92.4%, respectively. Pollutants released from the sediments was removed more than 80% of organics and 60-70 % of nutrients. Consequently, fine bubbles coagulation and flotation process could be effectively used as an alternative treatment method for the purpose of control of lake water quality.

• Journal of the Society of Naval Architects of Korea
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• v.55 no.4
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• pp.330-340
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• 2018

In the present works, the High-speed Cavitation Tunnel (HCT) has been designed and manufactured to have the large test section to conduct various supercavitation experiments. The large amount of air ventilated behind a cavitator produces lots of tiny bubbles, which prevent clear observation of supercavitation at the test section. To collect small bubbles effectively, a bubble collecting section of large volume is equipped upstream of the test section. HCT has the test section dimension of $0.3^H{\times}0.3^W{\times}3.0^L\;m^3$ and provides maximum flow speed of 20.4 m/s at the test section. The blockage and Froude effects on the ventilated supercavitation are investigated successfully at the test section. The basic studies such as the supercavitation evolution, drag measurements and cavity shape extraction with air flow rate are also carried out in HCT.

• Journal of the Korean Society of Marine Environment & Safety
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• v.18 no.5
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• pp.481-487
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• 2012

Recently, the construction of the small Al alloy ships is an increasing trend in viewpoint such as the disposal issue of a retired ship, the enhancement of environmental regulation and resources recycling etc. for FRP ships. However, Al alloy ship which can achieve high speed by light weight in marine environment is exposed to a problem on materials damage by cavitation-erosion which is generated by large impact pressure with the collapse of air bubbles due to cavitation. Consequently, in this study, water cavitation peening technology was applied in Al alloy for ship to enhance durability life by preventing cavitation damage. So, the water cavitaton peening application time that presented the excellent cavitation characteristic investigated. The weight-loss of 5456-H116, 5083-H321 and 5052-O Al alloy at the optimum water cavitation peening time were improved to 42.11 %, 50.0 % and 25.7 %, respectively.