• Wind and Structures
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• 제19권2호
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• pp.113-144
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• 2014

Separation bubble and conical vortices on a large-span flat roof were observed in this study through the use of flow visualization. The results indicated that separation bubble occurred when the flow was normal to the leading edge of the flat roof. Conical vortices that occur under the cornering flow were observed near the leading edge, and their appearance was influenced by the wind angle. When the wind changed from along the diagonal to deviating from the diagonal of the roof, the conical vortex close to the approaching flow changed from circular to be more oblong shaped. Based on the measured velocities in the conical vortices by flow visualization, a proposed two-dimensional vortex model was improved and validated by simplifying the velocity profile between the vortex and the potential flow region. Through measured velocities and parameters of vortices, the intensities of conical vortices and separation bubble on a large-span flat roof under different wind directions were provided. The quasi-steady theory was corrected by including the effect of vortices. With this improved two-dimensional vortex model and the corrected quasi-steady theory, the mean and peak suction beneath the cores of the conical vortices and separation bubble can be predicted, and these were verified by measured pressures on a larger-scale model of the flat roof.

• Korea-Australia Rheology Journal
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• 제18권2호
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• pp.109-118
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• 2006

A finite-element method was employed to analyze axisymmetric unsteady motion of a deformable bubble near the wall. In the present study a deformable bubble in a Newtonian medium near the wall was considered. In solving the governing equations a structured mesh generator was used to describe the collapse of highly deformed bubbles with the Arbitrary Lagrangian Eulerian (ALE) method being employed in order to capture the transient bubble boundary effectively. In order to check the accuracy of the present FE analysis we compared the results of our FE solutions with the result of the collapse of spherical bubbles in a large body of fluid in which solutions can be obtained using a 1D FE analysis. It has been found that 1D and 2D bubble deformations are in good agreement for spherically symmetric problems confirming the validity of the numerical code. Non-spherically symmetric problems were also solved for the collapse of bubble located near a plane solid wall. We have shown that a microjet develops at the bubble boundary away from the wall as already observed experimentally. We have discussed the effect of Reynolds number and distance of the bubble center from the wall on the transient collapse pattern of bubble.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.211-216
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• 2001

Flow motion and variation of thermal field around a bubble which attached at the upper cooled solid wall in a $B\dot{e}nard$ convection flow is studied experimentally using thermo-sensitive liquid-crystal tracers and image processing for flow visualization and analysis. The air is injected gradually by $0.1m\ell$ to make the bubble. As the growing of the bubble in a $B\dot{e}nard$ convection flow, the variation of temperature field and surface tension along the bubble, which in turn cause to change the thermal field patterns and the flow direction and patterns. 6 cells flow pattern is transformed into diverse flow pattern. At the large size of a bubble, it's only conduction mechanism under the region of the bubble because of low Ra number 1137, but the convection flow both sides of the bubble leads to another convection flow in the bubble influence area which has been remained stable stagnation.

• 대한조선학회논문집
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• 제53권4호
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• pp.300-306
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• 2016

The gas ventilated by supercavitation splits into smaller bubbles and follows the water passage of the cavitation tunnel. The bubbles quickly return to the test section by rather high speed flow, and interrupt the observation of the supercavitation. To secure clear observation in the test section, the bubble collecting section(settling chamber) of large volume is prepared to collect bubbles in the water passage ahead of the test section. The bubble collecting section should provide enough buoyancy effect to the bubbles for proper bubble collecting. However, rather high-speed oncoming flow produces non-uniform velocity distribution and deteriorates buoyancy effect in the bubble collecting section. In the present study, the bubble collecting space and three porous plates are designed and analyzed through numerical methods, and the bubble collecting function is experimentally validated by 1/10-scaled model in terms of the formation of uniformly low velocity distribution in the bubble collecting section.

• 한국유체기계학회 논문집
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• 제14권3호
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• pp.10-17
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• 2011

Free energy based lattice Boltzmann method (LBM) has been used to simulate the contact angle and the bubble necking with large density ratio. LBM with the proper contact angle model is able to reduce the spurious currents and eliminate the singularity in the contact lines. The numerical results of the contact angles are satisfied with the Youngs law. For bubble necking flows, simulations are executed for various viscosities and contact angles. The phenomena of the bubble necking are simulated successfully and the subsequent results are presented. The present method is also applicable to the nucleate boiling flows.

• 한국유체기계학회 논문집
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• 제13권4호
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• pp.31-36
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• 2010

Free energy based lattice Boltzmann method (LBM) has been used to simulate the rising bubble flows with large density ratio. LBM with compact discretization is able to reduce the spurious current of the static bubble test and be satisfied with the Laplace law. The terminal rise velocity and shape of the bubbles are dependent on Eotvos number, Morton number and Reynolds number. For single bubble flows, simulations are executed for various Eotvos number, Morton number and Reynolds number, and the results are agreed well with the experiments. For multiple bubbles, the bubble flow characteristics are related by the vortex pattern of the leading bubble. The coalescence of the bubbles are simulated successfully and the subsequent results are presented. The present method is validated for static, dynamic bubble test cases and compared to the numerical, experimental results.

• 한국음향학회지
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• 제41권3호
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• pp.255-267
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• 2022

본 논문에서는 오리피스형 공기분사기에서 생성된 기포소음 특성을 고찰하였다. 전반적인 기포소음 특성을 파악하기 위해 Strasberg와 Blake가 각각 제안한 기포소음 스펙트럼을 살펴보았고, 기포소음 계측을 위해 선박해양플랜트연구소의 대형캐비테이션터널에서 공기분사 실험을 수행하였다. 본 실험은 5종의 공기분사기를 이용하여 정지 유체 조건과 유동 조건에서 수행되었다. 계측결과로부터 실험 조건에 따른 기포소음 스펙트럼 특성을 관찰하였고, 회귀분석을 통해 기포소음에 대한 인자별 영향을 분석하였다. 끝으로 회귀분석 결과를 기반으로 기포소음 추정식을 제안하였고 제안된 추정식은 계측결과와 잘 일치함을 확인하였다.

• Korea-Australia Rheology Journal
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• 제11권3호
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• pp.247-253
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• 1999

The growth of a spherical vapor bubble contained in a large body of upper convected Maxwell fluid is theoretically analyzed under the devolatilization condition of polymer by using a Galerkin FEM in the Lagrangian frame. Using the finite element technique, a fully explicit numerical scheme is developed both for the calculation of pressure distribution and for the tracking of bubble surface. Oscillatory behavior in bubble radius is observed during growth and the oscillatory behavior is found to be due to the interaction of mass transfer resistance and elasticity. It is found that the elasticity of fluid accelerates the growth and removal of volatile component. It is also found that the bubble growth in the devolatilization of polymers is affected by both mass transfer resistance and viscoelasticity of fluids.

• 설비공학논문집
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• 제5권2호
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• pp.85-93
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• 1993

The rise of a large gas bubble or slug in a Micro Two-Phase Closed Thermosyphon with a thin wire insert has been analiged by the potential flow theory. The effect of the interfacial surface tension is explicitly accounted by application of the Kelvin-Laplace equation and solved for the bubble shape. The solution is expressed in terms of the Stokes stream function which consists of an infinite series of Bessel functions. The conditions of the bubble movement in a Micro Two-Phase Closed Thermosyphon were theoretically ascertained.

• 설비공학논문집
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• 제8권1호
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• pp.99-109
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• 1996

Experimental investigations are carried out for the characteristics of bubble rise in the Miniature Two-Phase closed Thermosyphon(MTPCT) with a thin wire insert. The working fluids applied as experimental media are of three kinds: water, methanol, and ethanol. The effects of combination of the inclination with diametric ratio $\alpha$(=d$_{0}$/D$_{I}$) on rising velocity of a large bubble in the thermosyphon are explicitly analyzed. The realm of a movable bubble and the critical value of $\alpha$ are iteratively pursued to interpret the region Figures-of-Break, rooted in the governing physics relations, according to the application of working fluid. Experimental results are compared with those of analysis and critical ranges for $\alpha$ and D$_{I}$ were ascertained from comparisons.isons.