• Proceedings of the Korean Nuclear Society Conference
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• 1996.05b
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• pp.241-246
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• 1996

An analytical model to calculate rate of vapor generation due to heterogeneous wall nucleation in flashing flow is developed. In the present model, an important parameter of the vapor generation term, i.e. nucleation site density is calculated by integrating its probability distribution function with respect to active cavity radius. The limits of integration are minimum and maximum active cavity radii, and these are formulated using an active cavity model for nucleate boiling. This formulation, therefore. can statistically account for the effect of surface specific thermo-physical and geometric conditions on the vapor generation rate and flashing inception. For verifying the adequacy of the present model, steady state two-fluid and the bubble transport equations are solved with applicable constitutive equations. The applicable region of the bubble transport equation is also extended to churn-turbulent flow regime to predict interfacial area concentration at high void fraction. Predicted results in terms of axial pressure and void fraction profiles along the channels are compared with experimental data of Super Moby Dick and BNL Reasonable agreements have been achieved and this shows the applicability of the present model to flashing flow analysis.

• Journal of Ocean Engineering and Technology
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• v.11 no.4
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• pp.122-129
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• 1997

Flow characteristics of two-dimensional closed square cavities near unsteady critical Reynolds numbers were studied numerically at five Reynolds numbers : 8${\times}10^3$, 8.5${\times}10^3$, 9${\times}10^3$, 9.5${\times}10^3$ and $10^4$ were investigated. A convection conservative difference scheme based upon SOLA to maintain the nearly 2nd-order spatial accuracy was adopted on irregular grid formation. Irregular grid number is 80${\times}$80 and its minimum size is about 1/400 of the cavity height(H) and its maximum is about 1/53 H. The result shows that the critical Reynolds number indicating the emergence of flow unsteadiness exists near Re=8.5${\times}10^3$ and their flow patterns reveal periodic fluctuation during transient and fully-developed stages.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.781-785
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• 2003

This work presents a numerical analysis of two-phase natural circulation flow in reactor cavity under external vessel cooling. Steady, incompressible, three-dimensional Reynolds-averaged Navier-Stokes equations for multiphase flows with zero equation turbulence model are solved to predict the shear key effect on the circulation rate of cooling water and the distribution of void fraction according to the different mass flow of inlet air. Results show that shear key has a positive effect on the circulation rate of cooling water and induce a local increase of void fraction below the shear key, but not remarkably.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.8 s.251
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• pp.796-805
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• 2006

In this study, the problem of using the mixing index as a measure of the mixing performance for a certain flow field has been discussed. The flow model subjected to this study is the two-dimensional unsteady lid-driven cavity flow. The transport equation for the concentration within the cavity was solved by using the finite volume method where the convective terms are discretized with the central difference scheme. It was shown that both the concentration dispersion and the mixing index depend highly on the initial distribution of the concentration, and therefore the mixing index obtained from the concentration dispersion equation loses its universal applicability.

• Solar Energy
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• v.20 no.4
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• pp.53-60
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• 2000

An experimental study was carried out in a cavity with upper channel and square heat surface by visualization equipment with Mach-Zehnder interferometer and laser apparatus. The visualization system consists of 2-dimensional sheet light by Argon-Ion Laser with cylindrical lens and flow picture recording system. Instant simultaneous velocity vectors at whole field were measured by 2-D PIV system(CACTUS'2000). Obtained result showed various flow patterns. Severe unsteady flow fluctuation within the cavity are remarkable and sheared mixing layer phenomena are also found at the region where inlet flow is collided with the counter-clockwise rotating main primary vortex. Photographs of Mach-Zehnder are also compared in terms of constant heat flux.

• Journal of the Korean Society of Visualization
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• v.21 no.3
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• pp.85-92
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• 2023

In this study, we present experimental results of cavitating flow around a vortex generator which is used to improve the flow in the wake of ships and enhance propulsion efficiency. We conducted experiments at the CNU cavitation tunnel on a total of six vortex generators, two different aspect ratios and three taper ratios. We recorded cavity patterns using a high-speed camera and quantitatively evaluated cavity fraction using OpenCV. The most important finding of this study is that the vortex cavity generated at a root leading edge of the vortex generator develops at a specific angle.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.196-199
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• 2005

Flow traveling over a cavity opening forms a vortex due to unstable shear layer and induces an aerodynamic pressure excitation from the diffusion of the vortex convecting out of the trailing edge of the opening. The interaction between the excitation force and the cavity response sustains resonance in the resonator(cavity) and locked-in vortex shedding at the leading edge of the opening. The aerodynamic excitation force can be described from the diffusion of the vortex over the trailing edge and the level of its diffusivity is related to the strength of vorticity seeded at the loading edge. In this study, the control scheme of the internal pressure oscillation was proposed from regulating the vorticity at the leading edge by use of an oscillating spoiler. It was found that the relative motion between the spoiler and the air mass at the cavity opening influenced vorticity strength and the control was achieved by direct feedback of the cavity pressure fluctuation to the actuator.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.11
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• pp.753-760
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• 2019

Sparkjet actuator, also known as plasma synthetic jet actuator, which is a kind of active flow control actuator is considered as being high possibility for the supersonic flow control due to ejecting stronger jet compared to the other active flow control actuators. Sparkjet actuator generates high temperature and high pressure flow inside the cavity by using arc plasma and leads momentum by ejecting such flow through orifice or nozzle. In this research, numerical calculation of sparkjet actuator with respect to the location of electrodes which exists inside the cavity is conducted and the change of the performance of sparkjet actuator is suggested. As the location of electrodes goes closer to the bottom of the cavity, impulse is increased and the average pressure inside the cavity maintains higher. When the location of electrode is 25% and 75% of the entire cavity height, impulse is 2.515 μN·s and 2.057 μN·s, respectively. Each impulse is changed by about 9.92% and -10.09% compared to when the location of electrodes is 50% of the entire cavity height.

• Korea-Australia Rheology Journal
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• v.20 no.2
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• pp.79-88
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• 2008

In this study, we focus on the improvement of the filling efficiency in injection molding by application of ultrasonic vibration. While studies about the filling efficiency of typical filling processes in the injection molding have been widely performed, there have been only few studies about the filling efficiency of an ultrasonic process. The effect of the ultrasonic vibration is an important process condition, which influences the flow characteristics of polymer melt. This new condition even affects well-known injection conditions such as cavity pressure, injection temperature and mold temperature. For this study, we carried out a numerical analysis by appropriate modeling and analysis of the ultrasonic process in the filling process. To verify this numerical analysis, we compared the numerical results with the experimental data. Also, we analyzed the filling process in a thin cavity using this numerical analysis. To understand the flow characteristics of polymer melt in the ultrasonic process, we substituted real and complex vibration conditions with simplified and classified conditions according to the position of vibrating cavity surfaces and the phase difference between two opposing cavity surfaces. We also introduced MFR (melt flow ratio) as a new index to estimate the filling efficiency in the ultrasonic process.

• Transactions of Materials Processing
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• v.12 no.1
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• pp.3-10
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• 2003

The formation of surface defects associated with Jotting in injection molding is related to the geometries of cavity and fate, operational conditions and the rheological properties of polymer. In this study we have examined jetting phenomena in injection molding process for the throe kinds of PCs which have different molecular weights and structures, PBT and PC/ABS alloy with several injection speeds. We have used various cavity shapes those are tensile, flexural and impact test specimens with various gates and cavity thicknesses. Through this study we have observed that the jetting is related to the dic swell of material. This means that the jotting is strongly affected by the elastic flow property rather than the viscous flow property in viscoelastic characteristics of molten polymer. Different resins have different elastic properties, and elastic flow behavior depends on the shear rate of flow, i.e. injection speed. Large die swell would eliminate jetting however, the retardation of die swell would stimulate jetting. In the point of mole design, reducing the thickness ratio of cavity to gate can reduce or eliminate jetting regardless of amount of elasticity of polymer melt.