• Journal of the Korean earth science society
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• v.34 no.3
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• pp.189-194
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• 2013

In this study, the patterns of barotropic vortices and their structural rotation were investigated through laboratory experiments. Both stable and unstable barotropic vortices were formed in a rotating water tank with a rotating circular plate depending on the diameter, direction, and speed of rotating circular plate. The patterns of stable vortices turned out to be tripolar, triangular, rectangular, and monopolar vortex. These vortex patterns were affected by the gap between the circular plate and the wall of the water tank. Many unstable vortices were formed by anticyclonically and highly rotating circular plate. These results were caused by the centrifugal instability. The structural angular velocity of the tripolar vortex increased with the tangential velocity of the circular plate. The anticyclonic tripolar vortex had higher structural angular velocity than the cyclonic vortex. The tripolar vortex in the water tank was very similar with the real oceanic tripolar vortex from the view point of the Rossby number and the structural rotation.

• Polymer(Korea)
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• v.25 no.4
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• pp.536-544
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• 2001

We investigate, in the viewpoint of mathematical stability, the validity of the time-strain separability hypothesis employed in polymer viscoelasticity on the basis of experimental results. There have been suggested two distinct stability criteria such as Hadamard related to quick response and dissipative stability conditions, and in the limit of high deformation rate we have proved that separable constitutive equations are either Hadamard or dissipative unstable. The fact that the separability is not valid in the short time region in stress relaxation experiments exactly coincides with the results of our analysis. Therefore, since the application of the separability hypothesis incurs thermodynamic inconsistency as well as mathematical instability, such application should be avoided in the formulation of constitutive equations. In addition, careful attention should be paid to the limit of its validity even in experiments. It is also proved that there is neither theoretical nor physical validity of using the damping function.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.3B
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• pp.231-237
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• 2009

This study presents a numerical model to simulate density currents developing two dimensionally. The ${\kappa}-{\varepsilon}$ model is used for the turbulence closure. Elliptic flow equations are solved by the finite volume method. In order to investigate the applicability of the numerical model, discontinuous density currents are simulated numerically. The vortices due to the instability at the interface are simulated, showing a good agreement with the experimental visualizations in the literature. It is also investigated that the transition from slumping phase to inertial phase occurs when a bore generated at the end wall overtakes the front. However, the propagation of the density current is retarded compared with the experimental results. Two-dimensional modeling seems to have an effect on underestimating the front velocity of the density current.

• Journal of Radiation Protection and Research
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• v.39 no.2
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• pp.70-80
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• 2014

The interpretation on the diffusion of radiation contaminants in air is usually to apply a Gaussian plume equation that obtains normal distributions in stable air flow conditions to draw a conservative conclusion. In this study, a numerical study using computational fluid dynamics methods was performed to interpret the air flow pattern and the diffusion of the radiation contaminants at the Wolseong nuclear power plants, and a more detailed solution can be obtained than the Gaussian plume equation, which is difficult to use to simulate complex terrains. The results show that a significant fluctuation of air flow in the terrain appears in the case of a northwester and southeaster because of the mountain located in the northwest and the sea located in the south-east. The northwesterly air flow shows the most unstable flow in the vertical direction when it passes over the terrain of mountain. The stable southeasterly air flow enters into the nuclear power plant from the sea, but it becomes unstable rapidly because of the interference by the building and the terrain. On the other hand, in the case of a northeaster and southwester, a small interruption of air flow is caused by the terrain and wake behind the buildings of nuclear power plants.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.8
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• pp.1067-1074
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• 2010

In this study, a loosely coupled fluid-structure interaction (FSI) analysis was conducted for a low-pressure (LP) final-stage rotor blade. Preliminary FSI analyses of a $15^{\circ}$ sweptback wing and a NASA Rotor 37 compressor blade were performed for verifying the boundary conditions. The results were compared with the established literatures for each model. The FSI analysis of the $15^{\circ}$ sweptback wing was carried out under both stable and unstable conditions. The excessive deformation of the wing was observed within 0.05 s under the unstable condition which is higher than the divergence speed of a wing compared with the stable condition. On the basis of the results of a steady-state study, an unsteady state FSI analysis was conducted for a NASA Rotor 37. Different deformations were observed at trailing edge of the blade in the static FSI and dynamic FSI analysis. A 3D FE model of a LP rotor was generated from the span-wise section data. In order to develop a reasonable model, an impact test was performed and compared to the FE model. Using this FE model, the steady-state FSI analysis was performed successfully.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.4
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• pp.449-455
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• 2012

During the measurement of the flow rate of gases such as natural gas, flow hunting is observed in most orifice meters but the intensity of flow hunting at each metering system shows different characteristics. In order to investigate why such a difference occurs and whether the difference actually influences metering error, pipeline network analysis on the main factors and characteristics of flow hunting was carried out in a previous study. Following this, in this study, computational fluid dynamics (CFD) analysis was carried out to clarify the relation between flow instability and flow hunting and determine the factors influencing the orifice meter depending on the intensity of upward pressure fluctuation, time interval, and flow rate. Finally, we showed that the pressure hunting rate is a function of the ratio of the pressure difference before and after an orifice meter. On the basis of CFD analysis results, we also presented some major factors and relations influencing flow hunting.

• Journal of the Korean Vacuum Society
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• v.21 no.5
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• pp.249-257
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• 2012

Nonlinear dynamical behaviors of plasma in the Pierce diode are investigated by a numerical code developed using a one dimensional fluid model. The plasma in Pierce diode is alternately stable and unstable as Pierce parameter is changed. The dynamical characteristics of neutral and non-neutral Pierce system is examined analytically and numerically. It alternately has growing and oscillatory mode as Pierce parameter varies. As Pierce parameter is decreased, each oscillatory mode undergoes a sequence of subharmonic period-doubling bifurcation and then culminate in a chaotic strange attractor. The analysis for this nonlinear behavior can be used as a model for understanding of beam-plasma interaction in more complex geometries and a data for chaos control.

• Journal of Energy Engineering
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• v.2 no.2
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• pp.179-186
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• 1993

Heat pipes. applied to a flat plate solar collector, have a long and slender configuration with relatively low heat flux in the evaporator section. Such a heat pipe has a tendency to build-up a liquid pool at the lower part of the evaporator section. and at this pool occurs such complicated phenomena of evaporation and fluid dynamics as superheat, sudden generation of bubble, its likely explosive growth process and flooding, etc. In the present study. we tried to solve these problems by means of adjusting two principal design parameters, the liquid inventory and the installation region of the wick, using 4 heat pipes and 3 thermospheres. The corresponding results can be summarized as follows$\^$1)/. The effective thermal conductances of the heat pipe was greatly improved by eliminating the wick in the adiabatic and condenser sections$\^$2)/. The liquid inventory should be increased by about 40% larger than what is saturated the wick$\^$3)/. In the evaporator section the wick has a favorable effect to reduce both unstable operation by intermittent occurrence of nucleate boiling and response time at the initial start-up process.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.11
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• pp.761-767
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• 2019

In the present study, analyses of initial behavior of an air-launched projectile for varying initial conditions are performed by coupling computational fluid dynamics and 6 degrees of freedom calculations. Accuracy of the present numerical methods is validated by comparing the present result with the measured data. Launching safety analyses are carried out for various ejecting conditions by considering weight of the projectile and magnitude of front and rear ejector forces as the major parameters of initial behavior of the projectile. A response surface of the projectile launching safety is obtained in the range of the major parameters. In all the conditions of zero rear ejector force, unsafe launching behavior is observed. As the weight of the projectile decreases, the initial launching behavior becomes more unsafe.

• Journal of Aerospace System Engineering
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• v.13 no.1
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• pp.1-10
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• 2019

The flow characteristics in a stirred tank are very useful in a wide variety of industrial applications. Generally, the flow pattern, power consumption and mixing time in stirred vessels depend not only on the design of the impeller, but also on the tanks' geometry and internal structure. In this study, the analysis of an unstable and unsteady complicated flow characteristics generated by the interaction between the baffle shape and impeller were performed using the ANSYS FLUENT LES Turbulence Model. The study compared the predictions of CFD with the interaction between two types of rotating impellers (axial and radial flows) and the shapes of three baffles. The results of the comparison verified that the design model showed a relatively efficient trend in the mixing flow fields and characteristics around the impeller and baffles during agitation.