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

• Computational Structural Engineering
• /
• v.8 no.2
• /
• pp.153-161
• /
• 1995

The dynamic instability for snapping phenomena has been studied by many researchers. There is few paper which deal with the dynamic buckling under the load with periodic characteristics, and the behavior under periodic excitation is expected the different behavior against step excitation. In this study, the dynamic direct snapping of shallow elliptic paraboloidal shells is investigated under not only step excitation but also sinusoidal and seismic excitations, applied in the up-and-down direction. The dynamic nonlinear responses are obtained by the numerical integration of the geometrically nonlinear equations of motion, and examined by the Fourier spectral analysis in order to get the frequency-dependent characteristics of the dynamic instability for various load levels. The results show that the dynamic instability phenomenon carried out from stable to unstable region reveals considerably different mechanism depending on the characteristics of excitations.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.19 no.6
• /
• pp.447-456
• /
• 2007

Suspended particles behavior when they go through a vertical riser with heat transfer is of significant concern to system designers and operators in pneumatic transport, various processes such as in chemical, pharmaceutical and food industries. When it comes with the energy system, that knowledge is critical to the reliable design practices of related equipment as heat exchangers, especially in the phase of system scale-up. Without haying a good understanding of the related physics, many scale-up practices based on their pilot plant experience suffer from unexpected behaviors and problems of unstable fluidization typically associated with excessive pressure drop, pressure fluctuation and even unsuccessful particle circulation. In the present study, we try to explain the observed phenomena with related physics, which may help understanding of our unanswered experiences and to provide the designers with more reliable resources for their work. We selected hot exhaust gas with solid particle that goes through a heat exchanger riser as our model to be considered. The effect of temperature change on the gas velocity, thermodynamic properties, and eventually on the particles motion behavior is reviewed along with some heat transfer analyses. The present study presents an optimal riser length at full scale under given conditions, and also defines the theoretical limiting length of the riser. The field data from the numerical analysis was validated against our experimental results.

• Tunnel and Underground Space
• /
• v.5 no.1
• /
• pp.22-40
• /
• 1995

Thermomechanical characteristics of rocks such as thermal shock, thermal expansion, thermal cracking were experimentally investigaed using Iksan granite, Cheonan tonalite and Chung-ju dolomite to obtain the basic data for proper design and Chung-ju dolomite to obtain the basic data for proper design and stability analysis of underground structures subjected to temperature changes. The effect of thermal shock did not appear when the heating speed was under 3$^{\circ}C$/min. and there existed little difference between multi-staged cyclic heating and single-cycled heating. Thermal expansion of rocks was affected by mineral composition, crack porosity and the degree of thermal craking. In quartz-beraring multimineralic rocks such as Iksan granite and Cheonan tonalite, the thermal expansion coefficient increaseed continuously with temperature rise, but that of Chung-ju dolomite which was a monomineralic rock showed a constant value for the temperature above 250$^{\circ}C$, Chung-ju dolomite yielded the lowest critical threshold temperature(Tc) of 100$^{\circ}C$ and unstable thermal cracking was initiated above the new threshold temperature(Tc')of 300$^{\circ}C$. Above Tc' thermal cracks grew but they were not interconnected. Iksan granite showed closing of microcracks to the temperature of 100$^{\circ}C$, then expanded linearly to Tc of 200$^{\circ}C$. Above Tc, thermal cracking was initiated and progressed rapidly and almost all the grain boundaries were cracked at 600$^{\circ}C$. Cheonan tonalite also showed similar behavior to iksan granite except that Tc was 350$^{\circ}C$ and that thermal cracks propagated more rapidly. Thermal expansions calculated by Turner's equation were found to be valid in predicting the thermal expansion and cracking behavior of rocks.

• The Journal of Engineering Geology
• /
• v.25 no.3
• /
• pp.331-337
• /
• 2015

A new technique is presented to detect unstable slope behavior, based on Hotelling's T² analysis of pore pressure and water content obtained during flume tests using granitic and gneissic weathered soils. Three sets of pore pressure-water content values were simultaneously obtained during each test, and T² statistics at the 90.0% and 95.0% confidence levels were calculated based on the correlations between values. The results show that unsuccessful detection of some local failures of the flume slope depended on the sensor position. In the case of global slope failures, anomalous behavior was detected between several hundred and several thousand seconds before the event as T² statistics exceeded the confidence interval 90%. Hotelling's T² analysis provides a single control criterion because it enables correlations between diverse measured values within the same slope; the criterion also includes stepwise criteria for a forecasting and warning system based on confidence levels.

• Journal of the Korean Ceramic Society
• /
• v.49 no.6
• /
• pp.586-591
• /
• 2012

$Zr_2WP_2O_{12}$ powder, which has a negative thermal expansion coefficient, was synthesized by a solid-state reaction with $ZrO_2$, $WO_3$ and $NH_4H_2PO_4$ as the starting materials. The synthesis behavior was dependent on the solvent media used in the wet mixing process. The $Zr_2WP_2O_{12}$ powder prepared with a solvent consisting of D. I. water was fully crystallized at $1200^{\circ}C$, showing a sub-micron particle size. According to the results obtained from a thermal analysis, a $ZrP_2O_7$ was synthesized at a low temperature of $310^{\circ}C$, after which it was reacted with $WO_3$ at $1200^{\circ}C$. A new sintering additive, $Al(OH)_3$, was applied for the densification of the $Zr_2WP_2O_{12}$ powders. The cold isostatically pressed samples were densified with 1 wt% $Al(OH)_3$ additive or more at $1200^{\circ}C$ for 4 h. The main densification mechanism was liquid-phase sintering due to the liquid which resulted from the reaction with amorphous or unstable $Al_2O_3$ and $WO_3$. The densified $Zr_2WP_2O_{12}$ ceramics showed a relative density of 90% and a negative thermal expansion coefficient of $-3.4{\times}10^{-6}/^{\circ}C$. When using ${\alpha}-Al_2O_3$ as the sintering agent, densification was not observed at $1200^{\circ}C$.

• The Journal of Engineering Geology
• /
• v.18 no.4
• /
• pp.433-437
• /
• 2008

It's essential to build an earth retaining structure at the beginning and end point of a tunnel constructed in a colluvium area. A large scale of colluvial soil may cause a problem to the stability of the excavation ground. An excavation in colluvium has different behavior characteristics from those in a sandy soil due to unstable elements and needs counter measures for it. There are few systematic research efforts on the behavior characteristics of an earth retaining structure installed in colluvial soil. Thus this study set out to collect measuring data from an excavation site at the tunnel pit mouth in colluvium and set quantitative criteria for the safety of an earth retaining structure. After comparing and analyzing the theoretical and empirical earth pressure from the measuring data, the lateral earth pressure distribution acted on the earth retaining wall was suggested.

• Korean Journal of Community Nutrition
• /
• v.3 no.3
• /
• pp.358-367
• /
• 1998

The study purpose was to investigate psychosocial factors related to smoking among adolescent boys. The Theory of Planned Behavior provide the basis for the study. Twenty-five attitudinal beliefs, 9 normative beliefs and 20control beliefs were identified through questionnaire development. The data were analyzed using t-test and χ2-test. Thirty-three percent of 300 students were smokers. Most of the beliefs examined were significantly different between smokers(n=92) and nonsmokers(n=92). With respect to attitudinal beliefs, smokers responded less negatively on the items of bad health effects of smoking such as sore throat, headache, chest pain, risk of cancer and bad blood circulation(p<0.001), and decreased physical strength(p<0.05). Smokers believed less negatively on the items that smoking leads to bothering others, bad breath, yellow teeth and making them spend money(p<0.001). In contrast, smokers felt more positively on smoking as a means of stress management, relaxing, helping digestion(P<0.001). Smokers felt less pressure for not smoking from significant others. Especially mother, siblings, friends, girl friends, seniors of school were important sources of influence regarding subject's smoking. Smokers felt less confident in controlling the urge to smoke in several situations including; when they were with friends or asked to smoke by friends; after the meal, or drinking; when they were bored or stressed, upset, and when they felt unstable(p<0.001). Smokers also scored lower on specific skills to quit or control the urge to smoke as well as overall perceived control, compared to nonsmokers(p<0.001). These results suggest that interventions for adolescents incorporate diverse strategies to increase the perceived control over smoking in specific situations as well as overall perceived control, to help them realize and modify attitudinal beliefs, and to elicit support from significant others for not smoking. (Korean J Community Nutrition 3(3) : 358∼367,1998)

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.17 no.4
• /
• pp.30-37
• /
• 2013

A soft rock fracture zone is an important element for rock slope or earth retaining, however stability studies of earth retaining wall have been lack. Therefore, this study is analyzed for a behavior of earth retaining wall with condition of fracture zone or no fracture zone and then a numerical analysis (Finite Element Method) was performed considering interaction with field monitoring data between ground and structures. As a result, applied horizontal displacement on retaining wall is correspond between result of numerical analysis and field monitoring data and displacement point stress distribution with fracture zone condition analyzed to be stable side but no fracture zone condition is expressed to be unstable side. The results of this study is purpose for applying safety construction as a top priority at field when designing for future.

• Korean Journal of Metals and Materials
• /
• v.46 no.12
• /
• pp.771-779
• /
• 2008

An effective way of increasing the strength and fracture toughness of reactor pressure vessel steels is to change the material specification from that of Mn-Mo-Ni low alloy steel(SA508 Gr.3) to Ni-Mo-Cr low alloy steel(SA508 Gr.4N). In this study, we evaluate the effects of alloying elements on the microstructural characteristics of Ni-Mo-Cr low alloy steel. The changes in the stable phase of the SA508 Gr.4N low alloy steel with alloying elements were evaluated by means of a thermodynamic calculation conducted with the software ThermoCalc. The changes were then compared with the observed microstructural results. The calculation of Ni-Mo-Cr low alloy steels confirms that the ferrite formation temperature decreases as the Ni content increases because of the austenite stabilization effect. Consequently, in the microscopic observation, the lath martensitic structure becomes finer as the Ni content increases. However, Ni does not affect the carbide phases such as $M_{23}C_6 $ and $M_7C_3$. When the Cr content decreases, the carbide phases become unstable and carbide coarsening can be observed. With an increase in the Mo content, the $M_2C$ phase becomes stable instead of the $M_7C_3$ phase. This behavior is also observed in TEM. From the calculation results and the observation results of the microstructure, the thermodynamic calculation can be used to predict the precipitation behavior.