• Current Optics and Photonics
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• v.6 no.6
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• pp.583-589
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• 2022

Vanadium dioxide (VO₂) is a well-known material that undergoes insulator-to-metal phase transition near room temperature. Since the conductivity of VO₂ changes several orders of magnitude in the terahertz (THz) spectral range during the phase transition, VO₂-based active metamaterials have been extensively studied. Experimentally, it is reported that the metal nanostructures on the VO₂ thin film lowers the critical temperature significantly compared to the bare film. Here, we theoretically studied such early transition phenomena by developing an analytical model. Unlike experimental work that only measures transmission, we calculate the reflection and absorption and demonstrate that the role of absorption is quite different for bare and patterned samples; the absorption gradually increases for bare film during the phase transition, while an absorption peak is observed at the critical temperature for the metamaterials. In addition, we also discuss the gap width and VO₂ thickness effects on the transition temperatures.

• The Journal of the Korea institute of electronic communication sciences
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• v.13 no.1
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• pp.23-28
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• 2018

We derive a gap of specific heat discontinuity of superconducting crystals theoretically at the critical temperature $T_{CH}$ as an explicit function of applied magnetic field H by using the thermodynamic relations for Gibbs free energy and the linear model for the critical magnetic field $H_{CT}$. The derived a gap of specific heat discontinuity is compared with experimental results by J. Kacmarcik et al. for superconducting MgCNi3 crystal. Our specific heat gap function well explain the jump up phenomena of the superconducting crystals.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2004.03a
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• pp.774-778
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• 2004

The atomization process of a circular $SF_{6}$ liquid jet issued into an otherwise quiescent, high-pressure $N_2$ gas was observed to explore the breakup mechanism of liquid ligaments involved in turbulent atomization. Both liquid and gas temperatures were fixed at a room temperature but the gas pressure was elevated to more than twice the critical pressure of $SF_{6}$. Therefore, the liquid surface was in a thermodynamic state close to a critical mixing condition with suppressed vaporization. Since the surface tension and the surface gas density approach zero and the surface liquid density, respectively, phenomena equivalent to those which would appear when a very high speed laminar flow of water were injected into the atmospheric-pressure air can be observed by issuing $SF_{6}$ liquid at low speeds in micro-gravity environment which avoid disturbances due to gravity forces. The instability ob near-critical mixing surface jet was quantitatively characterized using a newly developed device, which could issue a very small amount of $SF_{6}$ liquid at small constant velocity into a very high-pressure $N_2$ gas.

• Earthquakes and Structures
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• v.9 no.4
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• pp.875-891
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• 2015

Earthquake can occur anywhere in the world and it is essential to design important members in special structures based on maximum possible forces that can be produced in them under severe earthquake. In addition, since the earthquake is an accidental phenomena and there are no similar earthquakes, therefore the possibility of strong earthquakes should be taken into account in earthquake-resistant design of important structures. Based on this viewpoint, finding the critical acceleration which maximizes internal forces is an essential factor in structural design. This paper proposes critical excitation method to compute the critical acceleration in design of important members in special structures. These critical accelerations are computed so that the columns' internal shear force at the base of the structure at each time step is maximized under constraints on ground motion. Among computed critical accelerations (of each time step), the one which produces maximum internal shear force is selected. A numerical example presents to show the efficiency of critical excitation method in determining the maximum internal shear force and base moment under variety of constraints. The results show that these method can be used to compute the resonant earthquake which have large enough effective duration of earthquake strong motion (between 12.86 sec to 13.38 sec) and produce the internal shear force and base moment for specific column greater than the same value for selected earthquakes in constructing the critical excitation (for different cases about 2.78 to 1.29 times the San Fernando earthquake). Therefore, a group of them can be utilized in developing the response spectrum for design of special structures.

• Journal of the Korean Home Economics Association
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• v.20 no.4
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• pp.193-204
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• 1982

This study attempts to clarify the relationship between mother;s child-rearing practices and children's critical thinking abilities. More specifically this study wants to answer the questions: (a) Is there any relationship between mother's child-rearing practices and children's critical thinking abilities? (b) What dimensions of child-rearing practices are more important for children's critical thinking abilities? (c) Does the developmental process of children's critical thinking abilities show any difference according to the mother's child-rearing practices? This study is based on the conceptual model of child-rearing practices from the cognitive disciplinary viewpoint proposed by Lee et al.(1973). They polarized the child-rearing practices into the“mediated”types. These types were specified along seven dimensions of child-rearing. In general terms, the two types refer to the degree to which mother resorts to the description of concrete and immediate phenomena and immediate expression of momentary impulses, and positively reinforces such behaviors of her children. Two kinds of data were collected from 60 primary school boys and girls, and their mothers. A child-rearing practices questionnaire with 28 items was sent to the mothers. And the children were administered a critical thinking abilities test containing measures of 4 dimensions from the Ahn's Critical Thinking Measurement Technique. The major findings are those (a) the children of mothers who reported to have used more“mediated”types of child-rearing show relatively higher level of critical thinking abilities than their counterparts; (b) of the seven dimensions of child-rearing practices, the most significant are“mode of description”,“rationality-orientation”and“reward and punishment”; and (c) the“mediated”type of child-rearing practices tends to facilitate critical thinking abilities development. In conclusion, it is believed that a further refinement of this study will contribute to the development of child-rearing training program for mothers.

• Fire Science and Engineering
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• v.17 no.3
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• pp.50-54
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• 2003

The backlayer phenomena of smoke in the road tunnel is evaluated through numerical experiments. A commercial code, PHOENICS is used to simulate smoke flow in the road tunnel. The independent and dependent variables are ventilation air velocity and the length of backlayer of smoke respectively. Hybrid scheme and $textsc{k}-\varepsilon$ turbulence model is adopted in the simulation process and mass residual is used as a convergence criterion. The experimental results say that the length of backlayer is reduced linearly with the increase of ventilating air velocity and that there is a critical air velocity which prevents from the onset of backlayering phenomena. One finds that there is a fresh air region near the bottom of tunnel which could make the passenger escape from the region polluted by smoke. These phenomena come from the severe vertical stratification of the smoke air mixture in the tunnel.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.4
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• pp.9-16
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• 2004

In the cavitation tunnel, the first corner playes role for the flow direction to execute 90-degree turn. So, energy loss is serious, and the cavitation phenomena well occur in the guide vane surface. In this paper, the flow in the first corner was numerically calculated. From the calculation result, cavitation phenomena mainly occurred in the suction side of the last guide vane and vicinity that vane and tunnel wall adjoin each other. And bubbles occurred from all guide vanes if the flow velocity in the test section reaches the any critical value. We could analogize with our experience in the water tunnel that bubbles that occurred in time not vanish, and become miniature in the flow although the pressure recover. So, they circulate with flow in the tunnel, and come into view in the test section. Therefore, first corner must be designed for bubbles not to appear in the test section according to the flow condition like velocity and pressure demanded by the experiment. We analyzed flow in case that the first elbow configuration was redesigned and some of the existing guides vanes were eliminated. And we presented that first elbow can be easely designed for the improvement of tunnel performance through the computational analysis.

• Computers and Concrete
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• v.29 no.1
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• pp.1-14
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• 2022

Several aspects influence corrosive processes in reinforced concrete (RC) structures such as environmental conditions, structural geometry and mechanical properties. Since these aspects present large randomnesses, probabilistic models allow a more accurate description of the corrosive phenomena. Besides, the definition of limit states in the reliability assessment requires a proper mechanical model. In this context, this study proposes a straightforward methodology for the mechanical-probabilistic modelling of RC structures subjected to reinforcements' corrosion. An improved damage approach is proposed to define the limit states for the probabilistic modelling, considering three main degradation phenomena: concrete cracking, rebar yielding and rebar corrosion caused either by chloride or carbonation mechanisms. The stochastic analysis is evaluated by the Monte Carlo simulation method due to the computational efficiency of the Lumped Damage Model for Corrosion (LDMC). The proposed mechanical-probabilistic methodology is implemented in a computational framework and applied to the analysis of a simply supported RC beam and a 2D RC frame. Curves illustrate the probability of failure evolution over a service life of 50 years. Moreover, the proposed model allows drawing the probability of failure map and then identifying the critical failure path for progressive collapse analysis. Collapse path changes caused by the corrosion phenomena are observed.

• Journal of the Korean Institute of Navigation
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• v.17 no.3
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• pp.85-92
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• 1993

In connection to the design of high speed vessels, the numerical simulation is carried out to make clear the property of flows and breaking phenomena around the catamaran. It is because the bradking phenome-non is closely related to the free-surface turbulent flow. The free-surface wave and transverse velocity vectors are calculated around the twin and demi hull of the catamaran. Computed results are applied to detect the appearance of sub-breaking waves around the hull. The critical condition for their appearance is studied at two Froude numbers of 0.45 and 0.95. The nu-merical analysis shows that the breaking is more serious near the twin hull rather the demi hull. To simu-late the flows, the Navier-Stokes solver is invoked with a free-surface. The computation is made only in half a domain because it is symmetric in the shape.

• Proceedings of the KSR Conference
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• 2011.10a
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• pp.643-648
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• 2011

Fatigue crack in most rails take place by rolling contact between wheel and rail in railway industry. Therefore, it is critical to understand the rolling contact phenomena, especially for the three-dimensional situation. In this paper the steady-state rolling contact problem of KTX wheel and rail (UIC60) has been studied with three-dimensional finite element analysis. The variation of contact pressure and contact stresses on rolling contact surface were obtained using the finite element method. The three-dimensional distribution of contact stresses on the contact surface are investigated. Results show that the distribution of shear stress and contact stress (von Mises) on the contact surface varies rapidly as a result of the variation of stick-slip region. The contact stress at the leading edge is greater than at the trailing edge because of stick and slip phenomena.