• Bulletin of the Korean Chemical Society
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• v.34 no.10
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• pp.2995-3004
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• 2013

The structural, electronic, and optical properties of poly(3-hexylthiophene) (P3HT) have been comprehensively studied by density functional theory (DFT) to rationalize the experimentally observed properties. Rather, we employed periodic boundary conditions (PBC) method to simulate the polymer block, and calculated effective charge mass from the band structure calculation for describing charge transport properties. The simulated results of P3HT are consistent with the experimental results in band gaps, absorption spectra, and effective charge mass. Based on the same calculated methods as P3HT, a series of polymers have been designed on the basis of the two types of building blocks, furofurans and furofurans substituted with cyano (CN) groups, to investigate suitable polymers toward polymer solar cell (PSC) materials. The calculated results reveal that the polymers substituted with CN groups have good structural stability, low-lying FMO energy levels, wide absorption spectra, and smaller effective masses, which are due to their good rigidity and conjugation in comparison with P3HT. Besides, the insertion of CN groups improves the performance of PSC. Synthetically, the designed polymers PFF1 and PFF2 are the champion candidates toward PSC relative to P3HT.

• Structural Engineering and Mechanics
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• v.74 no.1
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• pp.91-104
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• 2020

A mathematical analysis has been carried out for understanding the traversal attributes of torsional waves in a Voigt-type viscoelastic porous layer bounded with corrugated surfaces resting over a heterogeneous transversely isotropic gravitating semi-infinite medium. Both the media are assumed to be under the effect of initial stresses acting along horizontal directions. In the presumed geometry, continuous and periodic type of corrugation has been considered. The condensed form of dispersion relation has been obtained analytically with the aid of the Whittaker's function and suitable boundary conditions. The influence of viscoelasticity, porosity, initial stresses, heterogeneity, gravity, undulation and position parameters on the phase and damped velocities has been illustrated graphically. In addition, relative examination investigating the impact of corrugated and planar bounded surfaces on the dispersion and damping characteristics is one of the important highlights of this study.

• Structural Engineering and Mechanics
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• v.28 no.6
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• pp.749-765
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• 2008

Free axisymmetric vibrations of layered cylindrical shells of variable thickness are studied using spline function approximation techniques. Three different types of thickness variations are considered namely linear, exponential and sinusoidal. The equations of axisymmetric motion of layered cylindrical shells, on the longitudinal and transverse displacement components are obtained using Love's first approximation theory. A system of coupled differential equations on displacement functions are obtained by assuming the displacements in a separable form. Then the displacements are approximated using Bickley-spline approximation. The vibrations of two-layered cylindrical shells, made up of several types of layered materials and different boundary conditions are considered. Parametric studies have been made on the variation of frequency parameter with respect to the relative layer thickness, length ratio and type of thickness variation parameter.

• Earthquakes and Structures
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• v.12 no.4
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• pp.409-423
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• 2017

This paper proposes a quick and simplified method to describe masonry vaults in global seismic analyses of buildings. An equivalent macro-element constituted by a set of six trusses, two for each transverse, longitudinal and diagonal direction, is introduced. The equivalent trusses, whose stiffness is calculated by fully modeled vaults of different geometry, mechanical properties and boundary conditions, simulate the vault in both global analysis and local analysis, such as kinematic or rocking approaches. A parametric study was carried out to investigate the influence of geometrical characteristics and mechanical features on the equivalent stiffness values. The method was numerically validated by performing modal and transient analysis on a three naves-church in the elastic range. Vibration modes and displacement time-histories were compared showing satisfying agreement between the complete and the simplified models. This procedure is particularly useful in engineering practice because it allows to assess, in a simplified way, the effectiveness of strengthening interventions for reducing horizontal relative displacements between vault supports.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.18 no.6
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• pp.106-112
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• 2019

Rotary swaging is a method of forging automotive drive shafts. In this paper, we propose a new two-hammer forging technique by applying the problem-solving approach TRIZ to improve the efficiency and productivity of the rotary swaging automation process. We will simplify the materials and hammers via the 3D modeling tool SolidWorks for high accuracy of a comparative analysis of existing and proposed methods under the same boundary conditions. In addition, we will compare the stress trends of the proposed model using ANSYS Workbench and verify the feasibility through a comparison of the simulation results using DEFORM. Relative to the existing method, the proposed method can decrease production costs and improve efficiency of the automation process by reducing the power source.

• Nuclear Engineering and Technology
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• v.55 no.4
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• pp.1400-1409
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• 2023

In this study, three condensation models of the CUPID code, i.e., the resolved boundary layer approach (RBLA), heat and mass transfer analogy (HMTA) model, and an empirical correlation, were tested and validated against the COPAIN and CAU tests. An improvement on HMTA model was also made to use well-known heat transfer correlations and to take geometrical effect into consideration. The RBLA was a best option for simulating the COPAIN test, having mean relative error (MRE) about 0.072, followed by the modified HMTA model (MRE about 0.18). On the other hand, benchmark against CAU test (under natural convection and occurred on a slender tube) indicated that the modified HMTA model had better accuracy (MRE about 0.149) than the RBLA (MRE about 0.314). The HMTA model with wall function and the empirical correlation underestimated significantly, having MRE about 0.787 and 0.55 respectively. When using the HMTA model, consideration of geometrical effect such as tube curvature was essential; ignoring such effect leads to significant underestimation. The HMTA and the empirical correlation required significantly less computational resources than the RBLA model. Considering that the HMTA model was reasonable accurate, it may be preferable for large-scale simulations of containment.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.5
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• pp.20-26
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• 2019

Where hydraulic structures are to be installed over the entire width of a river or stream, usually a bed protection structure is to be installed. However, a local scour occurs in which the river bed downstream of the river protection system is eroded due to the influence of the upstream flow characteristics. This local scour is dominant in the flow and turbulence characteristics at the boundary of the flow direction and in the material of the bed materials, and may gradually become dangerous over time. Therefore, in this study, we compared the turbulence patterns in the local scour hole at the downstream of the river bed protection with the results of the analysis of the mobile bed experiment, and compared with the application of OpenFoam, a three dimensional numerical analysis model. The distribution of depth-averaged relative turbulence intensities along the flow direction was analyzed. In addition to this result, the stabilization of scour hole was compared with the bed shear stress and Shields parameter, and the results were compared by changing the initial turbulent flow conditions. From the results, it was confirmed that the maximum depth of generation of the three-stage was dominantly developed by the magnitude of depth-averaged relative turbulence intensity rather than the mean flow velocity. This result also suggests that design, construction or gate control are needed to control the depth-averaged relative turbulence intensities in order to reduce or prevent the local scour faults that may occur in the downstream part of the bed protection.

• Atmosphere
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• v.25 no.4
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• pp.639-657
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• 2015

There is an increasing need to improve the air quality over South Korea to protect public health from local and remote anthropogenic pollutant emissions that are in an increasing trend. Here, we evaluate the performance of the WRF-Chem (Weather Research and Forecasting-Chemistry) model in simulating near-surface air quality of major Korean cities, and investigate the impacts of time-varying chemical initial and lateral boundary conditions (IC/BCs) on the air quality simulation using a chemical downscaling technique. The model domain was configured over the East Asian region and anthropogenic MICS-Asia 2010 emissions and biogenic MEGAN-2 emissions were applied with RACM gaseous chemistry and MADE/SORGAM aerosol mechanism. Two simulations were conducted for a 30-days period on April 2010 with chemical IC/BCs from the WRF-Chem default chemical species profiles ('WRF experiment') and the MOZART-4 (Model for OZone And Related chemical Tracers version 4) ('WRF_MOZART experiment'), respectively. The WRF_MOZART experiment has showed a better performance to predict near-surface CO, $NO_2$, $SO_2$, and $O_3$ mixing ratios at 7 major Korean cities than the WRF experiment, showing lower mean bias error (MBE) and higher index of agreement (IOA). The quantitative impacts of the chemical IC/BCs have depended on atmospheric residence time of the pollutants as well as the relative difference of chemical mixing ratios between the WRF and WRF_MOZART experiments at the lateral boundaries. Specifically, the WRF_MOZART experiment has reduced MBE in CO and O3 mixing ratios by 60~80 ppb and 5~10 ppb over South Korea than those in the WRF-Chem default simulation, while it has a marginal impact on $NO_2$ and $SO_2$ mixing ratios. Without using MOZART-4 chemical IC, the WRF simulation has required approximately 6-days chemical spin-up time for the East Asian model domain. Overall, the results indicate that realistic chemical IC/BCs are prerequisite in the WRF-Chem simulation to improve a forecast skill of local air quality over South Korea, even in case the model domain is sufficiently large to represent anthropogenic emissions from China, Japan, and South Korea.

• Journal of the Korean Geotechnical Society
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• v.31 no.3
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• pp.51-62
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• 2015

Shear behavior obtained by direct shear tests is dependent on shear box and boundary condition. The objective of this study is to analyze problems of conventional direct shear test (type-A) and provide the reliable results by developing type-C direct shear apparatus. Experimental tests are carried out for Ulleung sand by using type-A and -C direct shear devices. The soil specimens, which are prepared at the relative density of 60%, and are applied to vertical confining stresses of 50, 100, 200, 300, and 400 kPa, are sheared at a constant shear strain rate of 0.5 mm/min. By comparing the results obtained by type-A and -C direct shear apparatus under constant normal load (CNL) condition, the performance of new one is verified. In addition, two constrained conditions including constant normal load (CNL) and constant pressure (CP) are applied to type-C one. Experimental results show that type-A direct shear apparatus has some problems such as rotating of loading plate and upper shear box, and the frictional forces between soil and inner wall of upper shear box. Thus, the shear strengths obtained by type-A device are overestimated or underestimated depending on shear box and boundary condition. On the other hand, type-C device produces clear and consistent test results regardless of constrained conditions. This study represents that type-C direct shear apparatus not only can solve the problems of type-A direct shear apparatus but provide the reliable results.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.4
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• pp.1813-1818
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• 2014

This paper presents the stability analysis of the haptic system including a human impedance using the Routh-Hurwitz criterion. The reflective force is computed from a virtual spring model and is transferred to a human operator using the first-order-hold method. The stability boundary conditions are induced and the relation among a virtual spring ($K_w$), the mass ($M_h$), the damping ($B_h$) and the stiffness ($K_h$) of a human impedance is analyzed. Hence the stability boundary of the virtual spring ($K_w$) is proposed as $K_w{\leq}54413{\sqrt{(M_h+M_d)(B_h+B_d)}}-0.486K_h$ when the sampling time is 1 ms. The average relative error is about 0.5% when the mathematical analysis results are compared with the results of the stability boundary model.