• 한국신재생에너지학회:학술대회논문집
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• 2011.05a
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• pp.195.2-195.2
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• 2011

The borehole heat exchanger of Geothermal Heat Pump (GHP) system should be sustainable and cost effective for long term operation. To guaranty the performance of the system thermal Response Tests (TRTs) with simple recommended procedures have been applied in many countries. Korea government developed a standard TRT procedure in order to control the quality on GHP projects. In the TRT procedure interpretation method has a rule that data set has to be interpreted by the line source model(LSM). The LSM employes some assumptions that surrounding medium is homogeneous and the line source is infinite and constant heat flux, however real ground condition is unisotropic and heterogeneous, and showing regional or local ground water flows in many cases. We need to develope improved evaluation models to estimate accurate ground thermal conductivity with respect to geological and influence of ground water because current TRT standard test procedure has limitations to be applied for every locations and system. This study surveyed the uncertainty of the thermal parameters from the interpretation method considering different evaluation period. The interpretation of 208 TRT data sets represents limitations of LSM application that some obtained ground thermal conductivities are statistically unstable and convergence time of ground thermal conductivity over test period shows trends responding the length of test period. This evaluation study will be helpful to provide some effective procedure for the thermal parameter estimation and to complement current TRT standard procedure.

• 한국신재생에너지학회:학술대회논문집
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• 2007.06a
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• pp.378-382
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• 2007

To improve the reliability for a wind-turbine gearbox, the mechanical loads acting on the gearbox need to be monitored and analysed exactly. This study was conducted to identify the characteristics of torques and bending moments acting on the main shaft of the gearbox using the rainflow counting method and predict the fatigue life of the main shaft by using the modified Miner's rule. While the mean wind speed became 3.5 m/s, the life of the main shaft by the acting torques was predicted as 4.3${\times}10^6$ years, and it by the bending moments was as 2.3${\times}10^4$ years. If the life of the wind turbine was estimated as 20 years, the fatigue life of the main shaft was regarded as infinite. Also, it was suggested that the life of the main shaft must be predicted by not the torques but the bending moments.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.645-645
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• 2013

Applying a first-principles computational approach combining density-functional theory and matrix Green's function calculations, we have studied the effects [2+2] cycloaddition olligormerization of fullerene $C_{60}$ chains on their junction charge transport properties. Analyzing first the microscopic mechanism of the switching realized in recent scanning tunneling microscope (STM) experiments, we found that, in agreement with experimental conclusions, the device characteristics are not significantly affected by the changes in electronic structure of $C_{60}$ chains. It is further predicted that the switching characteristics will sensitively depend on the STM tip metal species and the associated energy level bending direction in the $C_{60}-STM$ tip vacuum gap. Considering infinite $C_{60}$ chains, however, we confirm that unbound $C_{60}$ chains with strong orbital hybridizations and band formation should in principle induce a much higher conductance state. We demonstrate that a nanoelectromechanical approach in which the $C_{60}-STM$ tip distance is maintained at short distances can achieve a metal-independent and drastically improved switching performance based on the intrinsically better electronic connectivity in the bound $C_{60}$ chains.

• Journal of the Korean Society of Safety
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• v.17 no.2
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• pp.8-15
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• 2002

In order to investigate the vibration characteristics of fluid-structure interaction problem, we modeled two identical rectangular plates coupled with fluid. A commercial computer code, ANSYS was used to perform finite element analysis and FEM solutions were compared with the experimental results to verify the finite element model. As a result, comparison of FEM and experiment showed good agreement, and the transverse vibration modes, in-phase and out-of-Phase, were observed alternately in the fluid-coupled system. The effect of fluid gap size on the fluid-coupled natural frequency were investigated. It was shown that the mode numbers increased, the normalized natural frequencies monotonically increased. And it was also found that an increase of the fluid gap reduced the coupled natural frequencies for the in-phase modes but increased the coupled natural frequencies for the out-of phase modes, and eventually converged to the results of an infinite fluid gap.

• Interaction and multiscale mechanics
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• v.4 no.2
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• pp.85-105
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• 2011

The influence of surface elasticity and surface residual stress on the elastic field of an isotropic nanoscale elastic layer of finite thickness bonded to a rigid material base is considered by employing the Gurtin-Murdoch continuum theory of elastic material surfaces. The fundamental solutions corresponding to buried vertical and horizontal line loads are obtained by using Fourier integral transform techniques. Selected numerical results are presented for the cases of a finite elastic layer and a semi-infinite elastic medium to portray the influence of surface elasticity and residual surface stress on the bulk stress field. It is found that the bulk stress field depends significantly on both surface elastic constants and residual surface stress. The consideration of out-of-plane terms of the surface stress yields significantly different solutions compared to previous studies. The solutions presented in this study can be used to examine a variety of practical problems involving nanoscale/soft material systems and to develop boundary integral equations methods for such systems.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2008.04a
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• pp.54-58
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• 2008

The structural intensities have been applied to understand a source point and the path of vibrational energy flows in interested structures by many researchers. In this paper, a feasibility study was carried out to investigate the characteristics of a damaged beam with a inflicted open crack using the structural intensities. The damaged beam was taken as a continuous system with equivalent bending stiffness and the flexural vibrations were only considered in numerical simulation and experiments. A four(4)-transducer array was used to measure the flexural vibrations of the beam and the structural intensities were estimated by means of cross spectral density method. As a result, the magnitude changes of the structural intensities could be observed in the vicinity of the damage location and a damage index was newly proposed to identify the damage zone. It has been confirmed that the measurement of the structural intensities was simple and effective method to find out the damage zone.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.10
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• pp.1598-1608
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• 1997

Based on the application of te theory of conjugate approximations and the Loubignac's iterative method in a local region, a method to improve the stress filed in a displacement-formulated finite element solution has been proposed. The validity of the proposed method has been tested through two examples : a thick cylinder under internal pressure loading and an infinite plate with a central circular hole subjected to uniaxial tension. As a result of analysis of the examples, it was found that the stress field obtained for the local region model by the proposed method approximates well for the whole domain model. In addition, it was found that because of a significant decrease in the computing time to obtain the improved stress field, the proposed method is efficient and useful for the detailed stress analysis in local regions.

• Journal of the Korean Society of Propulsion Engineers
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• v.1 no.2
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• pp.22-31
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• 1997

We developed a general purpose program for the analysis of flows in the combustor with recirculating flow regime and simulated the flows. The program uses non-staggered grids based on finite volume method and the primitive variables are cartesian velocities. The combustion model is irreversible one step reaction with infinite chemistry The Favre averaged governing equations are considered and the clipped gaussian distribution is considered as a probability density function of the conserved scalar. We calculated turbulent diffusion flame with recirculating flow regime. Simulation shows two recirculating regions like experimental results. Velocity, turbulent kinetic energy, temperature and concentration distribution in simulation agree well with experimental data.

• Nuclear Engineering and Technology
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• v.23 no.3
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• pp.275-284
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• 1991

It is important to predict the main feature of fully developed turbulent secondary flow through infinite triangular arrays of parallel rod bundles. One-equation turbulence model which include anisotropic eddy viscosity model was applied to predict the exact velocity field. For a constant properties, Reynolds equations were solved by the finite element method. Mean axial velocity near the wall is simulated by the law of the wall. The numerical results showed good agreement with avaiable experimental data. The strength of the secondary flow increased with Reynolds number but decreased with rod spacing, P/D (pitch-to-diameter). The secondary flow affects remarkably the distribution of the axial velocity, wall shear stress and turbulent kinetic energy in the closely packed rod array bundles.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.690-695
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• 2000

This article deals with the analytic results on the development and application of the Spherical CVT. The Spherical CVT is marked by its simple configuration, the infinite torque multiplication characteristic, and the smooth transitions between forward/neutral/reverse states of output speed. In this study, we describe the conceptual principles behind the Spherical CVT and some applications of it, which we developed recently. And, we propose the shifting algorithm based on the analytic consideration of CVT powertrain system. Contrary to conventional shifting algorithms using the OOL(optimal operating line) of the power source, the proposed shifting algorithm is represented as a $2^{nd}$ order equation in an explicit form, and it reveals the possibility of theoretic design of all optimal controller. As an example, we present numerical results that demonstrate the energy saving possible and the proposed shifting algorithm from the use of the Spherical CVT over standard reduction gear unit, using an ideal dc motor model.