• Smart Structures and Systems
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• v.3 no.4
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• pp.439-454
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• 2007

In this paper a damage imaging technique using pre-stack migration is developed using Lamb (guided) wave propagation in composite structures for imaging multi damages by both numerical simulations and experimental studies. In particular, the paper focuses on the experimental study using a finite number of sensors for future practical applications. A composite laminate with a surface-mounted linear piezoelectric ceramic (PZT) disk array is illustrated as an example. Two types of damages, one straight-crack damage and two simulated circular-shaped delamination damage, have been studied. First, Mindlin plate theory is used to model Lamb waves propagating in laminates. The group velocities of flexural waves in the composite laminate are also derived from dispersion relations and validated by experiments. Then the pre-stack migration technique is performed by using a two-dimensional explicit finite difference algorithm to back-propagate the scattered energy to the damages and damages are imaged together with the excitation-time imaging conditions. Stacking these images together deduces the resulting image of damages. Both simulations and experimental results show that the pre-stack migration method is a promising method for damage identification in composite structures.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.15 no.4
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• pp.30-39
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• 2016

There have been many cases of deaths from crushing caused by dense crowds. Numerous studies about pedestrian flow have performed various simulations, but the experimental data to prove the simulations are still not enough. In this paper, the evacuation of pedestrians for proving pedestrian flow simulation is observed. Due to the possibility of real casualties, it is difficult to experiment with humans directly. Therefore, ten C57BL/6NCrSIc mice have been used. It is assumed that C57BL/6NCrSIc mice act like humans in panic situations. Electrical Stimulus Experiments on mice are conducted for exits with various angles. ICY software is applied in this paper. As a result, the mice escape fast at a proper angle of 45 to 60 degrees.

• Atmosphere
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• v.20 no.3
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• pp.319-332
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• 2010

Sensitivity experiments of WRF model using different planetary boundary layer (PBL) and land surface model (LSM) parameterizations are evaluated for prediction of wind fields within the surface layer. The experiments were performed with three PBL schemes (YSU, Pleim, MYJ) in combination with three land surface models (Noah, RUC, Pleim). The WRF model was conducted on a nested grid from 27-km to 1-km horizontal resolution. The simulations validated wind speed and direction at 10 m and 80 m above ground level at a 1-km spatial resolution over the South Korea. Statistical verification results indicate that Pleim and YSU PBL schemes are in good agreement with observations at 10 m above ground level, while the MYJ scheme produced predictions similar to the observed wind speed at 80 m above ground level. LSM comparisons indicate that the RUC model performs best in predicting 10-m and 80-m wind speed. It is found that MYJ (PBL) - RUC (LSM) simulations yielded the best results for wind field in the surface layer. The choice of PBL and LSM parameterization will contribute to more accurate wind predictions for air quality studies and wind power using WRF.

• Journal of the Korea Safety Management & Science
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• v.22 no.3
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• pp.15-21
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• 2020

This study analyzed the evacuation time in indoor stadiums when exits that automatically open/close when the fire sensor is triggered are installed as a means to improve the problem of closing certain exits. Firstly, when spectators on the 2nd floor stands exit through the 1st floor exits, the RSET of all inhabitants was 529.8 seconds when the automatic opening/closing exits are broken and employees are not present. Secondly, when spectators on the 2nd floor stands exit through the 1st floor exits, the RSET of all inhabitants was 445 seconds when the automatic opening/closing exits with 750mm width are working but employees are not present. Lastly, when spectators on the 2nd floor stands exit through the 1st floor exits, the RSET of all spectators was 337 seconds when the automatic opening/closing exits with 1,500mm width are working and employees are present. As a result, it was revealed that the evacuation time is shortened when the automatic opening/closing exits are working. Additional comparative studies with actual simulations of people evacuating an indoor stadium and firefighting simulations considering smoke flow are necessary.

• Bulletin of the Korean Chemical Society
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• v.33 no.11
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• pp.3719-3726
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• 2012

Formation of Z-DNA, a left-handed double helix, from B-DNA, the canonical right-handed double helix, occurs during important biological processes such as gene expression and DNA transcription. Such B-Z transitions can also be induced by high salt concentration in vitro, but the changes in the relative stability of B-DNA and Z-DNA with salt concentration have not been fully explained despite numerous attempts. For example, electrostatic effects alone could not account for salt-induced B-Z transitions in previous studies. In this paper, we propose that the B-Z transition can be explained if counterion entropy is considered along with the electrostatic interactions. This can be achieved by conducting all-atom, explicit-solvent MD simulations followed by MM-PBSA and molecular DFT calculations. Our MD simulations show that counterions tend to bind at specific sites in B-DNA and Z-DNA, and that more ions cluster near Z-DNA than near B-DNA. Moreover, the difference in counterion ordering near B-DNA and Z-DNA is larger at a low salt concentration than at a high concentration. The results imply that the exclusion of counterions by Z-DNA-binding proteins may facilitate Z-DNA formation under physiological conditions.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.37.2-37.2
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• 2014

Gas in disk galaxies interacts nonlinearly with a underlying stellar spiral potential to form galactic spiral shocks. Numerical simulations typically show that these shocks are unstable to the wiggle instability, forming non-axisymmetric structures with high vorticity. While previous studies suggested that the wiggle instability may arise from the Kelvin-Helmholtz instability or orbit crowding of gas elements near the shock, its physical nature remains uncertain. It was even argued that the wiggle instability is of numerical origin, caused by the inability of a numerical code to resolve a shock that is inclined to numerical grids. In this work, we perform a normal-mode linear stability analysis of galactic spiral shocks as a boundary-value problem. We find that the wiggle instability originates physically from the potential vorticity generation at a distorted shock front. As the gas follows galaxy rotation, it periodically passes through multiple shocks, successively increasing its potential vorticity. This sets up a normal-mode that grows exponentially, with a growth rate comparable to the orbital angular frequency. We show that the results of our linear stability analysis are in good agreement with the those of local hydrodynamic simulations of the wiggle instability.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.26 no.3
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• pp.73-79
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• 2012

A counterpoise is commonly employed in grounding systems installing near the ground surface of low resistivity soils and radial-type counterpoises are used in the limited space. Recently some studies on the evaluation of ground impedance of paralleling ground electrodes have carried out, but the data for providing the frequency-dependent ground impedances considering potential interferences are not yet sufficient. In order to provide the information about the design of grounding systems for surge protection, the simulations of the frequency-dependent ground impedance of various shaped counterpoises are carried out by using the distributed parameter circuit model including the effect of potential interferences. This paper presents the theoretical simulations and actual experiments of the frequency-dependent ground impedance of paralleling and 3 or 4-arms star counterpoises. The accuracy of the simulation methodology is examined by the comparison with the measured results, and the results show a good agreement between the simulation and the experiment.

• Advances in aircraft and spacecraft science
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• v.1 no.4
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• pp.409-425
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• 2014

Converging flows of a gas and a liquid at a microchannel cross junction, under proper conditions, can result in the formation of periodic, dispersed microslugs. This microslug formation phenomenon has been proposed as the basis for a fuel injection system in a novel, 'discrete' monopropellant microthruster designed for use in next-generation miniaturized satellites. Previous experimental studies demonstrated the ability to generate fuel slugs with characteristics commensurate with the intended application during steady-state operation. In this work, numerical and experimental techniques are used to study the effect of valve actuation on slug characteristics, and the results are used to compare with equivalent steady-state slugs. Computational simulations of a valve with a 1 ms valve-actuation cycle show that as the ratio of the response time of the valve to the fully open time is increased, transient effects can increase slug length by up to 17%. The simulations also demonstrate that the effect of the valve is largely independent of surface tension coefficient, which is the thermophysical parameter most responsible for slug formation characteristics. Flow visualization experiments performed using a miniature valve with a 20 ms response time showed less than a 1% change in the length of slugs formed during the actuation cycle. The results of this study indicate that impulse bit and thrust calculations can discount transient effects for slower valves, but as valve technology improves transient effects may become more significant.

• Korean Journal of Agricultural Science
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• v.47 no.3
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• pp.509-517
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• 2020

Geographic information system (GIS) sewer network data are a fundamental input material for urban inundation modeling, which is important to reduce the increasing damages from urban inundation due to climate change. However, the essential attributes of the data built by a local government are often missing because the purpose of building the data is the maintenance of the sewer system. Inconsistent simplification and supplementation of the sewer network data made by individual researchers may increase the uncertainty of flood simulations and influence the inundation analysis results. Therefore, it is necessary to develop a basic algorithm to convert the GIS-based sewage network data into input data that can be used for inundation simulations in consistent way. In this study, the format of GIS-based sewer network data for a watershed near the Sadang Station in Seoul and the Oncheon River Basin in Busan was investigated, and a missing data supplementing algorithm was developed. The missing data such as diameter, location, elevation of pipes and manholes were assumed following a consistent rule, which was developed referring to government documents, previous studies, and average data. The developed algorithm will contribute to minimizing the uncertainty of sewer network data in an urban inundation analysis by excluding the subjective judgment of individual researchers.

• Journal of The Korean Astronomical Society
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• v.36 no.3
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• pp.105-110
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• 2003

Recent observations of galaxy clusters in radio and X-ray indicate that cosmic rays and magnetic fields may be energetically important in the intracluster medium. According to the estimates based on theses observational studies, the combined pressure of these two components of the intracluster medium may range between $10\%{\~}100\%$ of gas pressure, although their total energy is probably time dependent. Hence, these non-thermal components may have influenced the formation and evolution of cosmic structures, and may provide unique and vital diagnostic information through various radiations emitted via their interactions with surrounding matter and cosmic background photons. We suggest that shock waves associated with cosmic structures, along with individual sources such as active galactic nuclei and radio galaxies, supply the cosmic rays and magnetic fields to the intracluster medium and to surrounding large scale structures. In order to study 1) the properties of cosmic shock waves emerging during the large scale structure formation of the universe, and 2) the dynamical influence of cosmic rays, which were ejected by AGN-like sources into the intracluster medium, on structure formation, we have performed two sets of N-body /hydrodynamic simulations of cosmic structure formation. In this contribution, we report the preliminary results of these simulations.