• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.49.3-50
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• 2015

Synchrotron radiation of relativistic electrons is an important radiation mechanism in many astrophysical sources. In the sources where the synchrotron cooling timescale is shorter than the dynamical timescale, electrons are cooled down below the minimum injection energy. It has been believed that such fast-cooling electrons have a power-law distribution in energy with an index -2, and their synchrotron radiation has a photon spectral index -1.5. On the other hand, in a transient expanding astrophysical source, such as a gamma-ray burst (GRB), the magnetic field strength in the emission region continuously decreases with radius. Here we study such a system, and find that in a certain parameter regime, the fast-cooling electrons can have a harder energy spectrum. We apply this new physical regime to GRBs, and suggest that the GRB prompt emission spectra whose low-energy photon spectral index has a typical value -1 could be due to synchrotron radiation in this moderately fast-cooling regime.

• Proceedings of the Korean Society of Agricultural Engineers Conference
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• 1999.10c
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• pp.301-306
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• 1999

Over 4.1 billion metric tons of non-hazardous solid wastes are generated in the United States anually. Therefore, there is a need to reuse or reprocess as many products or by -products as possible. In this report, the aggregates produced from Naval Shipyard and Charlexton area dredge spoils are designated as NC and DS, respectively. The major objective of this portion was to determine the feasibility of the use of these aggregates in concrete mixtures. According to test results, the unit weight of concrete mixtures containing NC and DS (vitrified glass) was lower than the control mixture, and the control mixture produced higher compressive and bending strength compared to mixtures containing vitrified glass. However the mixtures containing vitrified glass still could be used in the field based on the minimum criteria set for the mix desgin.

• Magazine of the Korean Society of Agricultural Engineers
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• v.41 no.6
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• pp.90-96
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• 1999

Over 4.1 billion metric tons of non-hazardous solid wstes are generated in the United States annually. Therefore, there is a need to reuse or reprocess as many products or by-products as possible. In this report, theaggregates produced from Naval Shipyard and Charleston area dredge spoils are designated as GP-21-23 and GP 4-5, repectively. The major objective of this study is to determine the feasiblity of the use of these aggregates in concrete. The followings are some of the conclusions, observations and recommendations regarding this portion of the research project. In most cases, the unit weight of concrete containing GP 21-23 and GP 4-5 (vitrified galss) lower than the control concrete, and the concrete containing vitrified glass produced lower compressive and bending strength compared to the control. However the concrete containing virtrified glass could be used in the field based on the minimum criterial set for the mix design. The use of vitrified glass in concrete is recommended in several applications except parking lots, side walks and main roads, because abrasion resistace is not high.

• Tribology and Lubricants
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• v.32 no.4
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• pp.132-139
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• 2016

The piston of a marine diesel engine works under severe conditions, including a combustion pressure of over 180 bar, high thermal load, and high speed. Therefore, the analyses of the fatigue strength, thermal load, clamping (bolting) system and lubrication performance are important in achieving a robust piston design. Designing the surface profile and the skirt ovality carefully is important to prevent severe wear and reduce frictional loss for engine efficiency. This study performs flexible multi-body dynamic and elasto-hydrodynamic (EHD) analyses using AVL/EXCITE/PU are performed to evaluate tribological characteristics. The numerical techniques employed to perform the EHD analysis are as follows: (1) averaged Reynolds equation considering the surface roughness; (2) Greenwood_Tripp model considering the solid_to_solid contact using the statistical values of the summit roughness; and (3) flow factor considering the surface topology. This study also compares two cases of skirt shapes with minimum oil film thickness, peak oil film pressure, asperity contact pressure, wear rate using the Archard model and friction power loss (i.e., frictional loss mean effective pressure (FMEP)). Accordingly, the study compares the calculated wear pattern with the field test result of the piston operating for 12,000h to verify the quantitative integrity of the numerical analysis. The results show that the selected profile and the piston skirt ovality reduce friction power loss and peak oil film pressure by 7% and 57%, respectively. They also increase the minimum oil film thickness by 34%.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.8 no.2
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• pp.99-107
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• 1988

A limited number of cubical triaxial tests with independent control of the three principal stresses were performed on an overconsolidated clay. The cubical undisturbed specimens with overconsolidation ratio of 5 were prepared in triaxial chamber after sampling in field. It was found that the intermediate principal stress influences on the stress-strain, undrained strength effective strength, effective friction angle and pore pressure of the overconsolidated clay. When the magnitude of the intermediate principal stress is not same as the minimum principal stress, the failure strength of the overconsolidated clay is underestimated by use of Mohr-Coulomb failure criterion while it can be estimated quite well by use of Lade failure criterion. And the undrained strength of the overconsolidated clay does not coincide with that obtained by Tresca failure criterion.

• Journal of Korean Tunnelling and Underground Space Association
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• v.11 no.4
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• pp.387-394
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• 2009

The objective of this study is to evaluate the durability and deterioration of concrete lining in the seven conventional tunnels. These tunnels were constructed about 40~70 years ago, and closed about 10~40 years ago. The field investigation and various laboratory testings were performed for this study. It was observed from the visual, examinations that the concrete linings of 7 tunnels were severely deteriorated, such as, cracks, leakages, desquamation, and exploitations. The compressive strengths obtained from rebound hardness method and uniaxial compressive strength test on core specimens largely differed depending on the locations in the tunnel. The maximum compressive strength of concrete lining was greater about 2 times than the minimum compressive strength of concrete lining in the same tunnel. The results of micro-structural analysis showed that the substances deteriorating the concrete lining, such as ettringite and thaumasite, were detected in the concrete lining of tunnel.

• Journal of the Korean GEO-environmental Society
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• v.1 no.1
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• pp.65-70
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• 2000

Recently, soil-bentonite mixtures are frequently used as impervious liners for waste disposal sites. In the present work, bentonite composite liner systems have been developed by utilizing Korean zeolitic bentonites. The geomechanical properties of the liner systems, such as strength hydraulic conductivity, etc. have also been studied. The laboratory and field test results showed that uniaxial strengths of the system were improved by addition of bentonite and CaO-based additive to the upper and lower layer of the liner systems, respectively. Hydraulic conductivity values measured on field liner systems showed less $1{\times}10^{-3}cm/s$, which is considered to be minimum regulation requirement for waste disposal sites.

• The Journal of Korea Robotics Society
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• v.7 no.2
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• pp.150-159
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• 2012

In this paper, we propose a cost-aware Rapidly-exploring Random Tree (RRT) path planning algorithm for mobile robots. A mobile robot is presented with a cost map of the field of interest and assigned to move from one location to another. As a robot moves, the robot is penalized by the cost at its current location according to the cost map. The overall cost of the robot is determined by the trajectory of the robot. The goal of the proposed cost-aware RRT algorithm is to find a trajectory with the minimal cost. The cost map of the field can represent environmental parameters, such as temperature, humidity, chemical concentration, wireless signal strength, and stealthiness. For example, if the cost map represents packet drop rates at different locations, the minimum cost path between two locations is the path with the best possible communication, which is desirable when a robot operates under the environment with weak wireless signals. The proposed cost-aware RRT algorithm extends the basic RRT algorithm by considering the cost map when extending a motion segment. We show that the proposed algorithm gives an outstanding performance compared to the basic RRT method. We also demonstrate that the use of rejection sampling can give better results through extensive simulation.

• Ocean Systems Engineering
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• v.11 no.1
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• pp.43-58
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• 2021

In the present work, a 3D numerical model is proposed to study local scouring around single vertical piers with different cross-section shapes under steady-current flow. The model solves the flow field and sediment transport processes using a coupled approach. The flow field is obtained by solving the Unsteady Reynolds Averaged Navier-Stokes (URANS) equations in combination with the k-ω SST turbulence closure model and the sediment transport is considered using both bedload and suspended load models. The proposed model is validated against the empirical measurements of local scour around single vertical piers with circular, square, and diamond cross-section shapes obtained from the literature. The measurement of scour depth in equilibrium condition for the simulations reveal the differences of 4.6%, 6.7% and 13.1% from the experimental measurements for the circular, square, and diamond pier cases, respectively. The model displayed a remarkable performance in the prediction of scour around circular and square piers where horseshoe vortices (HSVs) have a leading impact on scour progression. On the other hand, the maximum deviation was found in the case of the diamond pier where HSVs are weak and have minimum impact on the formation of local scour. Overall, the results confirm that the prediction capability of the present model is almost independent of the strength of the formed HSVs and pier cross-section shapes.

• Journal of the Institute of Electronics and Information Engineers
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• v.49 no.10
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• pp.128-134
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• 2012

The performance evaluation and fabrication of integrated-optic electric-field sensor utilizing $Ti:LiNbO_3$ asymmetric Mach-Zehnder intensity modulator with a push-pull lumped electrode and a plate-type probe antenna to measure an electric field strength is described. The modulator has a small device size of $46{\times}7{\times}1\;mm$ and operates at a wavelength $1.3{\mu}m$. The devices are simulated based on the BPM software and fabricated utilizing Ti-diffused $LiNbO_3$ channel optical waveguides. The minimum detectable electric field is 1.02 V/m and 6.91 V/m, corresponding to a dynamic range of ~35 dB and ~10 dB at the frequencies of 500 KHz and 5 MHz, respectively.