• Journal of the Korean Society of Manufacturing Process Engineers
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• v.21 no.5
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• pp.53-60
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• 2022

In this study, the conceptual design and performance evaluation of a personal air vehicle (PAV) is presented, which is a potential futuristic individual transportation. The blade element theory (BET) is employed to compute a rotational velocity. A computational fluid dynamics (CFD) simulation is performed to investigate the difference in the thrust performance in the rotor axis distance of a quad-copter PAV in hovering. Modal analysis is performed to create a Campbell diagram to investigate critical speed. Consequently, a quad-copter PAV changes the aerodynamics thrust and critical velocity according to the rotor axis distance.

• Journal of Ocean Engineering and Technology
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• v.26 no.2
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• pp.26-32
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• 2012

In this paper, the effects of irregular bead shapes on fatigue life were investigated. A modified S-N curve method was used to estimate the fatigue life, which considered the inherent multiaxiality caused by the geometrical feature produced by the welding process. The point method of the critical distance method was used to determine the fatigue effective stress. Three types of fillet joint models were tested in the fatigue experiments. For each model, real bead shapes were collected using a 3D laser scanner, and finite element analyses were performed. The results of the analyses with actual bead shapes were compared with those using an idealized bead shape model. The results of the present analytical methods showed good agreement with the experimental results.

• Journal of Communications and Networks
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• v.14 no.4
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• pp.451-460
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• 2012

Natural disasters often lead to regional failures that can cause network nodes and links co-located in a large geographical area to fail. Novel approaches are required to assess the network vulnerability under such regional failures. In this paper, we investigate the vulnerability of networks by considering the geometric properties of regional failures and network nodes. To evaluate the criticality of node locations and determine the critical areas in a network, we propose the concept of ${\alpha}$-critical-distance with a given failure impact ratio ${\alpha}$, and we formulate two optimization problems based on the concept. By analyzing the geometric properties of the problems, we show that although finding critical nodes or links in a pure graph is a NP-complete problem, the problem of finding critical areas has polynomial time complexity. We propose two algorithms to deal with these problems and analyze their time complexities. Using real city-level Internet topology data, we conducted experiments to compute the ${\alpha}$-critical-distances for different networks. The computational results demonstrate the differences in vulnerability of different networks. The results also indicate that the critical area of a network can be estimated by limiting failure centers on the locations of network nodes. Additionally, we find that with the same impact ratio ${\alpha}$, the topologies examined have larger ${\alpha}$-critical-distances when the network performance is measured using the giant component size instead of the other two metrics. Similar results are obtained when the network performance is measured using the average two terminal reliability and the network efficiency, although computation of the former entails less time complexity than that of the latter.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2003.06a
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• pp.1358-1361
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• 2003

In this study, intrinsic static/dynamic fracture toughness of Al 7175-T74 are evaluated from the apparent static/dynamic toughness of notched specimen. The notch/crack critical average stress fracture model is suggested to establish the relationship to predict the intrinsic fracture toughness from the apparent fracture toughness of a notched specimen. The notch/crack critical average stress fracture model is established using the relation between the notch root radius and the effective distance calculated by finite element analysis. It is conclude that the true fracture toughness can be estimated from test results of apparent fracture toughness measured by using a notched specimen. Also, critical notch root radius can be predicted by notch/crack critical average stress fracture model.

• Proceedings of the SAREK Conference
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• 2009.06a
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• pp.154-158
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• 2009

The vortex ventilation system (VV) which uses a rotating finned swirler installed coaxially with the exhaust duct is a very effective local ventilator. VV can enhance the capture depth by a factor of 3-5 compared to the conventional exhaust hood, in the absence of any solid walls nearby. In real situations there may exist ceiling, side wall and floor, all of which can affect the flow field and suction performance by way of the no-slip condition on the walls. 3D CFD simulation was performed in order to see the effect of the floor on the capture performance of the VV. The presence of floor reduced suction flow velocity, and increased the critical rotational speed which is the rotational speed required for stable vortex formation. Flow velocity profile along the axis could be well approximated by a universal functional form when the distance from the exhaust inlet is non-dimensionalized by the distance to the floor. Capture depth, define by the distance from the exhaust inlet to a point of velocity decreased to 10% of that at the inlet, is reduced by about 10% when the floor distance is 6 times the exhaust hood diameter.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.05a
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• pp.7-8
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• 2009

Non-contact critical current measurement apparatus was developed using hall probe which measures the magnetic field distribution across the width of superconducting tape. The hall probe consists of 7 independent hall sensors which lie in a line 600 ${\mu}m$. The difference between maximum and minimum magnetic field in the magnetic filed distribution is a main parameter to determine the critical current. As preliminary research, we calculated the magnetic field intensity at the middle sensor, which is a minimum magnetic field and generated by the circular shielding current modeled by Bean model. We confirmed that there are some parameters that affect on the minimum magnetic field; the distance between superconducting layer and hall sensor, the width of superconducting tape, and the critical current distribution across the width of superconducting tape. Among these parameters, the distance between superconducting layer and hall sensor highly influences on the minimum magnetic field.

• Journal of ILASS-Korea
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• v.26 no.3
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• pp.149-156
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• 2021

In this study, the behavior of the droplet colliding with parallel wires was analyzed by time-delay photography. The impact behavior modes and the critical capture speed were analyzed by changing fluids, the droplet velocity, the wire diameter and the distance between wires. Seven typical modes of impacting droplet on parallel wires were observed. The tendency of mode change was generally similar when the wire diameter was changed, but the increase of the wire diameter caused the increase of the droplet velocity at which the mode changed. The modes at the highest droplet velocity were the splitting mode when the wires were closest, the passing and splitting mode in the middle, and the passing mode when the wires were farthest apart. The critical capture speed increased as the wire diameter increased and the distance between wires decreased. The ethanol droplet showed the lowest critical capture speed.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.10 no.10
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• pp.2810-2818
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• 2009

In order to examine the characteristic of shallow failure in cut slopes composed of weathered granite soil, this study calculated critical permeability coefficient according to rainfall characteristic in Korea, performed stability analysis according to the representative physical properties of weathered granite soil distributed in Korea such as horizontal distance to the failure surface of cut slope, slope inclination, slope height, and the depth of wetting by rainfall, and analyzed the results. In the results of analyzing critical permeability coefficient, when the local rainfall characteristic was considered, the maximum critical permeability coefficient was $7.16{\times}10^{-4}cm/sec$. We judged that shallow failure according to wetting depth should be considered when rainfall below the critical rainfall intensity lasts longer than the minimum rainfall duration in cut slopes composed of weathered granite soil, which had a critical permeability coefficient lower than the maximum critical permeability coefficient. Furthermore, using simulated failure surface, this study could understand the characteristic of shallow failure in cut slopes based on the change in slope safety factor according to horizontal distance, wetting depth, and strength parameter.

• Transactions of Materials Processing
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• v.20 no.1
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• pp.5-10
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• 2011

The main objective of the present study is to predict the process window of injection-compression molding corresponding to the capability of an injection machine for fabricating 7 inch LGP. The open distance and volume filled after injection stage were found to be two important factors that affect critical requirements such as flow length, injection pressure and clamping force for the process. Process window for the key factors was also predicted by response surface method. As a result, predicted process window for open distance and volume filled after injection stage satisfying the critical requirement with a given injection machine was in the range of 60 ~ 75%, and 104.00 ~ 104.25%, respectively.

• Tribology and Lubricants
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• v.12 no.1
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• pp.22-28
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• 1996

This study was undertaken to investigate the dry wear characteristics and mechanism of KP-4M steel for plastic molding against SKD 61 hardened by heat treatment. The wear test was carried out under different conditions such as sliding speed, contact pressure, sliding distance, with frictional tester of pin on disc type. The wear loss on variation of sliding speed was little in lower speed range below 0.5 m/sec and in higher speed range above 1.5 m/sec,'but wear loss was high in intermediate speed range. The critical sliding speed, which showed the maximum value of specific wear rate, became lower with increased contact pressure. Increasing the contact pressure, the critical sliding distance Lcr which the wear mechanism changes from severe wear to mild wear was increased due to the decrease of oxidation reaction velocity. Through this study we suggested a model of generation and elimination process of wear debris of KP-4M steel for plastic molding.