• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.19 no.2
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• pp.146-154
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• 2009

Dynamic response of a hull mounted sonar(HMS) to shocks transmitted through hull structures is analyzed and then the structural reliability of the sonars is evaluated. Finite element model of the hull mounted sonar is established and the transient responses to the shock is calculated using MSC.NASTRAN. According to BV043, the maximum allowable accelerations at the foundation of the sonar are converted from the shock spectra allowable for HMS. They are applied vertically and horizontally, respectively, using the large mass method. The structural reliability is evaluated by comparing the von-Mises stresses with the material yield stress. The drum for sensors shows a high reliability owing to mounts by which the shock waves from the base structure are well protected. However, the mounts between the base structure and the drum to mount sensors show a high stress intensity. The base structure also reveals a high stress intensity at the connection points to the hull.

• Wind and Structures
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• v.34 no.4
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• pp.385-394
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• 2022

Global Warming has been driven majorly by the consumption of fossil fuels. Harnessing energy from wind is viable solution towards reducing carbon footprint created due to burning such fuels, However, wind turbines have their problems of flow separation and aerodynamic stall to tackle with. In an attempt to delay the stall angle and improve the aerodynamic characteristics of the NACA 0015 symmetrical aerofoil, lateral cylindrical ridges were attached to its suction surface, at chord positions ranging from 0.1c to 0.5c. The characteristics of the original and ridged aerofoils were obtained using simultaneous pressure readings taken in a wind tunnel, at a free stream Reynolds number of R_e = 2.81 × 10⁵ for a wide range of free stream angles of attack ranging from -45° to 45°. Depending on the ridge size, a delay in stall angle varying from 5° to 20° was achieved together with the maximum increase in lift in the post-stall phases. Additionally, efforts were made to identify the optimum position for each ridge.

• Composites Research
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• v.22 no.3
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• pp.55-59
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• 2009

Microstrip antennas are low profile, are conformable to planar and nonplanar surfaces, are simple and inexpensive to manufacture, mechanically robust when mounted on rigid surfaces and are compatible with MMIC(Monolithic microwave integrated circuit) designs; they have been used in diverse communication systems. The rectangular microstrip patch antenna is designed for a central frequency of 12.5 GHz, and the final product is a $4{\times}1$ array antenna with curvature radius of 200 mm. The microstrip antenna is embedded in a sandwich structure which consists of skin and core material. After impact, the performance of damaged antenna is estimated by measuring the return loss and radiation pattern. The antenna performance was not affected by this impact damage.

• Nuclear Engineering and Technology
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• v.55 no.3
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• pp.947-957
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• 2023

Curved cylindrical structures such as elbows have a non-uniform thickness distribution due to their fabrication process, and as a result have a number of complex mode shapes, including circumferential and axial nodal patterns. In nuclear power plants, material degradation is induced in pipes by flow accelerated erosion and corrosion, causing the wall thickness of carbon steel elbows to gradually thin. The corresponding frequencies of each mode shape vary according to the wall thinning state. Therefore, the thinning state can be estimated by monitoring the varying modal characteristics of the elbow. This study investigated the varying modal characteristics of artificially thinned carbon steel elbows for each thinning state using numerical simulation and experimental methods (MRIT, Multiple Reference Impact Test). The natural frequencies of specified mode shapes were extracted, and results confirmed they linearly decreased with increasing thinning. In addition, by comparing single FRF (Frequency Response Function) data with the results of MRIT, a concise and cost effective thinning estimation method was suggested.

• Applied Microscopy
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• v.42 no.3
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• pp.124-128
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• 2012

Leaf surface structures were investigated in the Chinese juniper Juniperus chinensis by scanning electron microscopy. Adult scale leaves were collected from the tree, air-dried at room temperature, and sputter-coated with gold without further specimen preparation. Approximately fi ve stomata were locally distributed and arranged in clusters on the leaf surface. Stomata were ovoid and ca. 40 ${\mu}m$ long. The epicuticular wax structures of J. chinensis leaves were tubules and platelets. Numerous tubules were evident on the leaf regions where stomata were found. The tubules were cylindrical, straight, and ca. 1 ${\mu}m$ in length. They almost clothed the stomatal guard cells, and occluded the slit-shaped stomatal apertures. Moreover, the wax ridges were flat crystalloids that were connected to the surface by their narrow side. They did not have distinct edges, and their width/height ratio varied. In particular, the wax ridges could be discerned on the leaf regions where stomata were not present nearby. Since the wax ridges did not have distinct edges on their margin, they were identified as platelets. Instances were noted where platelets were oriented either parallel to each other or perpendicular to the cuticle surface. These results can be used in biomimetics to design the hierarchical structures for mimicking the plant innate properties such as hydrophobicity and self-cleaning effects of the leaf surface.

• Journal of the Korean Society for Advanced Composite Structures
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• v.3 no.2
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• pp.1-10
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• 2012

In the present study, an Element-Based Lagrangian Formulation for the nonlinear analysis of shell structures is presented. The strains, stresses and constitutive equations based on the natural co-ordinate have been used throughout the Element-Based Lagrangian Formulation of the present shell element which offers an advantage of easy implementation compared with the traditional Lagrangian Formulation. The Element-Based Lagrangian Formulation of a 9-node resultant-stress shell element is presented for the anisotropic composite material. The element is free of both membrane and shear locking behavior by using the assumed natural strain method such that the element performs very well in thin shell problems. The arc-length control method is used to trace complex equilibrium paths in thin shell applications. Numerical examples for laminated composite curved shells presented herein clearly show the validity of the present approach and the accuracy of the developed shell element.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2005.11a
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• pp.35-38
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• 2005

The Objective of this work was to design Composite Antenna Structures (CAS) and investigate impact behavior of CAS which was various curvature. This term, CAS, indicates that structural surface becomes antenna. Constituent materials were selected considering electrical properties, dielectric constants and tangent loss as well as mechanical properties. For the antenna performance, microstrip antenna layers inserted into structural layers were designed for satellite communication at the resonant frequency of 12.5 GHz and final demonstration article was. After making five kinds of curved CAS, which radii of curvature are flat, 200, 150, 100, 50 mm. The antenna performance changed in accordance with variation of curvature. The Reflection coefficient was independent of curvature but the gain decreased with the radius of curvature. The impact test equipment was Dyna-8250 drop weight tester. The impact characteristic in accordance with curvature is maximum absorb energy is same each other. The impact energy was 8.5 J. For various Impact energy test, five energy levels 3 J, 5 J, 7 J, 10 J, 20 J were used. The performance of impact damaged antenna was estimated by measuring the return loss and the radiation pattern. It was revealed that the performance of antenna was not related to the impact damage. Because the impactor did not damage the patch directly. CAS have good impact stability for the antenna performance.

• Smart Structures and Systems
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• v.18 no.4
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• pp.819-837
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• 2016

The quality of vibration pattern reproduction of elastic structures by the modal expansion method is influenced by the modal expansion method and the sensor placement as well as the accuracy of measured natural modes and the total number of vibration sensors. In this context, this paper presents an improved numerical method for reproducing the vibration patterns by introducing a block-wise modal expansion method (BMEM), together with the genetic algorithm (GA). For a given number of vibration sensors, the sensor positions are determined by an evolutionary optimization using GA and the modal assurance criterion (MAC). Meanwhile, for the proposed block-wise modal expansion, a whole frequency range of interest is divided into several overlapped frequency blocks and the vibration field reproduction is made block by block with different natural modes and different modal participation factors. A hollow cylindrical tank is taken to illustrate the proposed improved modal expansion method. Through the numerical experiments, the proposed method is compared with several conventional methods to justify that the proposed method provides the improved results.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.05a
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• pp.421-427
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• 2010

This paper is concerned with Anisogrid composite lattice structures whose load bearing shell is formed by systems of geodesic unidirectional composite ribs made by automatic wet winding process. Lattice structures are usually made in the form of cylindrical shell and consist of systems of helical and hoop ribs fabricated by continuous filament winding from carbon and epoxy composites. Design variables of the structure which are the angle of helical ribs, ribs spacings, and cross sectional areas are determined by the method of minimization of satety factors whick is described in the paper. And, fabrication methods and actual experimental results are presented.

• Journal of Korea Water Resources Association
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• v.47 no.11
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• pp.1067-1076
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• 2014

In this study, a series of hydraulic experiments are conducted to measure wave pressure on vertical structures with incident solitary waves that well represent characteristics of tsunamis. The pressure transducers measure time histories of wave pressure according to wave height to see pressure distribution. The force of incident solitary wave is estimated from integrated pressure distributions and represented with square and cylindrical columns. Experimental measurements are compared with the predictions of existing empirical formulas frequently used to design of coastal structures.