• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.45-48
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• 2003

The plume interference is a complex phenomenon, consisting of plume-induced boundary layer separation, separated shear layer, multiple shock waves, and their interactions. The base knowledge of plume interference effect on powered missiles and flight vehicles is not yet adequate to get an overall insight of the flow physics in plume-freestream flow field. Computational studies are performed to better understand the flow physics of the plume-induced shock and separation for Simple, Rounded, Porous-extension test model configurations. The present study simulates highly underexpanded exhaust plume effect on missile body at the transoni $c^ersonic speeds. In order to investigate the plume-induced separation phenomenon, Simple, Rounded and Porous-extension plate are attacked to the missile afterbody. The computational result shows that the rounded afterbody and the porous-extension wall attached at the missile base can alleviate the plume-induced shock wave and separation phenomenon and improve the control of the missile body.dy.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.3
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• pp.281-287
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• 2001

For the conventional two-dimensional source location of acoustic emission (AE) based on the threshold crossing, wave velocity has to be measured in the actual structure to calculate the arrival-time difference and thus to form the two hyperbolae. Velocity is dependent on the fiber orientation, however, due to the dependence of elastic modulus on fiber orientation in anisotropic materials such as compost#e plates. This tan affect the accuracy of AE source location and make the source location procedure complicated. In this study, we propose a method to reduce the location error in anisotropic plates by using the numerical solution of nonlinear equations, where the velocity term has been removed by employing the fourth sensor. The efficiency and validity of the proposed method has also been experimentally verified.

• Smart Structures and Systems
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• v.13 no.4
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• pp.587-614
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• 2014

In recent years the interest in online monitoring of lightweight structures with ultrasonic guided waves is steadily growing. Especially the aircraft industry is a driving force in the development of structural health monitoring (SHM) systems. In order to optimally design SHM systems powerful and efficient numerical simulation tools to predict the behaviour of ultrasonic elastic waves in thin-walled structures are required. It has been shown that in real industrial applications, such as airplane wings or fuselages, conventional linear and quadratic pure displacement finite elements commonly used to model ultrasonic elastic waves quickly reach their limits. The required mesh density, to obtain good quality solutions, results in enormous computational costs when solving the wave propagation problem in the time domain. To resolve this problem different possibilities are available. Analytical methods and higher order finite element method approaches (HO-FEM), like p-FEM, spectral elements, spectral analysis and isogeometric analysis, are among them. Although analytical approaches offer fast and accurate results, they are limited to rather simple geometries. On the other hand, the application of higher order finite element schemes is a computationally demanding task. The drawbacks of both methods can be circumvented if regions of complex geometry are modelled using a HO-FEM approach while the response of the remaining structure is computed utilizing an analytical approach. The objective of the paper is to present an efficient method to couple different HO-FEM schemes with an analytical description of an undisturbed region. Using this hybrid formulation the numerical effort can be drastically reduced. The functionality of the proposed scheme is demonstrated by studying the propagation of ultrasonic guided waves in plates, excited by a piezoelectric patch actuator. The actuator is modelled utilizing higher order coupled field finite elements, whereas the homogenous, isotropic plate is described analytically. The results of this "semi-analytical" approach highlight the opportunities to reduce the numerical effort if closed-form solutions are partially available.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.4
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• pp.425-433
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• 2001

A correlation between fatigue damage and acousto-ultrasonic (AU) parameters has been obtained from signals acquired during fatigue loading of the single-lap joints of a carbon-fiber reinforced plastic (CFRP) laminates and A16061 plate. The correlation showed an analogy to those representing the stiffness reduction $(E/E_0)$ of polymer matrix composites by the accumulation of fatigue damage. This has been attributed to the transmission characteristics of acoustic wave energy through bonded joints with delamination-type defects and their influence on the change of spectral content of AU signals. Another correlation between fatigue cycles and the spectral magnitude of acoustic emission (AE) signals has also been found during the final stage of fatigue loading. Both AU and AE can be applied almost in real-time to monitor the evolution of damage during fatigue loading.

• Journal of the Korean Society for Nondestructive Testing
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• v.30 no.4
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• pp.345-351
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• 2010

The oblique incidence technique for ultrasonic nonlinear characterization was studied in stainless steel 316L alloy subjected to high cycle fatigue. A dog-bone plate specimen was prepared to make different faitgue-driven deformation at each position where the stress concentration could occur in the middle of specimen. In addition to the normal transmission technique, the oblique incidence technique which is newly suggested in this study, was used to measure ultrasonic nonlinear parameter. The fatigued specimen shows higher ultrasonic nonlinear parameter than the virgin specimen for both techniques. Ultrasonic nonlinear parameter highly increases in the middle of test specimen where the stress concentration existes. Relative nonlinear parameter has strong correlation with fatigue damage. Consequently, the oblique incidence technique with longitudinal wave can be potential to characterize high cycle fatigue damage.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.40 no.5
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• pp.375-384
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• 2012

This paper presents numerical results comparing the performance of the 2008 Wilcox $\mathcal{k}-{\omega}$ turbulence model to the one of the 1988 Wilcox $\mathcal{k}-{\omega}$ model for supersonic flows. A comparison with experimental data is offered for a shock wave/turbulent boundary layer interaction case and two ramp injector mixing cases. Furthermore, a comparison is performed with empirical correlations on the basis of skin friction for flow over a flat plate and shear layer growth for a free shear layer. It is found that the maximum injectant mass fraction of some ramp injector cases is better predicted using the 1988 Wilcox model. On the other hand, the 2008 model performs better in simulating shock-boundary layer cases.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.1
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• pp.67-73
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• 2017

This study has addressed to evaluate the effects of radius of curvature and skew angle on the negative reaction in a plate girder bridge with LRB (Lead Rubber Bearing) supports. As analytical parameters, various radius of curvatures and skew angles were selected and two seismic loads of El-Centro and artificial earthquakes were applied to the bridge in the longitudinal and transverse directions. As results of 3D analysis, the possibility of negative reaction is shown at the part of acute angle and inner side of the curved bridge, and becomes increased when seismic load is applied in the transverse direction. In addition, the occurrence of negative reaction is found to be increased as both radius of curvature and skew angle decrease, which means that curved bridge has higher possibility of negative reaction than straight one. Conclusively, all of earthquake wave, gradient, radius of curvature and skew angle should be considered together to investigate the possibility of negative reaction at the bridge support subject to seismic load.

• Korean Journal of Optics and Photonics
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• v.17 no.3
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• pp.273-277
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• 2006

In this paper, we constructed the Littman type and fixed Littrow type tunable external-cavity diode laser systems. The laser output, which is the 0th-order diffracted beam from the diffraction grating in an external cavity, was the single longitudinal mode. Its FWHM was measured as less than 9MHz. With the diode driving current of 140mA and operating temperature of $25^{\circ}C$, the coarse tuning range of 5.375nm was measured for the Littman type, and of 13.65nm was measured for the fixed Littrow type. A fine tuning experiment in which an external mirror was rotated by a PZT driven by a sawtooth wave was performed, and its tuning range of 0.042nm was measured for both types. The fixed Littrow type tunable external-cavity diode laser system was an improvement on the conventional Littrow type tunable laser system in which the output direction varies due to the grating embedded in the mirror plate.

• Korean Journal of Optics and Photonics
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• v.8 no.6
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• pp.486-491
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• 1997

A division-of-amplitude type photopolarimeter has been constructed for the analysis of the dynamic optical characteristics of liquid crystal panels, one of the essential components of liquid crystal displays. In this instrument an incoming light ray, whose state of polarization is to be determined, is divided into three rays of nearly same intensities and of identical polarization state by using NPBS'(non-polarizing beam splitters). Each of the three rays is further divided into two components of orthogonal polarization states by using a PBS(polarizing cube beamsplitter) or by using a combination of a quater wave plate followed by a PBS. The intensity of each ray is measured by using a photodiode to produce a set of six photo-signals, which in turn are converted into four Stokes parameters describing the state of polarization of the incoming ray. Performance test of the insrument shows that its time resolution is 80 $mutextrm{s}$, accuracy $\pm$0.3 degrees when the state of polarization of the incoming ray is representated on the Poincare sphere.

• Journal of the Institute of Convergence Signal Processing
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• v.9 no.1
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• pp.10-17
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• 2008

The purpose of this thesis is to reduce of error for source location through acoustic emission(AE) signal, generated elastic wave from crack growth to leak for facility diagnosis. Especially, in order to overcome noise from original signal, this paper proposed enhancement of source location by using noise reduction based on wavelet transform. To evaluate actual performance in experiments, Pencil Lead Break is used crack signal source on the aluminum plate and drain valve of air compressor is used as substitute pressure vessel to generate leak signal. In signal processing, wavelet shrinkage and soft threshold are used to discriminate signal source and then source location techniques have been effectively used with group velocity using material property and time difference between sensor using cross correlation. Source location for crack and leak test have some difference, but the result show that improved 30% with a average length within 10.46mm in crack test and improved 2% compare with average filter in leak test when we applied wavelet transform.