• Journal of the Korean Society for Nondestructive Testing
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• v.19 no.6
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• pp.420-425
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• 1999

Time of flight diffraction(TOFD) method is used in nondestructive tests of piping and pressure vessels because of its advantages over a pulse echo technique: its speed, objectivity, repeatability and its insensitivity to specimen surface conditions and discontinuity orientation. But it is the one of weak points in TOFD method that it has the dead zone in sub-surface resolution induced by lateral waves. We solved the dead-zone problem near the sub-surface by using the deconvolution method and the developed ultrasonic testing system showed high performance.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.37 no.12
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• pp.35-42
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• 2000

A new dynamic passband filter enable to control the bandwidth using converging readout beam in photorefractive diffraction grating with high-efficiency is proposed and experimentally demonstrated. In this proposed scheme, the 3dB passband of a volume grating in crystal is theoretically predicted and the experimental results are obtained in consistent with our derivation that the bandwidth is controlled from 0.35nm to 3.3nm at the center wavelength of 670. 1nm.

• Journal of the Optical Society of Korea
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• v.18 no.1
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• pp.60-64
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• 2014

We report the generation of 3.3-mW single-cycle terahertz (THz) pulses at 1-kHz repetition rate via optical rectification in MgO-doped prism-cut stoichiometric LiNbO3. Efficient pulse-front tilting of 800-nm pulses was realized by an optimized single-lens focusing scheme for radially-symmetric propagation of THz beams. In this geometry, nearly-diffraction-limited THz Gaussian beams with electric field strength as high as 350 kV/cm were generated. The pump-to-THz energy conversion efficiency of $1.36{\times}10^{-3}$ and the extremely high signal-to-noise ratio of ~1:15000 achieved are among the best results for 1-kHz single-cycle terahertz pulse generation ever demonstrated in room temperature operation.

• Journal of the Optical Society of Korea
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• v.20 no.6
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• pp.653-662
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• 2016

An optimized multilayer superlens is designed, using a rigorous and efficient approach based on the method of moments (MoM) in conjunction with a simulated annealing (SA) algorithm. For the MoM solution, fast evaluation of closed-form Green's functions (GFs) in the spatial domain is performed by applying the complex-image (CI) technique, which obviates the time-consuming numerical evaluation of Sommerfeld integrals. The imaging capability of the superlens is examined with the correlation coefficient; results show that using circular polarization for the incident wave can improve this coefficient. To validate the proposed method, finite-element-based simulations are exploited, which reveal the method's accuracy and computational efficiency. Simulation results indicate that the designed structure is capable of producing two-dimensional sub-diffraction-limited images in the visible range, which may make it more versatile for practical applications. Finally, as a considerable finding, it is demonstrated for the proposed design that using circularly polarized illumination provides improved super-resolving performance, compared to linearly polarized illumination.

• Journal of Korea Water Resources Association
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• v.32 no.6
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• pp.675-683
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• 1999

The reflection coefficients of monochromatic waves propagating over a shelf with varying dimensions are theoretically calculated. The diffraction of waves by an abrupt depth change is formulated by the eigenfunction expansion method. Not only propagating mode but also evanescent modes are considered in formulation. The role of evanescent modes in reflection coefficients is investigated in detail. Water waves are obliquely as well as normally incident to the region. The obtained reflection coefficients are verified by checking conservation of wave energy.

• Bulletin of the Society of Naval Architects of Korea
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• v.26 no.2
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• pp.49-63
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• 1989

Wave drift forces which are small in magnitudes compared to the first order wave exciting forces can cause very large motion of a vessel in waves. In this paper a theoretical and experimental analysis is made of the mean and slowly varying wave dirft forces on the semi-submersible platform. Theoretical calculations are performed by using near field method with three dimensional diffraction theory and model tests are carried out in regular and irregular waves with a 1/60 semi model. Test results are compared with theoretical calculations and the mooring spring effects in the test are discussed.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.20 no.3
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• pp.291-299
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• 2008

The present study is to estimate the effects of diffracted wave fields in a harbor and around harborentrance due to random waves, when a navigation channel is dredged in the vicinity of the a harbor entrance. The cross sections of harbor boundary are considered to be partial or full reflection in this study. The numerical simulation has been performed by the boundary element method, which is to discrete segments of pit- and harbor- boundary with the algorism of auto generated elements. The incident wave conditions are specified using discretized forms of the Mitsuyasu's frequency spectrum and directional function. The results of the present numerical simulation agreed well with those of the published experimental data. It is shown that the ratios of wave height reduction are about 20% for the case of fully reflecting boundary, and 10% for the case of partially reflecting boundary by the effect of placing a pit, respectively.

• Journal of the Korean Society for Marine Environment & Energy
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• v.1 no.2
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• pp.27-39
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• 1998

The interaction of monochromatic incident waves with a horizontal flexible membrane is investigated in the context of two-dimensional linear hydro-elastic theory. First, analytic diffraction and radiation solutions for a submerged impermeable horizontal membrane are obtained. Second, the theoretical prediction was compared with a series of experiments conducted in a two-dimensional wave tank at Texas A&M University. The measured reflection and transmission coefficients reasonably follow the trend of predicted values. Using the developed computer program, the performance of surface-mounted or submerged horizontal membrane wave barriers is tested with various system parameters and wave characteristics. It is found that the properly designed horizontal flexible membrane can be an effective wave barrier.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.10 no.3
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• pp.359-367
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• 1999

The analysis of a surface wave antenna with the corrugated ground plane fed by a parallel-plate waveguide is considered. An equivalent theorem Is employed to subdivide the original problem into three regions for the simple analysis : one concerning the geometry inside the shorted parallel-plate(Internal region 1), one concerning the geometry of the corrugation(Internal region 2), and one concerning the geometry of the conducting wedge(External region). The hybrid method of moment(MOM)/uniform geometrical theory of diffraction(UTD) method is applied to analyze a corrugated surface wave antenna with the end-fire radiation. Our numerical results are very well matched with those of the previous experiment, better results are obtained when compared with those of the previous simple equivalent current approach. Also, we can obtain the parameters to design an effective end-fire corrugated surface wave antenna.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.2 no.1
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• pp.18-27
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• 1990

The calmness inside a harbor plays an important role in the appropriate disposition of har-bor structures. However, it is not easy to get the accurate computational results because they are affected by many factors concerning with the wave transformation. Successful solution also depends on determining the boundary values appropriately. This paper presents the numerical model which is able to calculate wave heights inside a harbor It is based upon the time-dependent mild-slope equation involving wave refraction, diffraction, shoaling effect and reflection. In particular, the arbitrary reflectivity is used at the boundary in order to simulate the real harbor reflection condition. This numerical model is applied for Hupo-Harbor and its validities are investgated by comparing with experimental values from the hydraulic model test as well as computational results from Taka-yama's numerical model (1981). It is shown that the model results are in good agreement with results from hydraulic model and Takayama＇s.