• Journal of Coastal Disaster Prevention
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• v.4 no.spc
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• pp.245-253
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• 2017

In this study, surface currents measured by small lagrangian GPS drifters (Aquadrifter) in Anmok coastal waters were analysed to account for the variability of nearshore surface current and wave-induced current to understand sediment transport mechanism near the crescentic bars in the surf-zone and near Kangneung breakwater and submerged breakwater in Anmok. The 8 times lagrangian drifter experiments were conducted mostly during in 2nd, 3rd, 4th intensive measurements in winter, summer, and spring seasons with long-term wave observation at the station W1. The analysed surface currents near the breakwaters in Anmok show that wave-induced currents at the middle of the submerged breakwater were separated and flowed toward the shoreline but offshore currents were dominant through the channels between the breakwaters. The longshore currents near the shoreline were flowed to the northwest (southeast) depending on the incoming waves from ENE (NNE). The surface nearshore offshore currents were generated mostly by waves and winds in case of high and low wave energy environments. Using the small-size lagrangian surface drifter experiments, we successfully measured longshore and offshore wave-induced currents in the surf-zone and near submerged breakwater close to Kangneung breakwater. The drifter experiment results show the availability of direct observation of nearshore surface currents to understand the mechanism of sediment transport analysing observed wave-induced current and ebb-current in the surf-zone generated by incoming waves and local winds.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.287-291
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• 2005

Two-dimensional velocity tomograms of P- and S-waves were obtained by inverting traveltimes of first arrivals. The two sections of shear-wave velocity show similar features as a whole, with smaller values on the section from surface wave dispersions. Difficulties in picking SH-wave phases due to noise and later arrivals than P waves and PS converted waves are experienced. In addition, a flat layer model based on the surface wave inversion prohibits applications of the method where sgear wave velocities vary strongly in the lateral direction.

• 한국전산유체공학회:학술대회논문집
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• 2004.03a
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• pp.119-125
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• 2004

Unsteady nonlinear wave motions on the free surface over a plane beach of constant slope are numerically simulated using a finite difference method in rectangular grid system. Two-dimensional Navier-Stokes equations and the continuity equation are used for the computations. Irregular leg lengths and stars are employed near the boundaries of body and free surface to satisfy the boundary conditions. Also, the free surface which consists of markers or segments is determined every time step with the satisfaction of kinematic and dynamic free surface conditions. Moreover, marker-density method is also adopted to allow plunging jets impinging on the free surface. The second-order Stokes wave theory and solitary wave theory are employed for the generation of waves on the inflow boundary. For the simulation of wave breaking phenomena, the computations are carried out with the plane beach of constant slope in surf zone. The results are compared with each other. The marker-density method is better then the hybrid method. Also they are compared with other existing experimental results. The Agreement between the experimental data and the computation results is good.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.3
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• pp.257-263
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• 2013

Acoustic nonlinearity of surface waves is an effective method to evaluate the micro damage on the surface of materials. In this method, the $A_1$ (magnitude of the fundamental wave) and $A_2$ (magnitude of the second-order harmonic wave) are measured for evaluation of acoustic nonlinearity. However, if there is another source of second-order harmonic wave other than the material itself, the linear relationship between $A_1{^2}$ and $A_2$ will not be guaranteed. Therefore, the second-order harmonic generation by another source should be fully suppressed. In this paper, we investigated the initial second-order harmonic generation in narrowband surface waves by multi-line laser beams. The spatial profile of laser beam was considered in the cases of Gaussian and square-like. The temporal profile was assumed to be Gaussian. In case of Gaussian spatial profile, the generation of the initial second-order harmonic wave was inevitable. However, when the spatial profile was square-like, the generation of the initial second-order harmonic wave was able to be fully suppressed at specific duty ratio. These results mean that the multi-line laser beams of square-like profile with a proper duty ratio are useful to evaluate the acoustic nonlinearity of the generated surface waves.

• Journal of Ocean Engineering and Technology
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• v.16 no.3
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• pp.34-39
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• 2002

본 논문에서는 수면파(Surface wave)와 수중파 (Internal wave)간의 동적 상관관계에 관하여 수행된 연구결과를 정리하였다. 표면파의 비선형 문제는 파의 경사매개변수를 2차원으로 가정하여 해석하였으며, Cauchy 문제는 불균일 조류상의 균일 수면중력파에 대하여 해석하였다. 또한, 파의 경사, 주기의 범위(Frequency range) 그리고 자유표면하의 조류의 분포들간의 조화에 대한 연구가 수행되었으며 해류 및 이동파와 연계되어 수중파의 최전 후방에 형성될 수 있는 정적 파형 (Steady wave pattern)이 수면파형에 포함되었다.

• Smart Structures and Systems
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• v.11 no.2
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• pp.185-197
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• 2013

We developed a simultaneous strain measurement and damage detection technique using a pair of surface-mounted piezoelectric transducers and a fiber connecting them. This is a novel sensor configuration of the fiber acoustic wave (FAW) piezoelectric transducer. In this study, lead-zirconate-titanate (PZT) transducers are installed conventionally on a plate's surface, which is a technique used in many structural health monitoring studies. However, our PZTs are also connected with an optical fiber. A FAW and Lamb wave are simultaneously guided in the optical fiber and the structure, respectively. The dependency of the time-of-flight of the FAW on the applied strain is quantified for strain sensing. In our experimental results, the FAW exhibited excellent linear behavior and no hysteresis with respect to the change in strain. On the other hand, the well-known damage detection function of the surface-mounted PZT transducers was still available by monitoring the waveform change in the conventional Lamb wave ultrasonic path.

• Water for future
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• v.10 no.1
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• pp.101-117
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• 1977

The statatistical characteristics and spectra of sea waves at Mookho were analysed by several statistical methods. As the results, the following conclusions are obtained: 1. Values of surface elevation of sea wave are better fitted to Gram Charlier distribution than Gaussian distribution. This proves that sea waves have not only characters of irregularity but also non-linearity. 2. Distribution of maxima of surface elevation practically follows the distribution of Cartwright and Longuet-Higgins, also spectral width parameter is found to be increased with the increase of root mean square of surface elevation. 3. Sea wave may have spectrum of broad frequency band, however distributions of wave heights and periods follow the Rayleigh distribution which is derived from the assumption of narrow frequency band. 4. Ratios among mean wave heights from observed data show good agreements with theoretical values from Rayleigh distribution. 5. Spectral density and spectral width parameter increase with increase of wind velocity. And wave period at optimum band gas higher value than significant wave period by about 10 percent.

• Journal of KSNVE
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• v.9 no.6
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• pp.1218-1226
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• 1999

In this paper, the extended Biot's semilinear model was developed. Combining the extended Biot model with the dynamic equation yields the nonlinear wave equation in poproelastic sound absorbing materials. Both perturbation and matching techniques are used to find solutions for nonlinear wave equations. By comparing results between linear and nonlinear wave solutions, characteristics of nonlinear waves in poroelastic sound abosrbing materials have been studied. Nonlinear waves were found to be attenuated faster than the linear ones. A maximum amplitude of the nonlinear wave occurred near its surface boundaries and decay quickly with distance from the surface. It has also been found that, if the amplitudes of linear waves are known at the surface boundaries, those of nonlinear ones can be determined. This will be the basis of finding effects of nonlinearity on the absorption coefficient and the transmission loss.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.336-341
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• 2004

This research investigates the feasibility of ultrasonic microscope for nondestructive assessment of thermal degradation in artificially aged commercial Co-base superalloy, FSX414. This alloy has been used for high temperature structure applications such as stationary gas turbine blade and nozzle chamber in fossil plant. Microstructural change was found that the fine carbides became coarser and spheroidized in matrix as aging time increased. The leaky surface acoustic wave velocity gradually decreases by a maximum of 4.7% with increasing aging time up to 4,000hours. However, the longitudinal wave velocity has a little change. Also, it has a good correlation between leaky surface acoustic wave velocity and Vickers hardness. Consequently, LSAW can be used to examine the degree of degradation in thermally aged Co-base superalloy.

• Journal of Mechanical Science and Technology
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• v.19 no.12
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• pp.2263-2269
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• 2005

The ship wave phenomena in the restricted waterway were investigated by a numerical analysis. The Euler and continuity equations were employed for the present study. The boundary fitted and moving grid system was adopted to enhance the computational efficiency. The convective terms in the governing equations and the kinematic free surface boundary condition were solved by the Constrained Interpolated Profile (CIP) algorithm in order to solve accurately wave heights in far field as well as near field. The advantage of the CIP method was verified by the comparison of the computed results by the CIP and the Maker and Cell (MAC) method. The free surface flow simulation around Wigley hull was performed and compared with the experiment for the sake of the validation of the numerical method. The present numerical scheme was applied to the free surface simulation for various canal sections in order to understand the effect of the sectional shape of waterways on the ship waves. The wave heights on the side wall and the shape of the wave patterns with their characteristics of flow are discussed.