• Proceedings of the Korean Institute of Navigation and Port Research Conference
• /
• 2005.10a
• /
• pp.147-152
• /
• 2005

선형파 이론에 의한 파랑스펙트럼 분포에 의해서는 30m 크기의 파랑은 현실적으로 거의 발생 불가능하다고 인식되어 왔다. 그러나 최근의 위성 영상을 이용한 조사에 의해 3주간의 기간 통안 25m 이상의 거대파가 10개 이상 관측됨에 따라 실해역에서 빈번히 마주칠 수 있는 현상임이 입증되었으며 이에 따라 지금까지 이유 불명으로 치부되어 왔던 많은 해양 재난이 거대파에 의해 발생했던 것으로 추정되고 있다. 거대파의 발생원인은 파군 형성과 관련한 파고분포 특성의 변화, 전파하는 파군의 비선형 공명간섭 통이 제기되고 있으나, 그 출현의 복잡성과 자료의 부족 등으로 아직 명확하게 해명되지 못하고 있다. 본 연구에서는 실해역에서 발생하는 거대파의 특성 및 선형 및 비선형이론에 근거한 거대파 발생 기구를 고찰하고 비선형 파랑전파를 모사할 수 있는 수치모형을 개발하였다.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.29 no.4
• /
• pp.344-350
• /
• 2009

The goal of this study is to introduce the theoretical background of acoustic nonlinearity in surface wave and to verify its characteristics by experiments. It has been known by theory that the nonlinear parameter of surface wave is proportional to the ratio of $2^{nd}$ harmonic amplitude and the power of primary component in the propagated surface wave, as like as in bulk waves. In this paper, in order to verify this characteristics we constructed a measurement system using contact angle beam transducers and measured the nonlinear parameter of surface wave in an Aluminum 6061 alloy block specimen while changing the distance of wave propagation and the input amplitude. We also considered the effect of frequency-dependent attenuation to the measurement of nonlinear parameter. Results showed good agreement with the theoretical expectation that the nonlinear parameter should be independent on the input amplitude and linearly dependent on the input amplitude and the $2^{nd}$ harmonic amplitude is linearly dependant on the propagation distance.

• Journal of Navigation and Port Research
• /
• v.30 no.3 s.109
• /
• pp.181-187
• /
• 2006

In the wave spectrum distribution based on linear wave theory, the appearance of a giant wave whose wave height reaches to 30m has been considered next to almost impossible in a real sea However since more than 10 giant waves were observed in a recent investigation of global wave distribution which was carried out by the analysis of SAR imagines for three weeks, the existence of the giant waves is being recognized and it is considered the cause of many unknown marine disasters. The change of wave height distribution concerning a formation of wave train, nonlinear wave to wave interaction and so on were raised as the causes of the appearance of the giant waves, but the occurrence mechanism of the giant waves hasn't been cleared yet. In present study, we investigated appearance circumstances of the giant waves in real sea and its occurrence mechanism was analyzed based on linear and nonlinear wave focusing theories. Also, through a development of numerical model of the nonlinear $schr\"{o}dinger$ equation, the formations of the giant wave from progressive wave train were reproduced.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.6 no.1
• /
• pp.61-71
• /
• 1994

A modified method obtained by expanding Longuet-Higgins and Stewart's method (1964) is proposed. which can easily derive the group-bountied long wave due to the irregular were group as well as the regular wave group. The result of the proposed method agree well with those of both second order nonlinear theory and radiation stress theory. Particularly in the shallow water region, three equations from the proposed method, the second order nonlinear theory and the radiation stress theory become identical.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.36 no.1
• /
• pp.53-59
• /
• 2016

Ultrasonic nonlinear evaluation is generally utilized for detection of not only defects but also microdamage such as corrosion and plastic deformation. Nonlinearity is determined by the amplitude ratio of primary wave second harmonic wave, and the results of its comparison are used for evaluation. Owing to the experimental features, the experimental nonlinearity result contains system nonlinearity and material nonlinearity. System nonlinearity is that which is unwanted by the user; hence, it acts as an error and interrupts analysis. In this study, a bulk wave mixing technique is implemented in order to minimize the system nonlinearity and obtain the reliable analysis results. The biggest advantage of this technique is that experimental nonlinearity contains less system nonlinearity than that for the conventional nonlinear ultrasonic technique. Theoretical and experimental verifications are performed in this study. By comparing the results of the bulk wave mixing technique with those of the conventional technique, the strengths, weaknesses, and application validity of the bulk wave mixing technique are determined.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.30 no.5
• /
• pp.458-463
• /
• 2010

Since the acoustic nonlinearity is sensitive to the minute variation of material properties, the nonlinear ultrasonic technique(NUT) has been considered as a promising method to evaluate the material degradation or fatigue. However, there are certain limitations to apply the conventional NUT using the bulk wave to thin plates. In case of plates, the use of Lamb wave can be considered, however, the propagation characteristics of Lamb wave are completely different with the bulk wave, and thus the separate study for the nonlinearity of Lamb wave is required. For this work, this paper analyzed first the conditions of mode pair suitable for the practical application as well as for the cumulative propagation of quadratic harmonic frequency and summarized the result in for conditions; (1) phase matching, (2) non-zero power flux, (3) group velocity matching, and (4) non-zero out-of-plane displacement. Experimental results in aluminum plates showed that the amplitude of the secondary Lamb wave and nonlinear parameter growed up with increasing propagation distance at the mode pair satisfying the above all conditions and that the ration of nonlinear parameters measured in Al6061-T6 and Al1100-H15 was closed to the ratio of the absolute nonlinear parameters.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.15 no.1
• /
• pp.51-58
• /
• 2003

The magnitude of evanescent modes in terms of dynamics it investigated in case that the transformation of water waves is predicted by the linear wave theory. For the waves propagating over two steps, the eigenfunction expansion method is used to predict the amplitudes of reflected and transmitted waves by the component of evanescent modes as well as propagating modes. Then. the relative importance of evanescent modes to the propagating modes is investigated. The numerical experiments find that the evanescent modes are pronounced at the relative water depth of k$_1$h$_1$＝0.11$\pi$ and the water depth ratio of h$_2$/h$_1$ close to zero.

• Journal of the Society of Naval Architects of Korea
• /
• v.37 no.2
• /
• pp.1-13
• /
• 2000

In the present paper, an infinite-depth unified theory is applied to the computation of the linear and second-order hydrodynamic forces on slender bodies. No forward speed is assumed, which is valid for some types of ships, like FPSOs and shuttle tankers. Strip theory solution, which is essential for the extension to theory is extended to unified theory, was obtained using NIIRD program developed at MIT. The linear theory is extended to the computation of the second-order mean-drift forces and moment. Furthermore, Aranha's formular is applied to the prediction of wave drift damping coefficients. From this study, it is proved that unified theory provides an accuracy comparable with 3D panel method for the second-order forces as well as the linear solution with much less computational effort.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.19 no.4
• /
• pp.355-360
• /
• 2007

The process of the nonlinear shallow water wave transformation in a basin of a constant depth is studied. Characteristics of the first breaking of the wave are analyzed in details. The Fourier spectrum and steepness of the nonlinear wave are calculated. It is shown that the spectral amplitudes can be expressed using the wave front steepness, which allows the practical estimations.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.30 no.2
• /
• pp.139-145
• /
• 2010

Bending fatigue of aluminium alloy was characterized by acoustic nonlinearity of narrow band laser-generated surface wave. The higher harmonic components generated intrinsically by arrayed line laser beam were analyzed theoretically and acoustic nonlinearity was measured successfully on the surface of fatigue damaged aluminium 6061 alloy. The acoustic nonlinearity increased as a function of fatigue cycles and has close relation with damage level. Consequently, the nonlinear acoustic technique of laser-generated surface wave could be potential to characterize surface damages subjected to fatigue.