• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2005.10a
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• pp.147-152
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• 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
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• v.29 no.4
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• pp.344-350
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• 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 the Korean Society for Nondestructive Testing
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• v.36 no.1
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• pp.53-59
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• 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 Korean Society of Coastal and Ocean Engineers
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• v.23 no.5
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• pp.335-344
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• 2011

A curvilinear non-hydrostatic free surface model is developed to investigate nonlinear wave interactions in a circular channel. The proposed model solves the unsteady Navier-Stokes equations in a three-dimensional domain with a pressure correction method, which is one of fractional step methods. A hybrid staggered-grid layout in the vertical direction is implemented, which renders relatively simple resulting pressure equation as well as free surface closure. Numerical accuracy with respect to wave nonlinearity is tested against the fifth-order Stokes solution in a two-dimensional numerical wave tank. Numerical applications center on the evolution of nonlinear waves including diffraction and reflection affected by the curvature of side wall in a circular channel comparing with linear waves. Except for a highly nonlinear bichrmatic wave, the model's results are in good agreement with superimposed analytical solution that neglects nonlinear effects. Through the numerical simulation of the highly nonlinear bichramatic wave, the model shows its capability to investigate the evolution of nonlinear wave groups in a circular channel.

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

• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
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• 1997.10a
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• pp.25-30
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• 1997

쇄파대내의 파랑은 원래 비선형성이며 더욱이 불규칙적이다. 이러한 비선형 파랑은 수심의 변화에 의해 불안정하게 된다. 이때 분산 및 일산의 효과가 적절하다면 비선형성과 균형을 이루어 soliton과 같은 안정한 비선형 파동으로 안착하나 이것의 효과가 충분하지 못하면 파형 변동이 현저히 되어 때로는 쇄파가 생긴다. 쇄파하면 이에 의한 파의 흐트러짐이 발생하여 새로운 일산기구가 발생하므로 비선형이 약해지면서 균형을 이루게 되며, 최종적으로는 안정한 파열로 안착하게 된다. (중략)

• The Journal of the Acoustical Society of Korea
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• v.20 no.4
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• pp.54-61
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• 2001

Magnetostrictive materials have nonlinear elasto-magnetic properties. However the constitutive equations to describe the nonlinear properties are not available, yet. In this study we develope the equation in magnetostrictive materials by use of piezomagnetic constitutive equation which is quasi-linearized. With the wave equation, we determine the propagation velocity inside the magnetostrictive materials when a plane wave propagates along a given magnetic field. Validity of the calculated velocity is verified through comparison with experimental velocity measurement results for the most representative magnetostrictive materials. Terfenol-D.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.311-314
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• 2000

자왜 재료는 자계 포화 이하에서 비선형 특성을 갖는다고 알려져 있다. 그러나 지금까지 비선형 특성을 표현하는 자왜 재료의 구조 방정식을 유도한 사례는 전무한 실정이다. 본 연구에서는 자계 포화 이하에서 비선형 특성을 갖는 자왜 재료의 비선형 구조 방정식을 4차 텐서를 이용하여 유도하였다. 나아가 유도된 구조방정식을 이용하여 자왜 재료 내의 파동 방정식을 정식화하였다. 그리고 비선형 특성을 갖는 자왜 재료에서 평면파가 자계 방향을 따라 전파될 때 등방성 자왜 재료의 탄성파 속도를 구하였다. 이상의 결과를 검증하기 위해서 자왜 재료 중에서 가장 널리 사용되고 있는 Terfenol-D의 탄성파 속도를 측정하여 본 연구에서 유도한 자왜 재료 비선형 구조 방정식의 타당성을 일부 검증하였다. 향후에 자왜 재료의 정확한 특성을 측정하여 본 연구 결과와 비교함으로써 본 연구에서 유도한 비선형 구조 방정식의 타당성을 검증하고자 한다.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.7 no.1
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• pp.116-125
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• 1995

Higher order nonlinear transfer functions are applied to model the nonlinear responses obtained Inn dynamic analysis of single degree of freedom systems (SDOF) subjected to wave and current loadings. The structural systems are subjected to single harmonic, two wave combination and irregular wave loading. Three different sources of nonlinearities are examined for each of the wave loading condition and it is shown that the nonlinear response appear at the resonance frequencies of the SDOF even when virtually no wave energy exists at those resonance frequencies. Higher order nonlinear transfer functions based on Volterra series representation are used to model the nonlinear responses mainly f3r the flexible systems and clearly shows the degrees of nonlinearity either as quadratic or cubic.

• Journal of Ocean Engineering and Technology
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• v.12 no.1
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• pp.99-106
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• 1998

3차원 파수조에서 완전 비선형파를 시뮬레이션하기 위하여 우선 랜킨 소스를 기저로한 적분방정식을 고차경계요소법을 이용하여 이산화하였다. 그리고 방사경계조건은 파흡수 비치와 포텐셜 스트레칭 기법을 이용하여 모델링하였으며, 비선형 자유표면과 경계조건식은 고차 예측 및 보정 기법을 이용하여 시간 적분하였다. 파흡수 비치는 파의 진행방향 특성에 따라 수조내에 다양하게 배치할 수 있으며 비칭서 흡수가 덜된 파는 수조의 길이 방향 끝단에서 포텐셜 스트레칭 기법에 의하여 반사없이 진행하도록 하였다. 수치실험 결과 일-에너지 보존법칙과 모멘텀-임펄스 보존 법칙이 만족됨으로써 본 수치기법의 효용성이 검증되었다.