• Title/Summary/Keyword: Kirchhoff 근사법

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Derivation of Coherent Reflection Coefficient at Mid and Low Frequency for a Rough Surface (불규칙 경계면에 대한 중저주파수 간섭 반사 계수 유도)

  • Chu, Young-Min;Seong, Woo-Jae;Byun, Sung-Hoon;Kim, Sea-Moon
    • The Journal of the Acoustical Society of Korea
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    • v.28 no.3
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    • pp.174-186
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    • 2009
  • When we apply a propagation model to the ocean with boundaries, we can calculate reflected wave using reflection coefficient suggested by Rayleigh assuming the boundaries are flat. But boundaries in ocean such as sea surface and sea bottom have an irregular rough surface. To calculate the reflection loss for an irregular boundary, it is needed to compute the coherent reflection coefficient based on an experimental formula or scattering theory. In this article, we derive the coherent reflection coefficients for a fluid-fluid interface using perturbation theory, Kirchhoff approximation and small-slope approximation respectively. Based on each formula, we can calculate coherent reflection coefficients for a rough sea surface or sea bottom, and then compare them to the Rayleigh reflection coefficient to analyze the reflection loss for a random rough surface. In general, the coherent reflection coefficient based on small-slope approximation has a wide valid region. Comparing it with the coherent reflection coefficients derived from the Kirchhoff approximation and perturbation theory, we discuss a valid region of them.

Analysis of Ultrasonic Scattering from Side-drilled Holes (원주형 기공에 대한 초음파 산란 해석)

  • Jeong, Hyun-Jo;Park, Moon-Cheol
    • Journal of the Korean Society for Nondestructive Testing
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    • v.24 no.6
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    • pp.559-565
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    • 2004
  • Two different methods were used for the scattering analysis of side-drilled holes(SDH). The scattering models include an explicit model based on the Kirchhoff approximation and the solution of the exact separation of variables. The far-field scattering amplitude was calculated and their time-domain results were compared for the case of shear vertical wave. The exact solution predicts the existence of the creeping wave. The Kirchhoff approximation agreed to the exact solution, except the case of the creeping wave. Two measurement models were introduced to predict the response from the SDHs for the case of immersion, pulse-echo testing. The received voltage was calculated for the case of the shear vertical waves with the incident angle of $45^{\circ}$ to the SDH with the diameter of 1mm, and compared with the experimental results.

SH Wave Scattering from Cracks: Comparisons of Approximate and Exact Solutions (SH파의 균열 산란장 해석: 근사해와 엄밀해의 비교)

  • Jeong, Hyun-Jo;Park, Moon-Cheol;Song, Sung-Jin;Schmerr, L.W.
    • Journal of the Korean Society for Nondestructive Testing
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    • v.24 no.4
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    • pp.354-361
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    • 2004
  • This Paper describes a crack scattering model for SH wave based on the boundary integral equation(BIE) method, where the fundamental unknown is crack opening displacement(COD). When a time harmonic plane wave was incident on a 2-D isolated crack (slit) of width 2a, the COD distributions were numerically calculated as a function of ka. The calculated COD agreed well with results obtained with other methods. The far-field scattering amplitude, which completely characterizes the flaw response, was calculated in two ways. The Kirchhoff approximation and the BIE-COD exact formulation were compared in terms of incidence angle and frequency ka in a pulse-echo mode. Maximum response was obtained for both methods at the specular reflection direction. Away from the specular direction, the Kirchhoff approximation becomes less accurate. The time domain crack response was also calculated using a band-limited spectrum of center frequency 10 MHz. At oblique incidence to the crack both methods show the existence of an antisymmetric flash points occurring from the crack edge. The Kirchhoff approximation provides an exact time interval between flash points, although it unrealistically gives the same amplitude.

Analysis of Scattered Fields Using High Frequency Approximations (고주파수 근사 이론을 이용한 결함으로부터의 초음파 산란장 해석)

  • Jeong, Hyun-Jo;Kim, Jin-Ho
    • Journal of the Korean Society for Nondestructive Testing
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    • v.20 no.2
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    • pp.102-109
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    • 2000
  • This paper describes two different theories used to model the scattering of ultrasound by a volumetric flaw and a crack-like flaw. The elastodynamic Kirchhoff approximation (EKA) and the geometrical theory of diffraction (GTD) are applied respectively to a cylindrical cavity and a semi-infinite crack. These methods are known as high frequency approximations. The 2-D elastodynamic scattering problems of a plane wave incident on these model defects are considered and the scattered fields are expressed in terms of the reflection and diffraction coefficients. The ratio of the scattered far field amplitude to the incident wave amplitude is computed as a function of the angular location and compared with the boundary element solutions.

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Scattering characteristic analysis of Fresnel zone plate lens using TLM (TLM법을 이용한 프레넬 존 플레이트 렌즈 산란특성 해석)

  • 김태용
    • Proceedings of the Korea Society for Simulation Conference
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    • 2003.06a
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    • pp.15-18
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    • 2003
  • Most numerical techniques such as FEM, BEM, and MOM are able to analize electromagnetic scattering problems from arbitrary shapes. Although these methods could be applied to compute electromagnetic scattering problems in frequency domain, it was limited for electrodynamic problem in time domain. In this paper, electromagnetic scattering problem from Fresnel zone plate lens are considered. Some numerical results computed by TLM are compared with Kirchhoff's approximation and PO method.

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Development of Radar Cross Section Analysis Program for Complex Structures (복합 구조물의 레이더 반사면적 해석 프로그램 개발)

  • Kwon, Hyun-Wung;Hong, Suk-Yoon;Song, Jee-Hun
    • Journal of the Korean Society of Marine Environment & Safety
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    • v.20 no.4
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    • pp.435-442
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    • 2014
  • In this paper, radar cross section (RCS) analysis program, RACSAN has been developed to predict RCS of complex structures. RACSAN is based on the high frequency range analysis method of Kirchhoff approximation in physical optics (PO). This program can present RCS including multi-bounce effect in complex structures by combination of geometric optics (GO) and PO method. GO method has a concern in the evaluation of the effective area, and PO method is involved in the calculation of RCS for the final effective area that is evaluated by GO method. Comparisons of the predicted results and analytical solutions showed that the developed program could be an effective tool for predicting RCS in complex structures.

Characteristics of Elliptic Fresnel Zone Plate Antenna (타원형 프레넬 존 플레이트 안테나 특성)

  • Kim, Tae-Yong;Jo, Heung-Kuk
    • Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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    • 2011.10a
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    • pp.49-50
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    • 2011
  • Receiving sensitivity of the power gain by using Soret typed FZPL antenna should be worse when obliquely incident wave is illuminated on the FZPL. To solve this problem, elliptic Fresnel Zone Plate Lens antenna system should be introduced. Some numerical results computed by PO method are compared with Kirchhoff's approximation and measurement result.

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Finite Element Analysis of Thermoforming Process for Axisymmetric System (축대칭형계 열성형공정의 수치모사연구)

  • 이재욱
    • The Korean Journal of Rheology
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    • v.8 no.3_4
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    • pp.215-225
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    • 1996
  • 평판형의 열가소성 수지를 유리전이온도 이상으로 가열한 다음 압력을 가함으로써 원하는 형상의 제품을 성형하는 열성형공정은 대상 수지가 큰 변형을 일으킬 뿐만 아니라 비선형적 거동을 보이게 된다. 따라서 수지의 변형거동 예측과 최적성형조건의 설정에 많은 어려움과 시행착오를 거치게 되는 바, 열성형 공정의 최적화를 위한 연구의 일환으로 원형 평판위 수지를 대상으로 수지의 부풀림 거동과 이에 따른 두께 분포를 예측할수 있는유한요 소법의 수치모사 알고리듬을 개발하고자 하였다. Piola-Kirchhoff 응력 텐서와 Green 변형 텐서 및 lagrangian 변형 텐서를 사용하여 평판상의 응력-변형에 대한 비선형의 에너지 수 지식을 수립하고 Newton-Raphson 반복수렴법을 이용하여 근사적으로 해석하였으며 수지의 유변학적 구성방정식으로는 neo-Hookean 모델, Mooney-Rivlin 모델 및 Ogden 모델등의 초탄성 모델을 사용하여 그결과를 비교하였다. 수치모사에는 두께가 매우 얇기 때문에 두께 방향의 응력변화를 무시할수 있는 membrane 가정을 도입한 2차원적 해석과 두께 방향의 응력 변화를 고려하는 3차원적 해석을 모두 수행하고 그 차이를 비교하였으며 3차원적 해석 의 경우에는 penalty법을 이용하여 비 압축성을 만족하였다. 일차적으로 내압을 받는 두꺼 운 원통계에 대한 수치모사 해석을 수행하고 완전해와 비교함으로써 개발된 수치모사 알고 리듬의 열성형 공정에의 적용 타당성을 검증하였으며 이를 이용하여 원형 평판의 자유부풀 림거동을 예측한 결과 Treloar 등의 실험결과와 잘 부합함을 확인하였다. 또 간단한 형상의 금형이 있는 경우와 반지름 방향으로의 온도변화에 따른 수지의 변형거동을 해석함으로써 실제 열성형 공정에서 요구되고 있는 성형품의 두께 분포를 균일하게 하기 위한 방안을 제 시하였다.

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Simulation of Time-Domain Acoustic Wave Signals Backscattered from Underwater Targets (수중표적의 시간영역 음파 후방산란 신호 모의)

  • Kim, Kook-Hyun;Cho, Dae-Seung;Seong, Woo-Jae
    • The Journal of the Acoustical Society of Korea
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    • v.27 no.3
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    • pp.140-148
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    • 2008
  • In this study, a numerical method for a time-domain acoustic wave backscattering analysis is established based on a physical optics and a Fourier transform. The frequency responses of underwater targets are calculated based on physical optics derived from the Kirchhoff-Helmholtz integral equation by applying Kirchhoff approximation and the time-domain signals are simulated taking inverse fast Fourier transform to the obtained frequency responses. Particularly, the adaptive triangular beam method is introduced to calculate the areas impinged directly by acoustic incident wave and the virtual surface concept is adopted to consider the multiple reflection effect. The numerical analysis result for an acoustic plane wave field incident normally upon a square flat plate is coincident with the result by the analytic time-domain physical optics derived theoretically from a conventional physical optics. The numerical simulation result for a hemi-spherical end-capped cylinder model is compared with the measurement result, so that it is recognized that the presented method is valid when the specular reflection effect is predominant, but, for small targets, gives errors due to higher order scattering components. The numerical analysis of an idealized submarine shows that the established method is effectively applicable to large and complex-shaped underwater targets.

Investigation of Target Echoes in Multi-static SONAR system - Part II : Numerical Modeling with Experimental Verification (다중상태 소나시스템을 적용한 표적반향음 연구 - Part II : 수치모델링과 실험적 검증)

  • Ji, Yoon Hee;Bae, Ho Seuk;Byun, Gi-Hoon;Kim, Jea Soo;Kim, Woo-Shik;Park, Sang-Yoon
    • Journal of Ocean Engineering and Technology
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    • v.28 no.5
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    • pp.440-451
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    • 2014
  • A multi-static SONAR system consists of the transmitters and receivers separately in space. The active target echoes are received along the transmitter-target-receiver path and depend on the shape and aspect angle of the submerged objects at each receiver. Thus, the target echo algorithm used with a mono-static system, in which the transmitter and receiver are located at the same position, has limits in simulating the target echoes for a multi-static SONAR system. In this paper, a target echo modeling procedure for a 3D submerged object in space is described based on the Kirchhoff approximation, and the SONAR system is extended to a multi-static SONAR system. The scattered field from external structures is calculated on the visible surfaces, which is determined based on the locations of the transmitter and receiver. A series of experiments in an acoustic water tank was conducted to measure the target echoes from scaled targets with a single transmitter and 16 receivers. Finally, the numerical results were compared with experimental results and shown to be useful for simulating the target echoes/target strength in a multi-static SONAR system.