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시간 영역 음향 경계요소법에서의 비유일성 문제 해결을 위한 방법에 관하여
장해원,이정권,Jang, Hae-Won,Ih, Jeong-Guon 한국음향학회 2012 韓國音響學會誌 Vol.31 No.1
Kirchhoff 적분식을 이용하여 외부 음향 문제의 시간 영역 응답을 계산하는 경우, 주파수영역 해석과 마찬가지로 가상적인 내부 음향 모드에 기인한 비유일성 문제가 발생한다. 이를 해결하는 방법들 중의 하나로서 CHIEF(Combined Helmholtz Integral Equation Formulation) 방법이 쓰이는데, 이는 몇몇 내부 수음점의 응답을 0으로 추가하여 구속하는 조건을 부가하는 기법이다. 이 기법은 주파수 영역 경계요소법에서는 간편한 수식 때문에 많이 사용되고 있지만, 시간 영역에서는 사용된 예가 없다. 본 연구에서는 대상체 내부의 가상 수음점과 경계 표면의 절점들간의 최소 거리에 대한 지연시간을 고려하여, 계산하고자 하는 미지수인 현재 시간의 경계 표면 음장을 구속함으로써, 시간 영역 해석에 적합하도록 CHIEF 방법을 수식화하였다. 예제로서, 반지름 방향으로 진동하는 구의 음향 방사 문제를 다루었다. CHIEF 방법을 적용함에 따라 저차의 내부 음향 모드에 기인한 비유일성 문제를 해결할 수 있었고, 비요동 모드에 의한 수치적 불안정성을 피할 수 있었다. 그러나, 유효주파수 밖에 남은 내부 음향의 고차모드들에 의한 수치적 불안정성은 증가하였다. The time-domain solution from the Kirchhoff integral equation for an exterior problem is not unique at certain eigen-frequencies associated with the fictitious internal modes as happening in frequency-domain analysis. One of the solution methods is the CHIEF (Combined Helmholtz Integral Equation Formulation) approach, which is based on employing additional zero-pressure constraints at some interior points inside the body. Although this method has been widely used in frequency-domain boundary element method due to its simplicity, it was not used in time-domain analysis. In this work, the CHIEF approach is formulated appropriately for time-domain acoustic boundary element method by constraining the unknown surface pressure distribution at the current time, which was obtained by setting the pressure at the interior point to be zero considering the shortest retarded time between boundary nodes and interior point. Sound radiation of a pulsating sphere was used as a test example. By applying the CHIEF method, the low-order fictitious modes could be damped down satisfactorily, thus solving the non-uniqueness problem. However, it was observed that the instability due to high-order fictitious modes, which were beyond the effective frequency, was increased.
장해원(Jang Hae-Won),박순홍(Park Soon-Hong),서상현(Seo Sang-Hyun),한제헌(Han Je-Heon) 한국소음진동공학회 2016 한국소음진동공학회 학술대회논문집 Vol.2016 No.4
Composite sandwich structures are widely used in aerospace and civil infrastructure due to their low weight as well as high flexural and transverse shear stiffness. In this work, vibration reduction performance of a damping sheet on composite sandwich structures was evaluated by measuring the damping ratio of the main bending mode. By applying a single damping sheet on composite sandwich beam, the damping ratio was increased by 4.0%. Meanwhile, the effect of the additional damping sheet is not great enough to 2.0%, and the results according to the change in the excitation level is not significant with less than 0.5%. In the test of composite sandwich plate, the damping ratio was increased by 2.0%. It is also noted that by applying a weighty mock-up as a avionics hardware, the dynamic stiffness became significantly lower, and the damping ratio was increased by more than 4.0%. The test results will be used for the efficient design through the verification of the analysis model.