BEM-FEM Acoustic-Structural Coupling For Spacecraft Structure incorporating Treatment of Irregular Frequencies

Paper number

IAC-07-C2.3.04

Author

Mr. Harijono Djojodihardjo, Universitas Al Azhar Indonesia, Indonesia

Year

2007

Abstract

The understanding of vibroacoustics, the study of acoustic disturbances and the resulting structural vibration, is very critical in the aerospace industry. Exterior sound pressure levels on a spacecraft structure during launch can reach 150 dB depending on the vehicle and the launch configuration. The magnitude of the acoustic loads transmitted to the payload is a function of the external acoustic environment as well as the design of the spacecraft structure and its sound absorbing treatments. The high intensity acoustic fields produced during a launch of a Space Shuttle or an Expendable Launch Vehicle (ELV) can easily damage a spacecraft’s mission critical flight hardware, such as its avionics, antennas, solar panels and optical instruments. The loads transmitted to the spacecraft structure from the launch vehicle (LV) in the first few minutes of flight are far more severe than any load that a payload experiences on orbit. Therefore, payloads are qualified by subjecting them to loads whose magnitude and frequency contents are representative of the launch environment. A more severe environment increases the cost of placing the payload into orbit because it must be designed to withstand higher launch loads.
It is with such motivation that the present study is carried out. Following earlier work, structural-acoustic interaction is modeled and analyzed using boundary and finite element coupling. The analysis is founded on the idealization of the problem into three parts; the calculation of the acoustic radiation from the vibrating structure, the finite element formulation of structural dynamic problem, and the calculation of the acousto-elasto-mechanic fluid-structure coupling using coupled BEM/FEM techniques. The computational scheme developed for the calculation of the acoustic radiation as well as the structural dynamic response of the structure using coupled BEM/FEM has given satisfactory results for acoustic disturbance in the low frequency range, which was the range of particular interest in many practical applications. However, for larger frequency range, it is well known that while the solution to the original boundary value problem in the exterior domain to the boundary is perfectly unique for all wave numbers, this is not the case for the numerical treatment of integral equation formulation, which breaks down at certain frequencies known as irregular frequencies or fictitious frequencies. This problem is completely nonphysical because there are no discrete eigenvalues for the exterior problems. 
To this end, CHIEF (Combined Helmholtz Interior integral Equation Formulation) method, suggested by Schenck and that suggested by Burton and Miller will be applied. Applications of CHIEF method to a spherical shell geometry has given excellent results. Modification of the method developed in earlier work using CHIEF and Burton-Miller approach will be further elaborated and compared to earlier results.

Abstract document

IAC-07-C2.3.04.pdf

Manuscript document

IAC-07-C2.3.04.pdf (🔒 authorized access only).

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