1 BEM-FEM Acoustic-Structure Interaction For Modeling and Analysis of Spacecraft Structures Subject to Acoustic Excitation Harijono Djojodihardjo Professor, Universiti Sains Malaysia harijono@djojodihardjo.com and Irtan Safari Graduate Student, Universiti Sains Malaysia tan_safari@yahoo.co.uk Abstract Spacecraft structures are subject to acoustic load and high frequency vibration, particularly during launch, which can impose severe and adverse affect ot the structures of the spacecraft and their payloads. For many classes of structures exhibiting a plate-like vibration behavior, such as antennas and solar panels, their low- order mode response is likely to be of greatest importance. Based on these considerations, the present work is focused on modeling and analysis of spacecraft structure subject to acoustic excitation, and for this purpose, the acoustic-structure interaction problem is modeled and analyzed using boundary and finite element coupling. The analysis has been developed using three parts of approach: the calculation of the acoustic radiation from the vibrating structure, the finite element formulation of structural dynamic problem, and the calculation of the acoustic-structural coupling using coupled BEM/FEM techniques. The development of the computational scheme for the calculation of the structural dynamic response of the structure using coupled BEM/FEM will be elaborated. Some generic examples typical for spacecraft structure are elaborated. 1. Introduction The propagation of vibration through structures, the radiation of sound from vibrating structures, and fluid/structure interaction are all elements that are significant in structural acoustic problems on aerospace applications [1]. 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. The methods and related numerical computation codes in structural acoustics for the prediction of noise emitted by structural vibration in all the audible low-, medium- and high- frequency band, play a very important role in the design and the conception of industrial products. These methods allow the design to be improved before construction and optimization with respect to the acoustical problems. Acoustic loads are a major component of the launch environment for spacecraft structure. Exterior sound pressure levels on a spacecraft structure during launch as depicted in Fig.1a can reach 150 dB [1][2] 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. For many classes of structures exhibiting a plate-like vibration behavior, such as antennas and solar panels as depicted in Fig.1b, their low-order mode response is likely to be of greatest importance. (a) (b) Fig. 1 (a) Spacecraft structure during launch, and (b) spacecraft structure in acoustic test configuration In the Structural-Acoustic Analysis for Aerospace Structure Design problems, it is recognized that Computational Structural Mechanics (CSM) is efficient for structural-acoustic prediction in low-frequency ranges. Analysis of the detailed behavior of individual modes is possible using finite-element and classical methods. Addressing the acoustic structural interaction problem for dealing with the influence of acoustic excitation on the structure, the coupled finite element method/boundary element method (FEM/BEM) is a convenient means of computing responses for an arbitrarily elastic structure submerged in a fluid subjected to alternating external forces and/or acoustic excitation, which has been addressed by the authors in previous work [3][4][5] as well as by other investigators [6][7][8]. The FEM is used to describe the dynamic behavior of the structure, while the BEM is used to represent the surface acoustic loading on the structure. The coupling boundary conditions between the fluid and structure are the continuity of wetted surface normal velocity and the surface pressure acting as a loading on the structure. The formulation of the basic problem of acoustic excitation and vibration of elastic structure in a coupled fluid-