Vibroacoustic behaviour analysis of large structures coupled with cavities, like those used in common transport vehicle (aircraft, autobus, train,-) may leads to very large numerical systems to be solved. To avoid enormous time computation, a numerical sub-structure technique is needful. In this framework, initially we propose an approach which takes into account, in the beginning, the structureal part of the problem. It consists of a method which subdivides the global structure into elementary cells. Each cell is described by a finite elements method taking into accoumt its physical properties. A similar method is used for the global cavity which is partitioned into elementary sub-domains computed by a finite element mehtod taking into account its acoustical properties. The modal shapes of the global cavity and the globalstructure are determined by using the elementary cells (acoustical and structural) modal shapes. The comtinuity conditions at the differnt sub-domains intefaces are satisfied by introducting Lagrange multipliers into the formulation. To reduce the dimension of the systems to be solved, we sue a system to be solved, we sue a system of generalised coordinates by projecting the matrixes through a base of eigenvectors computed locally for each sub-domain with introduction of mixed boundary conditions at the sub-domain itnerfaces.
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