Abstract: The acoustic modal analysis of cavity is the important matter on vehicle NVH performance study. Identifying system mode is of importance to avoid acoustic resonance and reduce interior noise. The body structure model and the cavity model for a car are built using FEM. Fluid-structure interaction (FSI) model of body system that considers the interaction between structure and air is built. The FE equations for three systems are listed. Making use of MSC.Nastran software, modal analysis is simulated for the three models. The modal frequency and distortion section of structure, the acoustic modal frequency and sound pressure distribution of cavity as well as the change of the modal frequency and shape for the structure and cavity in FSI system are discussed in detail. The equal-sound-pressure- curves on the longitudinal symmetry surface of cavity are measured for validating the simulation. The results show that the interaction between structure and air can change the modal frequency and shape of original system (body structure or cavity), and can bring another system to vibrate in a particular pattern. The first acoustic mode of the system is a longitudinal mode. The distribution and trend of the modal shape, which come from test, are accord with the simulation result.

Key words: Fluid-structure interaction (FSI), Modal analysis, MSC.Nastran, Noise vibration harshness (NVH), Simulation