Acoustic-structural resonances of thin-walled structure-gas systems

This paper summarizes a theoretical study, which is a natural continuation of approximately 50 years of research in the field of acoustoelasticity. Recently, the researchers' interest has been directed towards considering compressible fluid in contact with thin walled structures as it was found that the acoustic-structural coupling significantly changes the dynamic behavior of the system. Despite the interesting findings the main results still need additional, numerical, or experimental verification. The present work is intended to cast more light on the acoustic-structure coupling of light fluid-shell systems using a numerical approach, namely 3-D finite element (FE) modeling. Two different acoustoelastic systems are considered. The first system is a thin circular cylindrical shell containing light fluid in a coaxial annular duct and the second system is a thin-walled vehicle passenger compartment interacting with the enclosed cavity. Both systems are studied using ANSYS finite element code. The modeling involved shell finite elements for the structure and 3-D acoustic elements for the cavity. The 3-D FE modal analysis used produced results visualizing the complex picture of acoustic-structure coupling. It was confirmed that (1) in both fluid-elastic systems the strongest acoustic-structural coupling exists if the resonances of uncoupled acoustic and mechanical systems are close and (2) the nature of the acoustic-structural coupling is identical in the two cases studied. However it was found that strong coupling between the thin-walled structure and the acoustic cavity exists in the vicinity of any uncoupled acoustic resonance. Thus, the coupled properties of the systems were found to be dominated by the uncoupled acoustic resonances. As the focus of this study is on the mode shapes of vibration, it was found that coupled acoustic-structure modes of vibration exist in the neighborhood of an uncoupled acoustic resonance, which means that the coupled system manifests a specific type of energy exchange. These modes were termed coupled "combined" modes to differentiate from the coupled component responses. It was also found that the coupled "combined" modes are clustered around a rigid-walled cavity mode, and any acoustic-structure resonance of a given group involves this particular uncoupled acoustic mode. In conclusion, it is shown that the acoustic-structure interaction causes the appearance of coupled "combined" modes not existing in the shell in vacuo or rigid-walled acoustic spectrum. It was found also that the subsystems preserve their capability of independent vibration responses, i.e., the response at the component modes is believed to be strong at their uncoupled frequencies.