Structural-Acoustic Mode Coupling in a Bolted Aluminum Cylinder

This work presents a modal test on a cylindrical bolted structure that initially appeared to be a routine model calibration experiment. However, while reviewing the test data the structure appeared to have two pairs of ovaling modes with identical shapes. Assuming this to be the result of an uninstrumented component of the test article, extensive efforts were conducted to identify this feature. When all options were exhausted, the interaction between the structure and the air contained within was investigated. Contrary to the typical assumption that the fluid-structure interactions are negligible for such a thick walled cylinder, analysis showed that for this test article the acoustic modes of the internal air significantly impacted the structural response. In this case, the acoustic and the structural modes coincided in frequency, causing the first ovaling modes to split into two pairs at different frequencies. Experimental and analytical results are presented that describe this structural-acoustic mode coupling phenomenon.