18 Numerical Modeling of an Enclosed Cylinder 193 Fig. 18.1 FE mesh of the shell structure (a) and final shell design in free-free test setup (b) Fig. 18.2 Experimental FRFs for the designed acoustoelastic system showing the effect on coupling of adding air damping, broadband (a) and zoomed in at the (2,1) mode (b) 18.2.4 Example Test Data An example modal hammer test frequency response function (FRF) is shown in Fig. 18.2. Here, a modal hammer impacts radially at the location shown in Fig. 18.1 and the response is measured by accelerometers at several locations. Included in this figure are drive point FRFs for two test configurations: the nominal, coupled structure and the structure with foam added to the cavity to damp out the acoustic modes, a technique previously demonstrated on a different coupled structure [2]. The two coupled modes can be seen around 1890 Hz and 2600 Hz, evidenced by an additional peak in the FRF which is removed when the air is damped with foam. A more detailed summary of the experimental examination of this hardware is found in [4]. If the structure and acoustic modes were very close in frequency, a split in the mode near 1800 Hz would be expected. As the coupling instead looks like an additional peak around 1890 Hz with little shift in frequency of the main peak, the hardware does not have perfectly aligned structure and acoustic modes.
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