Acoustic Resonances in Rectangular Open Cavities

Acoustic resonances are computed numerically in rectangular two-dimensional deep and shallow open cavities. Unconfined cavities as well as cavities confined in a two-dimensional infinitely long duct are considered. Due to radiation losses these resonances are in general complex valued. All computations are made for zero mean flow. Perfectly Matched Layer (PML) absorbing boundary conditions are employed to avoid unphysical reflections at the necessarily finite grid boundaries. In confined cavities a few resonant modes, so-called trapped modes, exist with zero radiation losses, and significant differences are observed between confined and unconfined cavity resonances. A comparison with the frequencies of the modes observed by Rossiter suggests that the dominant Rossiter modes lock onto resonant cavity frequencies.