MODIFIED GEOMETRICAL ACOUSTICS PHENOMENA EXHIBITED BY COMPUTATIONS FOR HEMI-CAPPED CYLINDERS IN THE MID-FREQUENCY RANGE

摘要

The relation between surface pressure and normal velocity on a slender capped cylinder is examined. The data for this study are obtained from the wavenumber-based version of the surface variational principle (SVP), whose representation of the acoustic field is shown to consist of a discretely infinite sum of waves propagating in the circumferential and meridian directions. The circumferential wavenumbers are uncoupled, whereas coupling of the meridional spectra is described by the wet-surface impedance, whose value is obtained within an SVP analysis. Typical mid-frequency range results for this quantity in the case of a hemi-capped cylinder are examined. It is shown that the individual waves are essentially uncoupled, except at wavenumbers close to the pair at which a helical wave traveling along the surface transitions from subsonic to subsonic. The same type of phenomena are shown to occur over broad range of frequencies, even for reasonably short configurations. Interaction between wavenumber pairs, and the need to compute impedance values are the primary new aspects not encountered in ray theory analyses.