Nearfield Acoustic Radiation from a Point-Excited Cylindrical Shell.

The surface force exerted by a dense medium such as water upon an elastic structure undergoing dynamic motion is often comparable to the inertial and damping forces found within the structure. Fluid loading may alter the structural response by strongly coupling the elastic and acoustic media. Any attempt to locally deform the structure may lead to a slowly decaying pressure field that excites the elastic structure further away. In this study, the acoustic nearfield and radial surface deformation of a cylindrical shell of infinite axial extent is examined. The shell is subject to external fluid loading and is excited by a radially acting, time harmonic point force. The shell is modeled using Flugge's thin shell theory. Eigenfunction and integral transform methods are used to formulate the shell's radial deformation and the radiated acoustic pressure in terms of an infinite Fourier series of inverse transform integrals. The frequency spectra of the propagating and non-propagating modes of vibration are described. The propagating modes are compared to those of the shell in vacuo. For each circumferential mode, real singularities are shown to exist only at frequencies between a lower and an upper cut-off frequency. For fixed driving frequency, a limited number of circumferential modes will exhibit a real singularity. The inverse transform integrals are solved by Cauchy's theorem. In the acoustic nearfield and at frequencies below the classical plate coincidence frequency, the solutions are shown to be dependent only upon the residue contribution of a single real singularity at each of a small number of modes. Strong acoustical coupling exists between the driving-point excitation and the acoustic nearfield. Above the coincidence frequency, the solutions are dominated by a branch line integral over subsonic axial wavenumbers that correctly exhibits the coincidence beaming effect. The acoustic coupling between the drive point and points outside this beaming region is weak. Predictions of the acoustic nearfield and the surface radial displacement are given over a wide frequency range.