Mid-Frequency Acoustic Scattering from Finite Internally-Loaded Cylindrical Shells Near Axial Incidence.
In this thesis, I interpret monostatic and bistatic acoustic scattering measurements for finite cylindrical shells in water to determine the influence of internal structures and the endcap. I study the interaction of a transient acoustic pulse with the endcap and investigate elastic wave scatter at both a slope discontinuity and at a ring of mass plus stiffness. The shell configurations include an empty shell, a shell with four internal ring stiffeners, a duplicate stiffened shell with sprung internals, and an internally-loaded shell with an external constrained layer damping treatment. The scattering measurements were conducted using transient acoustic pulses in the mid-frequency range (2 < ka < 11) where acoustic wavelengths are similar to the cylindrical shell radius a, and k is the acoustic wavenumber in water. The scatter is discussed in terms of axisymmetric compressional and flexural waves, which are dominant near axial incidence. Incidence to 25^circ relative to the shell axis is considered, where 25^circ delineates a transitional region of incidence beyond which forced wave scatter becomes important. Near axial incidence, the initial part of the backscatter is caused by radiation from the endcap of compressional waves whose energy decays rapidly with time. The onset of the flexural response is indicated by a significant reduction in the decay rates, where the transition time equals the flexural wave group delay to the first discontinuity beyond the insonified endcap. The frequencies considered encompass a transitional range in terms of the mechanism for acoustic coupling at the endcap. At higher frequencies (ka > 9), the interaction is a local to the spherical section of the endcap and compressional waves are excited via trace-matching. At lower frequencies, the endcap moves as a piston and excites both compressional and flexural waves. The rings primarily reflect the incident structural waves; however, strong coupling between neighboring rings creates pass bands at certain combinations of frequency and frame spacing. The sprung internals only weakly influence the scatter because they are essentially decoupled from the shell motions except at frequencies near the ring resonances.
- Pub Date:
- January 1995
- Physics: Acoustics; Engineering: Marine and Ocean