Characteristics of wave propagation, vibration transmission and acoustic emission in fluid-filled coaxial periodic shells

Abstract

The wave propagation, vibration transmission and acoustic emission characteristics in the two coaxial shells containing fluid are studied. A finite element method (FEM) is utilized to conduct the investigation. Elastic band gaps (BGs) are generated in this coaxial shell system with either light or heavy fluid loading if the cylindrical shell walls are design to be periodic structure. For light fluid loading, BGs of the fluid-filled coaxial shell system are almost the same as those for the inner and outer shells in vacuum. Because the coupling effect of the annular light fluid is so weak that it can be neglected; thus, the inner and outer shell may behave independently. In terms of heavy fluid loading, the confined annular fluid region acts as an effective energy transmission medium that links the two shell motion together. The “hydrodynamic” mass induced by the annular-fluid can be far larger than its actual mass when the annular gap is sufficient narrow. As a result, the fluid inertial loading on the other shell can be quite enormous, rendering the central frequencies of BGs for the coaxial periodic shell system notably decreased. The BG formation mechanism may be attributed to (i) phenomenon of interference of waves reflected by these repeated “periodicity cells”, (ii) cut-off frequencies and (iii) hybrid vibration motion modes of different shell sections, according to various circumferential modes and frequency ranges. The acoustic pressure in the narrow annular region is extremely high, if the filling fluid is dense. However, when the harmonic frequency is located at the BG, the acoustic pressure becomes weaker and weaker along the axial direction of the shell; in contrast, if frequency is in the wave pass bands, the acoustic pressure levels will remain as strong as at the front end of the shell throughout the shell.