S. Yoshikawa et al., VIBRATION OF 2 CONCENTRIC SUBMERGED CYLINDRICAL-SHELLS COUPLED BY THEENTRAINED FLUID, The Journal of the Acoustical Society of America, 95(6), 1994, pp. 3273-3286
The vibrational characteristics of a point-driven ''double shell'' (tw
o concentric submerged cylindrical shells coupled by the entrained flu
id) are investigated theoretically and experimentally. Of particular i
nterest are the shielding effects, if any, of the outer shell upon the
inner shell. The theory on the double shell is based on Flugge's infi
nite-shell equations, the Helmholtz wave equation, and boundary condit
ions at the fluid-structure interfaces. This theory is used to model a
finite double-shell structure in wave number-frequency space. Experim
ents are carried out in which generalized near-field acoustical hologr
aphy (GENAH) is employed to provide the experimental vibration charact
eristics in wave number-frequency space of the finite double shell. It
is confirmed theoretically and experimentally that the outer shell of
the double shell exhibits two separate dispersion curves: A higher-fr
equency dispersion curve exhibits in-phase vibrations with respect to
the inner shell, and a second lower-frequency curve, out-of-phase vibr
ation. The higher-frequency dispersion curve of the double shell is ve
ry similar to the dispersion curve of the single shell (the inner shel
l without the outer shell), and thus is identified as a forced wave nu
mber response. The lower-frequency curve seems to be dependent on the
free wave number response of the outer shell alone of the double shell
. A double-shell structure can usually reduce its vibrational amplitud
es by splitting the response of single-shell's forced vibration into t
he responses of inner-shell's forced vibration and outer-shell's induc
ed vibration. However, it radiates low-frequency underwater sounds ine
vitably according to the lower-frequency dispersion curve. Furthermore
, the appearance of the inner shell's dispersion curve on the outer sh
ell seems to indicate that the shielding influence of the outer shell
is not completely effective.