ACOUSTIC SCATTERING BY SUBMERGED CYLINDRICAL-SHELL STIFFENED BY AN INTERNAL LENGTHWISE RIB

An experimental study of sound scattering by a thin-walled cylindrical shell stiffened by an internal lengthwise rib is presented. The resul ts show that, in the explored frequency band, the positions of the res onance frequencies of the wave S-0 are not affected much by the presen ce of a stiffener. Measurements made during the transient state after the quasiharmonic excitation of the stiffened shell have shown that, a t the S-0 wave resonance frequencies, backscattering is at its maximum when the rib is located at the vibration node of the shell. On the ot her hand, when in the total backscattered pressure, forced vibration i s also taken into account, the maxima are detected when the rib is loc ated at the vibration antinodes. In the stiffened shell other addition al resonances appear which are not excited in the case of an unstiffen ed shell. These resonances are attributed to the propagation of the fl exural type waves which are not detected in an unstiffened shell. In a stiffened shell these resonances appear as a result of the interactio n of the rib and the shell at their structural joint. These additional resonances are better observed when the rib is located at the ''illum inated'' part of the shell which indicates that most of the energy is scattered by the rib and shell joint. In the case of a short-pulse exc itation, specular reflections from a joint can be observed. The experi mental results correlate very well with the theoretical models.