Numerical models of periodically stiffened axisymmetric submerged shells impinged by an acoustic plane wave have been developed in order to study the scattered pressure by this target. Along with experimental results, they show that Bragg, Bloch-Floquet and helical waves can be observed. Nevertheless, the influence of added non-axisymmetric internal frames on the propagation of these waves has not been intensively studied. One can wonder if the scattering from these waves can still clearly be seen in this case. However, adding non-axisymmetries in the system couples the circumferential orders of the Fourier series and considerably increases the computational costs. A sub-structuring approach called the Condensed Transfer Function (CTF) method has been recently developed to couple subsystems along linear junctions. The displacements and forces at the junctions are decomposed on a set of orthonormal functions called condensation functions. Condensed transfer functions are defined for each uncoupled subsystems, and the behaviour of the coupled system can be deduced thanks to the superposition principle for passive linear systems, the force equilibrium and displacement continuity. The main advantage of the method is that the uncoupled subsystems can be described by any method. Thus, in the case of a submerged stiffened shell with non-axisymmetric internal frames, the axisymmetric stiffened submerged shell is described on one hand by a method called Circumferential Admittance Approach (CAA) while on the other hand the non-axisymmetric internal substructures are modelled by the Finite Element Method (FEM). The CAA is a dedicated model which uses the axisymmetry to save computational costs while the FEM offers a great flexibility on the design of the internal structures. A periodically stiffened submerged cylindrical shell including non-axisymmetric structures will be given as an example and the scattered pressure will be compared to the axisymmetric case.
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