This paper studies analytically the effects of an external mean flow and an internal gap mean flow on sound transmission through a double-wall cylindrical shell lined with poroelastic materials. The shell motions are described by Love's theory, and the poroelastic lining is modeled as an equivalent fluid by considering only the energetically most dominant wave based on Biot's theory. The transfer matrix method is employed to solve the system by applying appropriate boundary conditions. The random incidence transmission loss of the double-wall cylindrical shell in a diffuse sound field is calculated numerically in a wide frequency range. The numerical results suggest that the external flow and the gap flow have similar effects of improving considerably the sound insulation performance of such a double-wall sandwich shell below the coincidence frequency. In the presence of the external mean flow, increasing the gap flow Mach number enhances the transmission loss only when it becomes greater than the external flow Mach number. The internal gap flow has great potential for active/passive noise control and hence this gap flow effect needs further research.
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