Free Vibration from Orthogonally Stiffened Infinite Cylindrical Shell in Water Chen Junming Huang YuyingIn the context of the coupling effect between medium and structure, the free vibration of an immerged orthogonally stiffened thin elastic shell is investigated. In the discussion, the coupling conditions on the surface of the shell and Dirac δfunctions are used to deduce the motion differential equations of free vibration of the coupling system, in which Kennard's thin shell theory and Helmholtz equation are employed. Further, the equations are solved by means of Fourier integral transformation, and then by the use of an operator function with periodicity, the dispersion equation of the system is derived. Using a method of searching along the real wavenumber axis, the free propagation vibrations in the structure are obtained.orthogonally stiffened cylindrical shell; coupling effect; free propagation vibration