Vibration analysis of cylindrical shells made of functionally graded porous materials with variable thickness in contact with fluid

This study presents a comprehensive semi-analytical framework to investigate the vibrational behavior of variable-thickness cylindrical shells (encompassing four continuous and one stepped profile) made of Functionally Graded Porous Materials (FGPMs) with three distinct porosity distribution patterns, in contact with a fluid. The core novelty of this research lies in integrating material and geometric complexities with internal hydrodynamic effects within a unified model. To this end, the governing equations of the Fluid-Structure Interaction (FSI) system were derived utilizing the First-order Shear Deformation Theory (FSDT) for the structure and velocity potential theory for the fluid. To solve the equations, an efficient method based on the Rayleigh-Ritz approach was developed, employing orthogonal polynomials generated via the Gram-Schmidt process to approximate the displacement fields. For model validation, the convergence of the solution method was first evaluated, and the results were subsequently benchmarked against existing literature and Finite Element Method (FEM) simulations. Furthermore, a comprehensive experimental campaign involving Experimental Modal Analysis (EMA) was conducted on isotropic steel specimens with uniform and stepped thicknesses, under both dry and fluid-filled conditions. The good correlation between the analytical results and experimental data, particularly in extracting natural frequencies, mode shapes, and Frequency Response Functions (FRFs), confirms the efficacy and accuracy of the proposed method. Ultimately, the results demonstrate that the strategic tuning of geometric gradients and porosity distributions can effectively counteract the frequency-reducing effect induced by the fluid “added mass,” thereby providing an effective design paradigm for optimizing lightweight fluid-containing structures. This is particularly relevant for optimizing internal fluid storage facilities and piping systems in ocean and aerospace engineering, where tuning dynamic characteristics is vital to prevent fluid-induced resonance.