Effects of internal flow on flow-induced vibrations of flexible cylinders with localized masses

To investigate the effects of internal flow on the flow-induced vibration of flexible cylinders subjected to external flow, this study develops a coupled dynamic model integrating computational fluid dynamics, the finite-element method, and strip theory, using a deep-sea mining riser with localized masses as the representative application. Numerical simulations systematically examine the effects of internal-flow velocity and localized-mass distribution on the vortex-induced vibration response. Results show that the lifting-pump position significantly affects the system dynamics. The maximum cross-flow (CF) amplitude and lift-coefficient amplitude occur when the pump is installed in the middle-lower section of the cylinder. Conversely, placing the pump near the mid-span significantly reduces the response amplitudes, identifying it as the preferred configuration. For the fixed external-flow velocity of v = 0.3 m/s considered in this study, increasing the internal-flow velocity from V = 1 to 5 m/s reduces the nondimensional CF and in-line amplitudes at the free end by approximately 73.2% and 72.0%, respectively, while the dominant response frequency decreases from approximately 7.02–6.66 Hz. The centrifugal and Coriolis terms induced by the internal flow modify the effective axial-force contribution and modal coupling of the system, providing a physical interpretation of the observed changes in vibration amplitude and dominant frequency. These findings provide transferable physical insight into the vibration control of slender flexible structures with localized discontinuities, with potential relevance to marine, aerospace, nuclear, and renewable-energy systems.