A generalized engineering approach to simulate vortex-induced vibrations

Vortex-induced vibrations is a complex fluid-structure interaction phenomenon characterized by a strong non-linear feedback between structure oscillations and fluid forcing. When vortex shedding frequency approaches a modal frequency of the structure, fluid-structure synchronization develops, leading to large cross-flow vibrations, especially for lightweight and low-damped structures. The paper, supported by wind tunnel tests on a rigid flexibly-mounted building-type structure, provide a comprehensive description of the mechanics and modelling of vortex-induced vibrations and forces. Under the assumption of a fully synchronized fluid-structure interaction (lock-in), a 1-dof analytic formulation of VIV mechanics is first presented and validated both in steady-state and transient regimes. The formulation provide the mechanical framework to develop a 1-dof model reproducing modal vortex-induced forces through a non-linear spring-damper system, requiring the experimental identification of only two parameters from typical free-motion tests: the fluid-dynamic damping ratio and the force-displacement relative phase. The same parameters allows identifying a generalized 2-dof model reproducing modal vortex-induced forces through the non-linear coupling between the structure and a wake oscillator. By tuning the natural frequency of the wake oscillator to the Strouhal frequency, wake-structure synchronization naturally develops for flow velocities within the lock-in range. By identifying the wake oscillator system thorugh sectional tests, and by placing it at different locations along the structure, a n-dof model can be used, reproducing all the three-dimensional fluid-structure interaction mechanisms.