Analysis of vortex-induced vibrations in slender steel structures using CFD and analytical methods

Sammanfattning

This thesis investigates wind-induced cross-wind vibrations in slender steel structurescaused by vortex shedding, and was inspired by a real industrial project that Sweco hasbeen working on for LKAB in Gällivare. The project includes a 150-meter-tall steel towerused in the production of fossil-free iron. Due to its height and slenderness, dynamic windeffects such as vortex shedding constitute one of the major challenges in the structuraldesign. The effect of wind-induced vibrations are commonly estimated using simplifiedanalytical methods given in design standards such as Eurocode and Boverket’s Snö- ochvindlaster (BSV 97). The aim of this study was to investigate how well simplified analyticalmethods predict the cross-wind response compared with Computational Fluid Dynamics(CFD) simulations. Two-dimensional CFD simulations of the tower’s two cross-sections were performed inANSYS Fluent, across a range of free-stream velocities and following domain, mesh, andtime-step verification studies. From the simulated lift and drag histories, the aerodynamicparameters governing vortex shedding were extracted and substituted into the BSV 97 andEurocode procedures in place of the constant code values. The results showed that the aerodynamic parameters obtained from CFD differed fromthe constant values recommended in BSV 97 and varied with wind velocity and Reynoldsnumber. The Strouhal number reached approximately 0.15 for the tower’s upper part,against the code value of 0.12, lowering the predicted critical wind velocity from about 29 to24 m/s, When the CFD-derived parameters were implemented in the analytical procedures,lower equivalent static wind loads were obtained compared with those calculated using thestandard code values.