CFD Analyses for Wind Load AssessmentCFD Analyses for Wind Load Assessment : Wind-induced vibrations in the Bomarsunds Bridge hangers

Abstract: The trend in recent years of building longer and slender bridge components has introduced new challenges to ensure their stability and strength. This master thesis focuses on the effects of wind-induced vibrations in the context of long, slender arch bridge components, particularly the recently constructed Bomarsunds Bridge in Åland, Finland. The primary goal of this study is to comprehensively analyze the dynamic wind effects on the hangers due to the vortex shedding phenomenon, as the resulting vibrations pose potential risks to its safety and structural integrity. The slenderhangers of the bridge, close to the centre of the span, have exhibited significant vibrations, necessitating an in-depth investigation to understand the bridge’s response to wind forces. Computational fluid dynamics (CFD) simulations wereperformed using ANSYS Fluent to estimate more accurate aerodynamic quantities. Using CFD analysis, the behaviour of a given hanger section subjected to wind flow can be described. In this way, it was possible to calculate the aerodynamic coefficients that characterize that given section (i.e. Strouhal number (St), drag coefficient (CD), etc). By integrating advanced computer simulations and CFD analysis, the research addresses the complex challenges of investigating the vortex-induced vibration (VIV) phenomena at different wind speeds. The results showed an inconsistent trend for drag coefficients at varying wind speeds and lower drag for geometries with rounded edges, with an average value of drag coefficient of 1.60. The study highlighted the significant dependence of theStrouhal number on wind speed, varying from 0.129 for a wind speed of 2.5 m/s to 0.063 for a wind speed of 30 m/s, challenging traditional geometry-based estimations for this parameter. The drag frequency for each wind speed investigated is twice as high as the lift frequency, showing that at wind speeds of 7.5 m/s a drag frequency close to the fundamental transversal frequency of 6.6 Hz of the longest hanger is reached. This leads to the conclusion that for this particular case study, the headwind response is much more critical than the crosswind response. These findings can be used to implement effective measures to mitigate wind-induced vibrations in the studied hanger along the critical direction. By analyzing the complex vortex shedding phenomenon, the study contributes valuable insights into the field of wind engineering. This research plays a key role in ongoing efforts to design robust, safe, and resilient bridge structures.