CFD MODELLING TO INVESTIGATE HYDRODYNAMIC FORCES ON BRIDGES WITH FLOW 3D - HYDRO

Increasingly frequent and intense flood events, combined with the remarkable industrialization process of cities, are placing transportation networks worldwide under stress. Road and railway infrastructures are currently subjected to loads higher than those originally designed for, and their state of aging is such that any disturbance (flood, earthquake, landslide) could lead to a total or partial interruption of traffic with consequent socioeconomic losses. Bridges represent the most vulnerable component of a transport system, and their failure could compromise the functionality of the entire network. During floods, bridges may become partially or completely submerged, having to withstand higher hydrodynamic loads that could cause to structural collapse. The presence of Large Woody Debris (LWD) accumulations, typically transported by the flow during floods, can generate additional forces on the bridge, affecting structural integrity and scour around the piers to the point of reducing their load-bearing capacity. Therefore, in order to ensure the stability of bridges during extreme hydrological events and avoid repercussions on evacuation plans and the organization of rescue operations, this study examined the behaviour of a specific system under various hydraulic conditions. Using Computational Fluid Dynamics (CFD) modelling, the three-dimensional flow field around the bridge was investigated, and the drag and lift forces acting on the deck were estimated for different submergence values. In particular, dimensionless coefficients were calculated for inundation ratios ranging from 0 to 6 and Froude numbers varying between 0.16 and 0.42, applying different turbulence models (RNG, k-ω). Furthermore, the effect on the drag coefficient of the LWD accumulations around the piers, characterized by different geometries (circular, ogival, semi-circular), was evaluated. The results of the numerical simulations, conducted under fixed riverbed conditions, were validated using available experimental data. The drag forces acting on the piles in the presence of debris were compared with the laboratory measurements present in the literature, showing a limited gap and emphasizing the reliability of the numerical approach.