PSI - Issue 62

Tommaso Lazzarin et al. / Procedia Structural Integrity 62 (2024) 625–632 Lazzarin et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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deformed bathymetry, and the complex shape of piers and abutment, which all contribute to modify the flow and the bed shear stresses in the vicinity of the structure. Differently from 2D depth-averaged models, 3D CFD models with free-surface tracking methods can be also used to accurately predict the flow field for pressurized flow regime at the bridge deck. This regime has been demonstrated to drastically increase the risk for bridges. Besides the large hydrodynamic actions exerted by the current directly on the deck, the high-velocity orifice flow forming beneath the bridge deck massively increases the bed shear stresses and the capacity of the flow to induce sediment entrainment over the entire bridge region. This represents a factual risk for the structure, as generally limited parts of the riverbed are protected against scour. One of the advantages of using eddy-resolving simulations is their ability to provide detailed information on the temporal fluctuations of the relevant variables, whereas simplified models, such as those based on RANS, only provide turbulence-averaged values. The comparison of the results provided by the RANS and the DES approaches showed that simulations with the DES approach are enriched by the estimation of the instantaneous flow fields, and by a more accurate description of the vortical structures associated with smaller scale unsteadiness. Despite this, RANS and DES simulations lead to similar results in terms of time-averaged flow features. Particularly, the high velocity orifice flow, which is the main driver of sediment erosion, is consistently described by the two methods. As a result, one expects that the predicted mean flux of entrained sediment by the two approaches will be similar. This differs from the free-surface case, where scour around piers and abutments is driven by the horseshoe vortices and the eddies shed in the wake, and large differences are observed when comparing DES and RANS results. While providing richer information and higher level of details, 3D models require large computational costs, proper computational resources, and advanced skills to produce suitable computational grids, which can hinder their diffusion and application for practical engineering purposes. Still, with the growing availability of computational power, such models are expected to become more and more powerful tools for investigating real bridges. References Cheng, Z., Koken, M., Constantinescu, G., 2018. Approximate methodology to account for effects of coherent structures on sediment entrainment in RANS simulations with a movable bed and applications to pier scour. 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