Advanced numerical investigation of flow field and morphological evolution around tandem piers

Bridge pier scour around tandem piers constitutes a complex hydrodynamic phenomenon necessitating sophisticated numerical modeling for accurate prediction and mitigation strategies. This study employed FLOW-3D Hydro with LES turbulence model and Q-criterion vortex identification methodology to elucidate vortex-induced scour mechanisms at the vicinity of tandem arrangements, T1 and T2 under varying flow conditions. Numerical model validation achieved accuracies of 1.30–5.30 % against experimental observations, revealing best agreement with scour depths across all analysed arrangements. Morphological analysis reveals substantial configurational dependencies, with T2 arrangement exhibiting maximum scour depth as compared to T1. Interference of WVs significantly reduced scour by 38 % (T1) and 56 % (T2) at rear piers, elucidating the critical influence of pier diameter sequencing on erosional patterns. Findings established correlation between scour patterns and hydrodynamic parameters including velocity profiles, RSS and Q-criterion vortex structures, which are fundamental in understanding scour development. The velocity profiles and RSS distributions were analysed at three key section to assess flow characteristics and vortex behaviour around tandem piers. The Q-criterion methodology identifies coherent vortex structure as regions where rotational motion dominates strain, providing detailed visualisation and quantification of vortical structures responsible for scour development. Q-criterion analysis adequately identified coherent vortex structures with varying intensities at both u/s and d/s pier locations. In the complex flow region between the front and rear pier, Q-criterion vortex structures effectively captured the sheltering phenomenon where WVs from the u/s pier disrupted coherent vortex formation at the d/s pier. These vortical interactions resulted in substantial scour depth reductions of 38 % and 56 % for T1 and T2 arrangements, respectively. This paper contributes to a fundamental understanding of vortex-induced scour dynamics around complex pier arrangement, which is critical for designing resilient bridge foundations.