Turbulent flow field around a 45° wing-wall bridge abutment in different scour conditions

We present the results of a numerical investigation of the flow field around a 45° wing-wall bridge abutment in three different scour conditions: beginning of the process, logarithmic phase and equilibrium stage. The flow field was computed using a wall-resolving large eddy simulation (a simulation where the near-wall viscous sub-layer is directly resolved) and the bathymetric data were taken from physical experiments with an equivalent geometry. The correlation between the flow field coherent structures and the bottom bed statistics was investigated, focusing mainly on the bed shear stress amplification and the near-wall pressure gradients. The results show the importance of the instantaneous flow field and its intermittency on the space-time distribution of the stresses at the bottom wall. The probability distributions of the wall shear stress was found to remain nearly similar to that of a straight duct flow up to the upstream edge of the scour hole; within the scour, in the upstream corner of the abutment the standard deviations of the wall shear stress was found to increase due to splat phenomenon of the downflow and to the presence of rare events with very intense values of wall shear stress. The horizontal pressure gradients evolution depends on the location whereas the vertical pressure gradients show a general increase in the logarithmic phase and a decrease going toward the equilibrium stage. The results of the present study may be helpful to formulate new physical-based local scour models to be used for practical evaluation of the scour depth around bridge abutments.