This paper presents the results of a numerical investigation on the oscillatory flow fields around the base of a circular pile mounted on a flat or scoured bottom. The flow fields were simulated using the three-dimensional Navier-Stokes equations expressed in the generalized curvilinear coordinate system. Descriptions are given of the basic flow structures in and around the cone-shaped scour hole and examinations are made on the effects of the scour depth and the Keulegan-Carpenter number on the vortex motion and bed shear stress. Detailed investigations are made on the differences in the development processes of the horseshoe vortex and wake vortices, by comparing the flat-bottom case with the scoured-bottom case. The three-dimensional structures of these vortices are well identified by the distributions of the vortex cores. The maximum velocity in the horizontal direction decreases near the bottom as the scour depth increases, while the vertical velocity increases with the scour depth. The reduction ratio of the bed shear stress with respect to its flat-bottom value is significant for large KC numbers.
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