Numerical simulation of scour around circular piles due to unsteady currents and oscillatory flows

Abstract

Numerical modeling of pile scour is performed due to the action of unsteady currents and oscillatory flow greens, using 3D Reynolds averaged Navier–Stokes (RANS) equations for the hydrodynamic description and the Exner equation for the morphological evolution of the bed, all of them incorporated into the computational fluid dynamics tool called REEF3D. As a first task, the numerical model was calibrated by comparing its results of scour against two sets of data, the first one obtained from numerical simulations and the second one from experimental tests. From these results it was possible to define the closure model of turbulence and a relaxation coefficient on the bed load equation necessary to increase the mobility of the sediments in the vicinity of the pile. Once the model was calibrated, a set of numerical simulations of pile scour under unsteady flow and oscillatory flow was performed using different hydrographs and wave conditions. There is a strong agreement between the simulated data and the experimental data reported by other authors for unsteady flow. In the case of scour associated with oscillatory flow, the numerical results showed the same statistical behavior as the experimental data previously published by others. When the numerical results are analyzed according to the Keulegan-Carpenter number, they represent the scour phenomenon in the same order of magnitude of the experiments. A conclusion of the study is the necessity to improve the sediment transport description, in order to avoid the use of a dedicated relaxation coefficient that reduces the threshold condition of the incipient transport.