Three-dimensional Computational Fluid Dynamics modelling of scour around a single pile due to combined action of the waves and current using Level-Set method

This paper presents three dimensional (3D) numerical modeling of local scour around a single pile under the combined action of waves and currents. A semi-coupled hydrodynamic and morphological model (REEF3D) is used in this study which solves the Reynolds-averaged Navier-Stokes (RANS) equation with k-w turbulence model closure. The Exner equation is used to calculate the changes in bed elevation. The use of the Level-Set method is done to capture the changes in the free surface. Realistic scour and deposition calculations are aided with the use of modified critical bed shear stress reduction formulation on a sloping bed supplemented with the use of Sand Slide algorithm in the morphological model. The scouring process is modeled in this study using a reduced-length numerical wave tank with wave generation and wave absorption implemented using active wave absorption method. The model is validated for the numerical modeling of local scour around a single pile in steady current, waves alone, and combined wave-current environment. The validated model is used to analyze the effect of Keulegan-Carpenter (KC) number and combined wave-current parameter (Ucw) on normalized equilibrium scour depth. It is found that the normalized equilibrium scour depth for scour under waves alone, waves combined with weak currents, and waves combined with mild current do not significantly differ for flow developed in a low KC number environment. However for waves combined with strong currents the normalized equilibrium scour depth increases significantly. For large KC numbers, the normalized equilibrium scour depth is found to have increased with an increase in KC number as well as combined wave-current parameter (Ucw).