Scour around twin-piles under combined wave–current flows

Abstract

The paper presents three dimensional simulations of scour around a pair of piles arranged side-by-side configurations under combined wave–current flow conditions using a computational fluid dynamics model. The Reynolds-averaged Navier–Stokes (RANS) equation is solved using the k-ω turbulence model in the present work. The Exner equation is used to measure the variations in bed elevation. The level-Set approach is used to capture the free surface realistically. The numerical model couples the hydrodynamic module with the morphological module to simulate the scour process. For accurate erosion and deposition calculations in the sediment bed, the morphological model employs a modified bed shear stress formula on a sloping bed in combination with a sand slide algorithm. In the present study, the simulations have been done in a truncated numerical wave tank with the Dirichlet boundary condition and active wave absorption method. The numerical model is validated with the experimental results of combined wave–current hydrodynamics and scour around a pair of piles. The validated numerical model is utilized to study the effect of the gap ratio and the effect of KC number in the various combined wave–current environment. In low KC conditions, normalized scour depth increments owing to waves alone, weak currents, and moderate currents are less, whereas the scour depth increases dramatically for waves with high currents. It is also found that the gap flow between the piles increases the depth of scour in a significant manner. For a given pile gap ratio, the scour depth is dependent on the KC number, which implies the larger the KC number, the larger scour depth. For a given KC number, the scour hole stretch in the flow direction reduces as the gap ratio increases, whereas the perpendicular stretch increases. It is also observed that the normalized scour depth decreases as the gap ratio increases for a fixed KC number and fixed Ucw. It is evident from the present research that the normalized scour depth increases with increase in KC number for a fixed wave–current parameter (Ucw).