3D Numerical Study of Scour around a Pile Group in the Staggered Arrangement under Combined Wave-Current Flows

The paper presents three-dimensional (3D) simulations conducted to study scour around a pair of twin piles arranged in different skew angle (alpha) configurations under combined wave-current flow conditions, utilizing a computational fluid dynamics (CFD) model, REEF3D. The Reynolds-averaged Navier-Stokes (RANS) equation is solved using the k-omega turbulence model in this study. The Exner equation is employed to measure variations in bed elevation, while the level-set approach accurately captures the free surface. The numerical model integrates a hydrodynamic module with a morphological module to simulate the scour process. The pressure component within the RANS equations on a staggered grid is characterized through Chorin's projection method. For discretizing the convective term of the RANS equations, the 5th order weighted essentially nonoscillatory (WENO) scheme is employed. van Rijn formula is used to calculate bed load transport, for suspended load transport standard convection diffusion formula is utilized. The numerical model is utilized to explore how the gap ratio (G/D=1 and 2) and the combined wave-current parameter (Ucw=0 and 0.2) impact piles arranged at different skew angles. Furthermore, it is observed that the flow gap between the piles substantially amplifies the scour depth. The scour depth is influenced by both the gap ratio and skewed arrangement (staggered with 30 degrees, 45 degrees, and 60 degrees approach flow angles) of piles. In the case of only wave conditions, the interference between piles vanishes when the skew angle is set at 30 degrees and the gap ratio (G/D) is 2. Conversely, under combined wave-current scenarios, the interference effect remains evident at a gap ratio of 2. Moreover, this interference effect persists at skew angles of 45 degrees and 60 degrees for both wave-only and combined wave-current conditions, when gap ratios (G/D) are 1 and 2. For a fixed Kuelegan-Carpenter (KC) number, the stretch of scour hole reduces as the gap ratio increases in combined wave-current flow, while the extent of scour hole perpendicular to the flow direction increases as the gap ratio and combined wave-current parameter increases. Additionally, it is noticed that the normalized scour depth decreases as the gap ratio increases when the KC number and Ucw are fixed.