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Three-dimensional modeling of hydrodynamics and scour with OpenFOAM

Scour is induced by the movement of sediment particles carried by flowing water. The scour is of great importance for water resources, the environment, and infrastructure. For a structure placed in flow and foundationed on loose material, it is critical to calculate the erosion and sedimentation around the object and predict the bed evolution. The scour process includes the interaction between turbulent flow and sediment transport. We developed and utilized a three-dimensional (3D) computational modeling in OpenFOAM to predict the turbulent flow field and bed morphodynamics. Existing numerical models typically use the Arbitrary-Lagrangian-Eulerian (ALE) method to capture the deformed bed. However, the ALE method cannot be easily used to deal with scour around complex objects, such as a complex bridge pier with a platform above multiple columns. To properly model the dynamic of the deforming bed, an improved adaptive immersed boundary (IB) method is applied, which gives smooth wall shear. The sediment mass transport rate is a function of wall shear stress. Thus, the smoothness of wall shear is essential for sediment transport modeling because the bed deformation depends on the divergence of sediment transport rate. In the IB method, the shear velocity is iteratively calculated with an image point and a wall function. The key to achieve smooth wall shear is to set the image point distance to be proportional to the size of IB cell and enforce the image points to be in the log-law layer. Consequently, all image points are roughly uniformly distributed in the log-law layer and follow the same wall function to provide smooth wall shear stress. The numerical model is demonstrated with several 3D cases with validation.