Numerical Simulation of Scour Hole Backfilling in Unidirectional Flow

Abstract

The computational fluid dynamics (CFD) solver FANS3D [Finite-Analytic Navier-Stokes code for three-dimensional (3D) flow] is coupled with a sediment transport model to simulate scour hole backfilling under unidirectional flows (current-only). FANS3D solves 3D, unsteady, incompressible Navier-Stokes equations in nonstaggered, general curvilinear coordinate systems. For the present study, the k-ε model is used for turbulence closure. The overset grid technique is utilized to generate the computational domain consisting of multiple blocks with different structures. The coupled model is validated with both clear-water and live-bed scour experiments. The numerical solution is also verified to be consistent against changes in time step size and grid density. Backfilling is initiated by first obtaining the equilibrium scour under a “flood” and then reducing the approach velocity to a “normal” flow, both of which conditions are in a live-bed scour regime. Using this approach, backfilling is successfully simulated around a cylindrical pier with three different hydrographs (variable velocity). The backfilled scour hole is compared with the equilibrium scour hole formed under normal flow (constant velocity). This study found that a given unidirectional flow velocity results in the same equilibrium scour depth regardless of the initial bed morphology. The location of the maximum depth inside the scour hole is also found to be a characteristic of the flow velocity.