CFD Modeling of Local Scour Around a Pair of Tandem Cylinders under Wave Conditions

Abstract

Local scour in a marine environment can lead to the failure of offshore and coastal structures. This is also the case, when marine infrastructure is built in Arctic conditions. The knowledge and understanding of erosion and sediment transport mechanisms and the correct prediction of the local scour magnitude is crucial for the structural design. In a first step towards numerically modeling the complex physics of local scour in an Arctic environment, local scour for wave conditions around a pair of tandem cylinders is modeled in the current paper. The numerical results are compared with experimental data, showing good agreement. A three-dimensional computational fluid dynamics model is used to calculate the detailed flow field and the resulting sediment transport pattern. The location of the free surface is represented using the level set method, which calculates the complex motion of the free surface in a very realistic manner. For the implementation of waves, the CFD code is used as a numerical wave tank. In order to provide an accurate prediction of propagating waves, the convection terms of the Navier-Stokes equations and the level set method are discretized with the 5th-order Weighted Essentially Non-Oscillatory scheme. The pressure is solved on a staggered grid, ensuring tight velocity-pressure coupling. The numerical model employs a Cartesian grid and complex geometries are treated with an immersed boundary method based on ghost cell extrapolation. Sediment transport is implemented through standard bedload and suspended load formulas. The resulting sediment discharges and concentrations are evaluated with the Exner equation, giving the local erosion and deposition pattern for each time step. For the geometric representation of the moveable sediment bed, the level set method is used.