The effect of boundary shear flow on hydrodynamic forces of a pipeline over a fully scoured seabed

The effect of boundary shear flow on hydrodynamic of a pipeline over a fully scoured seabed is numerically investigated by residual-based large eddy simulations. A generalized-a method is applied to solving variational multiscale formulations of the Navier-Stokes equations with a residual-based turbulence model. Flows with the same parameters as the experiment are considered in the study. The hydrodynamic forces computed by the in-house codes of the residual-based large eddy simulations are in good agreement with the experimental results. Characteristics of the flow distorted by the scour hole and its effect on hydrodynamic forces are examined by comparisons with the flow past a circular cylinder over a flat seabed. The results show that the scour hole causes lower velocity through the gap, weaker vortex shedding, smaller drag force, smaller RMS of lift force and smaller hydrodynamic forces along the seabed. In particular, the flows with different boundary layer thicknesses are considered in the study. It is found that the boundary layer thickness has evident effect on the flow characteristics and hydrodynamics of the cylinder. The size of the vortices downstream the cylinder become large and their vorticities become small as the boundary layer thickness of the flow increases. The mean drag, lift, RMS of lift and Strouhal number clearly decreases with the boundary layer thickness. Additionally, the variation of the vorticity contours downstream the cylinder and RMS of lift indicate that the vortex shedding become weak as the boundary layer thickness of the flow increases. The boundary layer thickness has little effect on the hydrodynamic forces along the seabed.