Numerical Study of Seabed Boundary Layer Flow Around Monopile and Gravity-Based Wind Turbine Foundations

Computational fluid dynamics (CFD) has been used to study the seabed boundary layer flow around monopile and gravity-based offshore wind turbine foundations. The gravity-based foundation has a hexagonal bottom slab (bottom part). The objective of the present study is to study the flow structures around the bottom-fixed offshore wind turbine foundations in order to provide essential hydrodynamic coefficients for engineering design and an assessment of potential scour erosion. Three-dimensional CFD simulations have been performed using Spalart-Allmaras Delayed Detached Eddy Simulation (SADDES) at a Reynolds number 4×10⁶ based on the free stream velocity and the diameter of the monopile foundation, D. A seabed boundary layer flow with a boundary layer thickness D is assumed for all the simulations. Vortical structures, time-averaged results of velocity distributions and bed shear stresses are computed. The numerical results are discussed by studying the difference in flows around the monopile and the gravity-based foundations. A distinct horseshoe vortex is found in front (upstream side) of the monopile foundation. Two small horseshoe vortices are found in front of the hexagonal gravity-based foundation, i.e. one is on the top of the bottom slab and one is near the seabed in front of the bottom slab. The horseshoe vortex size for the hexagonal gravity-based foundation (computed as the distance from the separation point to the foundation surface along the centerline on the seabed), is found to be smaller than that for the monopile foundation. The effects of different foundation geometries on destroying the formation of horseshoe vortices (which is the main cause of scour problems) are discussed.