Numerical Study of Seabed Boundary Layer Flow around Monopile and Gravity-based Wind Turbine Foundations
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- Institutt for marin teknikk 
Computational fluid dynamics (CFD) has been used to study the boundary layer flow around three different bottom-fixed offshore wind turbine foundation designs. Two of the designs are gravity-based foundations, where one has a hexagonal bottom slab and one a circular bottom slab (bottom part). The third design is a monopile. Three-dimensional analyses have been performed with Spalart-Allmaras Delayed Detached Eddy Simulation using a Reynolds number 4*10^6 based on the free stream velocity and the diameter of the monopile, D. The boundary layer size is D in all the analyses. Time averaged results for velocities, pressure and bed shear stress were obtained. The dependence of the results on the mesh resolution was investigated and comparisons with published data were made. The results were found to be reasonably accurate. A distinct horseshoe vortex was found in front (upstream side) of the monopile foundation. Vortex shedding was present in the wake of all the foundations. Two smaller horseshoe vorticies were found in front of the hexagonal gravity-based foundation, were one was on the top of the bottom slab and one was near the seabed in front of the bottom slab. Three horseshoe vortices in total were found in front of the circular gravity-based foundation, due to the presence of two horseshoe vortices near the seabed in front of the bottom slab. A large region of downflow exists in front of the monopile, reaching all the way down to the seabed. This causes a backflow in front of the foundation near the seabed due to conservation of mass. The gravity-based foundations were found to have two main regions of downflow, one in front of the cylindrical shaft (upper part) on top of the bottom slab and a smaller region in front of the bottom slab near the seabed. The gravity-based designs are found to limit the downflow near the seabed. Pressure distributions around the foundations were studied. A positive vertical pressure gradient was found in front of the monopile foundation. It was also found in front of the cylindrical shaft and in front of the bottom slab near the seabed on the gravity-based foundations. A larger volume of increased pressure exists in front of the monopile foundation than in front of the gravity-based foundations due to its geometry. The bed shear stress in the flow direction along the upstream symmetryline on the seabed was investigated. The horseshoe vortex size, measured as the distance from the separation point to the foundation surface along this line, was found to be 0.40D for the monopile foundation, 0.125D for the hexagonal and 0.22D for the circular gravity-based foundation. Bed shear stress distributions near the foundations were obtained. The magnitude of the bed shear stress, normalized by the far field bed shear stress, was used. A maximum value of 4.89 was found near the surface of the monopile foundation at phi = +-66.5 degrees, where phi is the angle measured from the stagnation point in front of the foundation. Similarly, 2.86 at phi = +-60.1 was found for the hexagonal gravity-based foundation. The larger values of the hexagonal foundation are concentrated around the corners at phi = +-60 degrees, and the rest of the seabed has shear stresses close to the far field shear stress. The results of the circular foundations were found to be slightly asymmetric, with a maximum value of 2.59 at phi = 68.9 degrees for the upper distribution (for positive phi) and 2.72 at phi = -85.4 degrees for the lower (for negative phi).