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research-article

NUMERICAL STUDY OF SEABED BOUNDARY LAYER FLOW AROUND MONOPILE AND GRAVITY-BASED WIND TURBINE FOUNDATIONS

[+] Author and Article Information
Muk Chen Ong

Department of Mechanical and Structural Engineering and Materials Science University of Stavanger NO-4036, Stavanger, Norway
muk.c.ong@uis.no

Eirik Trygsland

Department of Marine Technology, Norwegian University of Science and Technology NO-7491 Trondheim, Norway
eirik_trygsland@hotmail.com

Dag Myrhaug

Department of Marine Technology, Norwegian University of Science and Technology NO-7491 Trondheim, Norway
dag.myrhaug@ntnu.no

1Corresponding author.

ASME doi:10.1115/1.4036208 History: Received March 17, 2016; Revised March 02, 2017

Abstract

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 investigate the formation of horseshoe vortex and flow structures around two different bottom-fixed offshore wind turbine foundations in order to provide an assessment of potential scour for engineering design. 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 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 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.

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