In this study, an unresolved CFD-DEM was used to investigate a fluid flow and a behavior of sediment particles around a monopile. In order to consider the interaction between particles on the seabed and a current, an improved CFD-DEM solver was implemented within the OpenFOAM framework by proposing a void fraction method based on the kernel function. To validate computational methods, a settling velocity of a single particle, an angle of repose and an incipient motion of particles were simulated and compared with the existing experimental data. Finally, a scour around the monopole was predicted and discussed.
In relatively shallow water, most offshore wind turbines are based on monopile foundations as bottom-fixed structures. Theses bottom-fixed foundations installed on an erodible seabed are exposed to scour, which may lead to structural failure. Therefore, it is essential to understand how the hydrodynamic environments affects the foundation and the interaction between flow, structure and seabed (Sumer, 2014). In particular, the scour can be defined as the phenomenon that the seabed particles around the foundation structure are transported due to the interaction of the fluid flow and the structure. The scour is a threat to the stability of the structure exposed to currents and waves. In order to alleviate the scour problem, many studies have been conducted experimentally (Dargahi, 1989, Whitehouse, 1998; Sumer and Fredsoe, 2002). In particular, most of the work has been done on scour phenomenon with monopile foundations. It led to various empirical formulas and methods to predict scouring depth and extension (Matutano et al., 2013). On the other hand, the computation can be employed as an alternative tool to study the scour process around structure (Pang et al., 2016). Recently, several computational methods have been developed to estimate the equilibrium scour depth. The scour depth and extent were estimated based on the bed shear stress exerted by the flow field in the Eulerian-based approach (Park et al., 2017). However, the single-phase model usually was not able to consider interparticle interactions in the scour around structures. To deal with this problem, many studies have adopted a two-phase model (Yeganeh-Bakhtiary et al., 2011; Hajivalie et al., 2012). These models can be divided into two types, depending on the method used. One is the so-called Euler-Euler two phase model, which treats the fluid and sediment phases as separate continuous mediums. The other calculate the particle motion individually using a discrete element method (DEM). The advantage of this method is that it can analyze a large amount of particles based on a simple collision model and analyze the exact behavior of the particles. Thus, to consider the interaction between the fluid flow and the seabed soil, a CFD and DEM coupling method is needed. Recently, studies on the sediment transport using the CFD-DEM coupling method have been carried out (Schmeeckle, 2014; Sun and Xiao, 2016), and even the scouring around a submarine pipeline was performed to investigate the flow behavior and particle motions (Yeganeh-Bakhtiary et al., 2013; Zhang et al., 2015). However, there have been only a few studies to realize the scour phenomenon around the monopile using CFD-DEM coupling method.