Abstract
Vertical surface piercing cylinders, such as typical coastal wind turbine foundations and basic elements of many coastal structures, are often exposed to combined loading from waves and currents. Accurate prediction of hydrodynamic loads on a vertical cylinder in a combined wavecurrent flow is a challenging task. This work describes and compares two different approaches for numerical modelling of the interaction between focussed wave groups and a sheared current, and then their interactions with a vertical piercing cylinder. Both approaches employ an empirical methodology to generate a wave focussed at the location of the structure in the presence of sheared currents and use OpenFOAM, an open source Computational Fluid Dynamics (CFD) package. In the first approach, the empirical waveoncurrent focussing methodology is applied directly in the OpenFOAM domain, replicating the physical wavecurrent flume. This approach is referred to as the Direct Method. In the second approach, a novel Lagrangian model is used to calculate the free surface elevation and flow kinematics, which are then used as boundary conditions for a smaller 3D OpenFOAM domain with shorter simulation time. This approach is referred to as the Coupling Method. The capabilities of the two numerical methods have been validated by comparing with the experimental measurements collected in a wavecurrent flume at UCL. The performance of both approaches is evaluated in terms of accuracy and computational effort required. It is shown that both approaches provide satisfactory predictions in terms of local free surface elevation and nonlinear wave loading on the vertical cylinders with an acceptable level of computational cost. The Coupling Method is more efficient because of the use of a smaller computational domain and the application of the iterative wavecurrent generation in the faster Lagrangian model. Additionally, it is shown that a Stokestype perturbation expansion can be generalized to approximate cylinder loads arising from wave groups on following and adverse sheared currents, allowing estimation of the higherorder harmonic shapes and time histories from knowledge of the linear components alone.
Original language  English 

Pages (fromto)  6583 
Number of pages  19 
Journal  Coastal Engineering 
Volume  145 
Early online date  4 Jan 2019 
DOIs  
Publication status  Published  1 Mar 2019 
Keywords
 Focussed wave groups
 Harmonic reconstruction
 Lagrangian wavecurrent flume
 OpenFOAM
 Sheared currents
 Waveoncurrent focussing methodology
ASJC Scopus subject areas
 Environmental Engineering
 Ocean Engineering
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