Numerical Analysis of Wave and Current Interaction With Moored Floating Bodies Using Overset Method

Abstract

Numerical modelling of floating bodies is still being a very challenging issue, especially for large body displacements. Despite of the good performance of potential flow models in predicting floating body dynamics, there are still physical processes which are not well reproduced with that approximation. Their strong assumptions yield a lack of accuracy when high non-linear effects become predominant. In addition, the presence of restrictions to motion induced by mooring elements also introduces additional non-linear features which are sometimes out of the framework of the use of potential flow models. The use of CFD approach overcomes potential model limitations especially for non-linear effects. When CFD models are applied to solve waves and current interaction with floating bodies, several issues arise such as the numerical treatment of the floating element, mooring implementation and also the computational cost. Although several approaches are available in literature regarding the numerical implementation of floating bodies, the use of the Overset mesh appears as the more suitable one for large body displacement. Although accurate results have been obtained with re-meshing or even morphing techniques, large mesh deformation can yield into non-acceptable skewness and aspect ratio for the cells, consequently inducing numerical instabilities. In this work, we will present a numerical analysis of wave and current interaction with floating bodies. The objective of the work is to present a set of numerical implementations performed in OpenFOAM environment with the use of the Overset mesh method to study moored floating body dynamics due to the combined action of waves and current. The implementations, included in IHFOAM (www.ihfoam.ihcantabria.com) are a new set of boundary conditions to generate waves and current without the use of relaxation zones. The main consequence is that the computational cost can be reduced due to the use of smaller domains. In addition, the implementation of mooring will be also presented in order to extend the use of the model to realistic conditions. Numerical model predictions compared with laboratory data of wave interaction with moored floating bodies have been performed showing a high degree of agreement. Comparison of floating body displacement and mooring tension will be presented. The combined effect of waves and current, traveling in the same and in opposite directions than waves, and their interaction with floating bodies and mooring will be also studied. Results will show the applicability of current method to model floating bodies.