CFD modelling of ocean wave interaction with thin perforated structures represented by their macro-scale effects

Abstract

Fluid interaction with thin perforated structures is of interest in a range of contexts. Applications in marine engineering include current and wave interaction with aquaculture containers, breakwaters and, as a new application, platforms for floating wind turbines with perforated outer shrouds. Another more general application is for tuned liquid dampers with baffles for motion attenuation. Thus, there is significant interest in the challenge of simulating the effect of these thin porous structures using Computational Fluid Dynamics (CFD). This thesis proposes and assesses the use of a macro-scale approach to CFD modelling of wave interaction with thin perforated structures. The structures are not resolved explicitly but represented by their spatially averaged effects on the flow by means of a homogeneous porous pressure-drop applied to the Navier-Stokes momentum equation. Two options are explored where the pressure-drop is either applied as a volumetric porous zone or as a jump-condition across a porous surface. The wave modelling capabilities and the basis of the macroscopic porosity implementations are readily available in the open-source code OpenFOAM®, which is used in this work. Minor code modifications were necessary to introduce orthotropic porosity for a cylindrically shaped structure. More significant code development was required to implement accurate motion of a floating porous structure as a new capability as part of a custom motion solver. The method is applied to fixed perforated sheets and cylinders as well as a floating tension leg platform (TLP), and the overall fluid flow behaviour and global forces and motions are assessed. The validation against experimental and potential-flow results demonstrates that a macro-scale porosity representation can accurately reproduce large-scale flow, force and motion effects of all conditions investigated. As the most representative case, the CFD results of the horizontal force on the perforated cylinder differ between 2 and 12% from the experimental results. As part of this work, it is shown that, firstly, the Volume-Averaged Reynolds-Averaged Navier-Stokes (VARANS) equations can not only be used for large volumetric granular material, but also for thin perforated structures, and secondly, that the effects of applying a RANS turbulence model on the results are of minor significance and that the full Navier-Stokes equations give good results. The presented macro-scale approach offers greater flexibility in the range of wave conditions that can be modelled compared to approaches based on linear potential-flow theory and requires a smaller computational effort compared to CFD approaches which resolve the micro-structural geometry of the openings and the fluid flow across it explicitly. This approach can therefore be an efficient alternative to assess large-scale effects for engineering problems.