Two-phase Meshfree-based Numerical Analysis of Coastal Natural Blowhole Structures for Wave Energy Extraction Applications.

Abstract

Ocean structures can be classified as manmade or natural, where manmade structures mainly include offshore oil platforms, offshore wind platforms, and ocean wave energy converters (WECs), etc. and natural structures include blowholes, headland pillars, etc. These structures are often analysed using computational fluid dynamics (CFD) tools to understand wave-structure interactions (WSI) and forces exerted by sea waves. Single-phase hydrodynamic modelling is typically used for such simulations because it simplifies the process and reduces computational time. However, in some cases, two-phase hydrodynamic modelling is essential, especially when air compressibility plays a critical role in hydrodynamics, such as in the case of blowholes where the vertical upward water-air jet is generated mainly through air compressibility driven by the incident waves trapping air in a temporarily sealed compartment. Accordingly, this study focused on the two-phase hydrodynamic modelling of a blowhole structure with a 4 m diameter in a 2D environment. For the modelling work, mesh-free smooth particle hydrodynamics (SPH) based open-source DualSPHysics tool was used. The two-phase numerical model was validated using published experimental data of a WSI demonstration. 10 plus two-phase simulations were carried out on different scales according to the Froude scaling law to observe the scale effect of air compressibility in small-scale models and the prototype. The findings show that the scale of the air pocket and the air interface significantly affects the structure's performance. At smaller scales, the air exhibits incompressible behavior, and the air compressibility is the main driving force of this water jet that turns ocean wave energy into a powerful water jet. This study shows the importance of incorporating two-phase modelling for analysing such coastal natural structures, and identifying further improvement of these structures as wave energy converters with suitable further optimizations.