Investigation of wave impacts on porous structures for coastal defences

Abstract

There is great scientific interest in further understanding the underlying wave impact dynamics on solid and/or permeable structures for coastal defences. The accurate and validated simulation of the dynamics of the flow at microsecond temporal scale prior to, at, and after impact is an outstanding and challenging numerical problem in CFD. More advanced numerical models of free surface flow processes which include entrapment of large air pockets is required. These models will yield more insight into the trends of pulse-like forces involved at impact with solid and/or porous material and will enable the understanding of the mechanical stability and integrity of defence structures. Furthermore, the development of advanced numerical models for solving such problems will need to be made accessible as information systems to a wider community of civil engineers in order to achieve integrated design of structural defences (coastal, offshore oil and gas, hydraulic dams etc.). This research is on the development of free surface flow simulations, flow visualisation, analyses of forces of impact, and analyses of the integrity of offshore structures in an information system environment. A large dataset of compressible (and incompressible) numerical models have been generated to simulate waves impacting at solid and porous structures. Initial studies focus on the behaviour of wave impacts with a solid structure in a 2 dimensional domain. The simulations data are verified through a grid independence study. Numerical results are validated against two sets of experimental data. Air bubble entrapment and consequential multi-modal oscillatory pressure response trends are observed in the compressible simulations during wave impact. Frequency domain analyses of the oscillatory impact pressure responses are undertaken. The numerical model data sets are compared with results generated from analytic methods and experimental data with good agreement. These initial findings confirm the robustness of our numerical model predictions concerning the simulated air bubble formations when compared with theories on air bubbles at impact and their resonance frequency modes. The compressible numerical model is extended to a 3 dimensional simulation. A range of porous structure morphologies are incorporated into the domain to replace solid wall impact interface. A brief overview of previous research on the subject of fluid flow in porous media is presented. The characterisation of the porous model morphologies is examined. Various permeability flow models are discussed in detail. The methods for the generation of the various porous structures and their integration into the CFD model are described. The results from a soliton wave impact at the porous structure morphologies both with and without air entrainment effects at the free surface is investigated in detail.Finally future work to develop an experimentation specification for the analysis of fluid flow thorough a porous structure is discussed. It is envisioned that this experimental work with have dual outcomes. Firstly it will serve to validate the numerical models created over the course of this study and secondly the potential for clean, renewable energy harvesting from oscillatory pressures through the incorporation of smart sensor hardware within the porous structure will be investigated