Date of Award

2017

Document Type

Doctoral Thesis

Degree Name

Doctor of Philosophy

Department

Department of Mechanical, Biomedical and Manufacturing Engineering

First Advisor

Dr. Andrew Cashman

Second Advisor

Professor Ger Kelly

Abstract

This thesis focuses on the development of a state of the art modelling technique for offshore Oscillating Water Column (OWC) type Wave Energy Converters (WEC) using Computational Fluid Dynamics (CFD). Current literature indicates a limited amount of work has been completed on studying these devices containing non-linear time dependent flow phenomenon. Initially, a 2D Numerical Wave Tank (NWT) is studied to reduce discretisation error in order to reproduce accurately propagating waves. Further development into a 3D domain permits the geometrical requirements of an OWC type spar buoy to be included.

In parallel, a single Degree of Freedom (DOF) model is developed by incorporating a freely heaving barge into the 2D NWT. Response of the heaving barge is analysed with respect to a range of incident waves and compared to results in the literature to validate this modelling approach. A non-linear Power Take Off (PTO) boundary condition is developed to replicate the response of an impulse turbine, typically simulated by an orifice plate during small-scale testing. CFD simulations are completed with the PTO boundary and responses are compared to experimental data to further validate this step.

A dynamic CFD model is created by coupling together the 3D NWT, DOF modelling methodology and the non-linear PTO boundary condition. The restoring forces from a non-linear catenary mooring system are employed to enhance the model by including a surge mode. Linear monochromatic waves are allowed to propagate until responses from the model reach a quasi-steady state.

Responses are compared to experimental work conducted by MaRFI in the LIR- National Ocean Test Facility, UCC, Cork, under an FP7 MARINET project. Good correlations are observed for both simulated and experimental data sets. The developed numerical model is further tested for robustness and modified to permit a wide range of non-linear regular waves to be simulated. Interactions by the device to non-linear waves indicated good agreement to experimental responses. The model also demonstrates the ability to capture various device characteristics and performance trends with a high degree of accuracy. Finally, a design change to the PTO damping coefficient demonstrates a slight reduction in PTO damping can almost half the resulting structural stresses with marginal reductions in performance.

The project outcome presents a fully validated dynamic CFD model to analyse the performance of offshore OWCs with the inclusion of fluid structure interactions. Therefore, the developed model can be used to further analyse and optimise offshore WFCs for wide scale commercialisation.

Access Level

info:eu-repo/semantics/openAccess

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