An experimental and numerical investigation of a model breakwater concept

Abstract

Floating and free-surface breakwaters are identified as promising alternatives to conventional bottom-fixed breakwaters for deeper waters. The existing body of work regarding floating and free-surface breakwater experiments in wave tanks has been consolidated. A new free-surface breakwater incorporating both rectangular and cylindrical structures has been proposed. Cylindrical pipes are readily available and can be used to augment the wave attenuation behaviour and support activities such as compressed gas energy storage. An investigation involving both experiments and numerical simulations for the wave attenuation of the proposed hybrid structure at infinite mooring stiffness was carried out. Experiments at a scale factor of 1:75 were designed and models were constructed to test various piping configurations. Experiments were carried out in the wave tank in the Fluids Laboratory at the University of Malta. Wave surface elevation data was collected and analysed. Numerical models were developed using ANSYS® AQWA™ and the experimental and numerical results were compared. Validation against the experiments of other researchers was carried out to further consolidate the observations made. The limitations of small-scale model testing in a wave tank facility were well understood and documented. Experimental and numerical results were thus shown to be limited when interpreted individually, albeit following the same qualitative downward trends in transmission coefficient for shorter waves. Experimental results were shown to capture the near-surface energy bias and the increased water particle interaction with a denser partially porous structure. For conditions of overtopping, significant discrepanciues between experimental and numerical results were observed, indicative of the limitations of ANSYS® AQWA™ to capture the physics at the wave surface. Wave tank experiments have exhibited high sensitivity to standing wave phenomena resulting in staggered data points across the wave frequency range being tested. Longer wave tanks are therefore less susceptible to reflection effects. Consequently, it was not possible to consistently distinguish between wave tank effects and model wave attenuation phenomena. The combination of experimental and numerical models has been shown to be a very useful qualitative decision-making tool to predict relative improvements in the wave attenuation of different breakwater configurations under the proposed test conditions. The results should not be interpreted as absolute as per the limitations identified.