Micro-modelling of wave fields

Abstract

Physical models transfer part of the natural world into controlled laboratory conditions without the oversimplifying assumptions required by numerical models. However large scale hydraulic facilities are associated with large expenditure, require a high level of preparation, while at the same time access, to them is restricted for the majority of researchers and engineers. This thesis explores the potential of small scale physical models of waves (termed wave micro-models) as a modelling tool for coastal engineers and scientists. Length scales smaller than 1:50 are applied in micro-models and as such the requirements in facility size, technical and experimental personnel and experimental time (t = q 1 length scale for a Froude scaled model) are significantly reduced. However, the problem of scale effects must be assessed. Accordingly the first steps of the large experimental effort presented here, were focused on establishing the validity range of wave micro-models for both breaking and non-breaking waves. Using that as a starting base several micro-models were then designed and developed in order to 1. assess the effectiveness of culverts as flushing elements in marinas, 2. develop a new composite seawall for overtopping based wave energy conversion and 3. test geometry variations in an existed yacht harbour. The obtained experimental findings demonstrated that the micro-modelling of non-breaking waves is possible for water depths larger than 0.03m and wave periods longer than 0.35sec. On the contrary small scale breaking waves are strongly influenced by surface tension but when the freshwater was replaced by an isopropyl-alcohol and distilled water solution with reduced surface tension, very encouraging results were achieved. Undistorted plunging breakers were created, air entrainment was observed to increase significantly and the energy dissipation values exceeded even those reported in the literature for large scale experiments. It is believed that such a fluid could potentially be used in other areas of hydraulic research, e.g hydraulic jumps, dam break events, etc. Useful insights regarding the operation of culverts were also acquired and it was shown that efficient flushing is almost potentially impossible with symmetric culverts. In conjunction with this, the measurements of the hydraulic power potential of the composite seawall were very promising and when a modelling fluid with reduced surface tension was used run-up and overtopping at small scale were significantly improved. Finally, geometry effects on wave fields were observed with a novel, purposely developed, mapping method. The velocities of floating particles were used to extract wave heights and although inaccuracies were observed, partly because linear theory was employed, the 3D evolution of a wave field was mapped for the first time. Wave mapping in combination with wave micro-models appears attractive since the comparatively small areas allow for the measurement of the whole wave field rather than using single point measurements as it is usually applied in physical model tests.

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