Attili, Tommaso
(2023)
Numerical-laboratory modelling of waves interacting with dams and rigid-flexible plates.
PhD thesis, University of Nottingham.
Abstract
Fluid-Structure Interaction (FSI) is relevant for a range of mechanical processes, including wave impacts on offshore and coastal structures, wind-excited vibrations of tall buildings, fluttering of bridges and blood flows in arteries. Within the FSI phenomenon, Wave-Structure Interaction (WSI) involves wave impacts on dams, flood protection barriers, wave energy converters, seawalls, breakwaters, oil and gas platforms and offshore wind turbines. These structures are often challenged by extreme waves, e.g. tsunamis generated by landslides, rockfalls and iceberg calving, potentially leading to structural damage under exceptional conditions. For structures undergoing non-negligible deformations, referred to as Wave-Flexible Structure Interaction (WFSI) herein, the physical processes are even more complex. Unfortunately, accurate predictions of the wave effects, e.g. forces, on rigid and flexible structures are still challenging and laboratory models often involve scale effects.
This thesis explored a range of WSI phenomena based on the numerical model solids4foam, along with small-scale laboratory experiments. Two-Dimensional (2D) and Three-Dimensional (3D) tsunamis impacting dams were investigated first. The numerical wave loading agreed with predictions based on an existing approach and new empirical equations for wave run-ups and overtoppings of dams were proposed. The dynamic pressures were also investigated and correlated with new semi-theoretical equations. New insight into the 3D effects, including the dam curvature and asymmetrical wave impacts, were provided for selected cases. The combination of both these effects resulted in up to 32% larger run-ups compared to the 2D predictions.
2D wave impacts on offshore and onshore plates of different stiffnesses were then modelled, along with selected 3D tests. The plate stiffness had a negligible effect on the upwave forces for the majority of these tests. However, the offshore flexible plates resulted in up to 40% smaller total forces, compared to the rigid ones, due to increased downwave water depths following the plate deformations. For the onshore tests, the time series of the wave loading were characterised by two force peaks, according to previous studies. The second force peaks were up to 3.3 times larger than the first peaks. New semi-theoretical equations were proposed to predict the onshore wave forces and run-ups of a plate, as a function of the offshore wave energy.
Finally, a systematic investigation of the scaling approaches and scale effects for wave impacts on rigid and flexible plates was conducted based on numerical modelling supported by small-scale laboratory tests. The WFSI governing parameters were derived and successfully validated based on the numerical results. A number of simulations, involving non-breaking and breaking wave impacts, were then conducted for the prototypes and up to 40 times smaller models. These were scaled according to the scaling approaches (i) precise Froude (fluid and plate properties scaled), (ii) traditional Froude-Cauchy (fluid properties unscaled, plate properties scaled), (iii) traditional Froude (fluid and plate properties unscaled) and (iv) a new WFSI approach (partial conservation of the WFSI governing parameters). No scale effects were observed for (i). Non-breaking waves were correctly scaled by (ii), however, up to 132% scale effects were observed in the breaking wave pressures due to the unscaled fluid properties. Further, the plate displacements were up to 98% underestimated by (iii). The new approach (iv) successfully predicted non-breaking wave impacts, with less than 4.3% deviations for the maximum wave forces and plate displacements.
In conclusion, the findings of this PhD thesis are intended at enhancing the physical understanding of WSI to support the design and laboratory modelling of a range of offshore and onshore structures. Future studies should address a number of further aspects, such as the 3D effects on tsunami impacts and the role of the air compressibility on WFSI. Also, the WFSI governing parameters and the new scaling approach should be further validated using numerical and laboratory experiments.
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