Song, B; (2015) Fluid/Structure Impact with Air Cavity Effect. Doctoral thesis , UCL (University College London);.

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Abstract

Violent wave attacking offshore and coastal structures is a complex phenomenon frequently involving air entrapment. A study on fluid/structure impact with air cavity effect is carried out in the framework of velocity potential theory. The purpose is twofold. One is to develop methodologies to tackle the technical difficulties involved. The other is to achieve a better insight into the impact dynamics and the subsequent structure/water/air interaction process, as well as the associated air cavity effect and its acting mechanism. The study starts with axisymmetric problems. Impact by a liquid column on a rigid plate is studied analytically and numerically. The initial singularity at the body-free surface intersection is analysed in detail. The feature of the resulting long thin jet is revealed: providing field solution over larger wetted area without influencing the main impact dynamics. This is favourable in the study of some problems (e.g. steady state solution or local impact over a tiny region), and thus a decoupled shallow water approximation scheme is developed for the computation with long jet. Impact with air cavity of various parameters is studied systematically. Wave impact with air entrapment in practical engineering situations is then focused. A domain decomposition method together with a dual-system technique is developed to provide fully nonlinear simulation on the early impact stage by a plunging wave crest, tackling the large variation in scales involved. Local pressure peak is found to be generated by the sharp turn of the wave surface along the wall. The trapped cavity, governed by an adiabatic law, is found to cause oscillating loading on the wall. The local free jet drawn from the upper cavity surface in each re-contraction stage reveals its distortion and fragmentation mechanism. The initial dimensionless potential energy of the air cavity is found to largely influence its maximum pressure, and the scaling law revealed could be applied to the prediction of impact pressure in practical situations from a laboratory experiment.

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