Structures at sea, which can either be floating or bottom-founded, are always affected by several types of dynamic loads from wind, waves and currents; with floating offshore structures subjected to more motion and loading from these loads. Hence, necessitating more in-depth hydr
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Structures at sea, which can either be floating or bottom-founded, are always affected by several types of dynamic loads from wind, waves and currents; with floating offshore structures subjected to more motion and loading from these loads. Hence, necessitating more in-depth hydrodynamic analyses. Increasing size and weight of these floating structures together with industry demands have led to unsuitability of existing models – which make use of rigid body (no hydroelasticity) assumptions. This, in addition to other reservations about current practices, has led to the need for more realistic formulations of the interaction between fluids and our offshore structures, taking hydroelasticity into account. Extreme wave impacts, which are responsible for severe damage of offshore structures, have for a long period been only studied with experimental methods, as non-linear nature of the complex wave kinematics make solutions not straightforward. More recent advancements have seen the development of numerical techniques and programs, based on the Navier-Stokes equations, to better understand extreme waves and impacts on offshore structures.
This thesis focuses on the topic of experimental validation of fluid-structure interaction (FSI) in ComMotion – an advancement in the ComFLOW program (an improved Volume of Fluid (iVOF) Computational Fluid Dynamics (CFD) code) to capture motion of flexible structures in the presence of free surface effects and a two-way coupling between the fluid and the structure. This has led to the need to provide reliable data for validation purposes. The research is conducted in continuation of the studies carried out by Rizos (2016) in which an electromagnetic drive linear motor produced noise and variable amplitude enforcement obtained in the analyzed results, amidst other uncertainties in the measurement techniques used. The current methodology principally makes use of a different motion technique – rotary to linear motion to obtain more reliable data.
Firstly, rotary to linear motion was chosen (motivated from the internal combustion engine of an automobile) and then verified computationally with analytical derivations. The experiments were then designed, with the initial step being to verify the said linear motion. A sensitivity of the motion signal to weights in the system was carried out. Further, the experiment designed made use of a latex rubber membrane, applied with a clamped boundary condition at the bottom of a partially-filled rigid cylindrical structure. Tension existing in the membrane, due to water columns above it, was analytically obtained, as complete knowledge of system parameters was crucial. Linear oscillatory motion at the end of the converted rotary motion was applied to the fluid-structure system. Motion signals were obtained – rigid body measurements, to subtract them from the membrane motion measurements obtained. Motion measurements were acquired with laser triangulation displacement devices and a force transducer was used to monitor applied forces in the plunger during the experiments. The output of the transducer can offer validation data to the FSI solver.
Membrane measurements were taken at half radius at a 30° step over a 180° span, and at the centre. This was in a bid to capture maximum deflections of the membrane, as interest was to obtain the coupled frequency mode shape. Finally, the obtained membrane motion data were analysed to prove their reliability, and to draw conclusions from them. Volume (of liquid), frequency and polar dependence analyses were carried out. Recommendations were subsequently outlined, for future research.
