Fluid-Structure Interactions: Implosions of Shell Structures and Wave Impact on a Flat Plate

Abstract

The work in this dissertation examines the fluid-structure interaction phenomena in a series of three experimental studies. The first two sets of experiments were conducted in a large, water-filled pressure vessel with a nominal internal diameter of 1.77 m. Cylindrical shells were made from thin-walled aluminum and brass tubes with circular cross-sections (internal diameters D) and internal clearance-fit aluminum end caps. Implosion and explosion events were photographed with a high-speed camera (27,000 frames per second), and the waterborne pressure waves resulting from the implosion were measured simultaneously with underwater blast sensors. The natural implosions were generated by raising the ambient water pressure slowly to a value, Pc, just above the elastic instability limit of the models. For the models with larger L/D, where L is the internal length of the model, the model cross sections flattened during the implosion (mode 2). It was found that the amplitude of these mode 2 pressure waves scale with the pressure difference Pc-Pi (where Pi is the internal pressure of the air inside the cylindrical models) and the time scales with the characteristic bubble collapse time. The geometry and material properties of the structure seem to play only a secondary role. During the explosion experiments, the pressure vessel is pressurized to various pressure levels below the natural implosion pressure of the models and an explosive was set off nearby. It was found that the implosion is induced by one of two mechanisms: the shockwave generated by the explosion and the hydrodynamic pressure field of the explosion bubble during its collapse and re-expansion. In the final experimental study, the impact of a plunging breaking wave (wavelength,1.2 m) on a partially submerged cube (with dimensions L = 0.3048 m) is studied in a wave tank (14.8 m long). The water free surface shape upstream of the cube before and after the wave impact was measured with cinematic Laser-Induced Fluorescence (LIF), employing a high-speed digital camera, a laser light sheet and fluorescent dye mixed with the water. It was observed that for some cube positions, the free surface between the front face of the cube and the wave crest forms a circular arc that converges to a point and forms a high-velocity vertical jet (about 3m/s). Although these problems are intrinsically different, they are flows dominated by inertial forces (viscous effects are not important) where a rapidly collapsing interface shape produces high-pressure waves.