Energy balance of rapidly deforming foam filled cylindrical shells in a high pressure fluid environment

Abstract

The pressure differential between an air-filled structure and the surrounding fluid for underwater applications can potentially lead to instabilities and collapse of the structure. When air-filled structures collapse underwater, large pressure pulses are emitted into the surrounding water, which can damage nearby living organisms and structures. The present study aims to mitigate the high-pressure spikes generated in underwater implosions. Experiments were performed underwater in a pressure vessel. Thin-walled aluminum 6061 cylindrical shells were placed in the water filled pressure vessel, and the hydrostatic pressure was increased until the tubes reached instability and subsequently collapsed. To mitigate the pressure pulses radiated into the fluid by the implosion of the tubes, closed cell polyvinyl chloride foam rods of varying densities were placed inside of the aluminum tubes and assembled concentrically. The densities of the solid foam rods were 45 and 100 kg/m3, which were selected to investigate the effect of foam density on the mitigation capabilities of the foam rods. Three-dimensional digital image correlation was coupled with high-speed photography to obtain the full-field displacements and velocities of the aluminum tubes during implosion. Moreover, the pressure fields created during the structural collapse of the tubes were recorded with tourmaline pressure transducers. The 45 kg/m3 foam reduced the peak pressure pulse generated during the implosion event by 30%, whereas the 100 kg/m3 foam rods reduced the peak pressure pulse by 50%. Furthermore, the 45 kg/m3 foam decreased the energy imparted onto the fluid by the implosion of the tubes by 16%, whereas the 100 kg/m3 foam decreased the energy imparted onto the fluid by the implosion by 43%. Full field displacement data showed that both foam densities reduced the radial velocity of the tube during implosion, as well as reducing the peak centerpoint decelerations of the tube prior to wall contact by 45% and 62% for the 45 kg/m3 and the 100 kg/m3 foam, respectively. Fully coupled fluid–structure interaction finite element models were created using the Abaqus software to obtain the structural deformation energy in the aluminum tubes after implosion, as well as the energy absorbed by the foam rods. The models were in good agreement with the experiments in terms of radial displacements during implosion, as well as the final foam rod volume.