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Current practices for modeling the ocean floor in underwater explosion simulations call for application of an inviscid fluid with soil properties. A method for modeling the ocean floor as a Lagrangian solid, vice an Eulerian fluid, was developed in order to determine its effects on underwater explosions in shallow water using the DYSMAS solver. The Lagrangian solid bottom model utilized transmitting boundary segments, exterior nodal forces acting as constraints, and the application of prestress to minimize any distortions into the fluid domain. For simplicity, elastic materials were used in this current effort, though multiple constitutive soil models can be applied to improve the overall accuracy of the model. Even though this method is unable to account for soil cratering effects, it does however provide the distinct advantage of modeling contoured ocean floors such as dredged channels and sloped bottoms absent in Eulerian formulations. The study conducted here showed significant differences among the initial bottom reflections for the different solid bottom contours that were modeled. The most important bottom contour effect was the distortion to the gas bubble and its associated first pulse timing. In addition to its utility in bottom modeling, implementation of the non-reflecting boundary along with realistic material models can be used to drastically reduce the size of current fluid domains.

Fiber reinforced polymer composite structures have very comparable mass densities to that of water. As a result, the effect of the Fluid-Structure-Interaction (FSI) is much greater for those composite structures than for conventional metallic structures. As a result, this study investigates the effects of FSI on composite structures. A series of underwater impact testing has been conducted to understand the dynamic responses of composite structures coupled with the FSI effects. Both air-backed and water-backed underwater impacts are considered and their behaviors are compared to the dry impact test results in order to determine the FSI effects. Furthermore, multiphysics-based computational analyses have been undertaken so as to better understand and interpret the experimental results. The results of this study will provide knowledge necessary to apply composite materials to underwater structural applications.

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