THE EFFECTS OF POROUS SEA BOTTOMS ON THE PROPAGATION OF UNDERWATER SHOCK WAVES USING THE P-α EQUATION OF STATE

Open Access
Author:
Buxton, Rebecca
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Gary Hugo Koopmann, Thesis Advisor
  • Stephen A Hambric, Thesis Advisor
Keywords:
  • P-α equation of state
  • underwater explosion
  • shock reflection
  • porous media
Abstract:
Much is known about the linear acoustic properties of sea bottoms as well as the interaction of blast waves with porous media in air. However, little is known about how shock wave interaction with a sea bottom affects the pressure waves in the water environment. Non-contact explosions in underwater environments pose a significant threat to Navy vessels. The detonation of an explosive charge underwater generates a shock wave that can directly load a vessel but can also reflect from a sea bottom before loading. This necessitates a better understanding of shock reflections from a porous sea bottom. In this thesis, the Navy’s DYSMAS finite difference code is used for modeling of the propagation and reflection of underwater explosions. Within DYSMAS, the P-α equation of state is implemented to model porous materials. The equation of state uses the parameter α, the ratio of porous material specific volume to the specific volume of the solid, to create an irreversible compressibility simulating the crush of air pockets in porous sand. This thesis uses the P-α equation of state to model two dramatically different porosities of sand subjected to loading conditions corresponding to key values for P-α behavior. Measurement points are selected in the water field of an axi-symmetric model to record pressure and impulse intensity (the cumulative integral of pressure and a good indicator of potential for structural damage). It is observed that variation in porosities does not affect the peak pressures at any point in the water. Rather, porosity itself (as opposed to a rigid bottom) reduces peak pressures in a limited area near the water-sediment interface. This area increases in height with distance from the charge. Porosity variations do affect impulse intensity. Higher porosities create a greater reduction in maximum impulse intensity. Like the peak pressure, these reductions are limited to an area near the water-sediment interface. Unlike the peak pressures, the reduction area increases in height with proximity to the charge. Higher porosities lead to slower propagation in the sediment. Higher porosities and lower sand-loading pressures both lead to more defined tensile, low pressure reflections.