6. Underwater Implosion Mechanics: Experimental and Computational Overview

An implodable volume can be defined as a structural body which is acted upon by external pressure and internally contains gas at a lower pressure than the surrounding fluid. An underwater implosion occurs when the body suffers a sudden loss of structural stability and hydrostatic pressure drives the body to collapse inwardly upon itself. The result of the collapse is a rapid decrease in local pressure as the water expands to fill the void, and then a shock wave as the in-rushing water suddenly stops and is compressed. The physics of an implosion event is shown to be similar to the collapse of cavitation and underwater explosion bubbles. The pressure history resulting from an implosion event consists of several primary characteristics, namely an initial pressure drop in the surrounding fluid during the initial collapse, corresponding to the inward rush of the surrounding fluid, followed by a subsequent positive pressure spike and decay as the body collapses upon itself and the water motion is arrested. It is observed that the magnitude of both the pressure drop and subsequent peak pressures are dependent upon the hydrostatic collapse pressure. Additionally, the ductility of implodable volume material is shown to have an effect on the amount of energy released during the collapse with brittle implodables releasing larger amounts of energy into the surrounding fluid than ductile volumes. Finally, implodable volumes which are coated with energy absorbing materials display decreased energy in the implosion pulses.