A study of bubble collapse pressure pulse waves from small scale underwater explosions near the water surface

Explosive detonators have been used to study the effect of proximity of the water surface on the pressure wave generated by the collapse of an underwater explosive bubble. Transducers at four different depths recorded the pressure pulse, and a low resolution high speed video camera was used to observe the overall dynamics of the bubble. The detonators were fired at selected depths (z) within the range of 6.0-161.3 cm, corresponding to dimensionless depth (γ) values of approximately 0.3-9.0 where γ = z/Rmax, and Rmax is the maximum radius of the bubble during its first pulsation. Analysis of the recorded pressure waves was complicated by the superposition of the emitted wave with its reflection from the water surface. Despite this difficulty there was clear evidence of a dependency of the peak pressure (Pmax) and impulse (I) of the emitted wave on γ. The pressure pulse generated by the deeper events (γ > 3.4) did not appear to be strongly influenced by the water surface, exhibiting only a weak dependence on γ. However, Pmax and I decreased markedly for shallower detonations in the range γ ≈ 1.6-2.8. Imaging results showed that this decrease was associated with migration of the bubble away from the surface during its collapse. A markedly different behaviour was observed when γ was decreased further to 1.1. It was found that although the bubble was still repelled from the surface and I continued to decrease, Pmax increased dramatically. Details of the pulse and imaging results suggested a more complex bubble collapse behaviour at this depth. For shallower values of γ (<0.6) the bubble did not collapse and appeared to breach the water surface and ingest additional gas from the atmosphere.