Experimental investigation of bubble dynamics near the bilge with a circular opening

• Intriguing bubble motion features captured near the discontinuous boundary, highly dependent on D (distance from the bubble to the opening) and Φ (opening diameter).
• Jets are found shooting away from the structure due to the opening in certain conditions, or not formed at all.
• Bubble oscillations sorted out into 6 distinctive scenarios, with clear images. None the same with the 3 types of oscillation found without the opening.
• The occurrence conditions for each scenario are identified in terms of D and Φ. Scenario boundaries are well fitted by parabolic curves on the D–Φ plane.
• Noticeable trends found with the initiation times and the speeds of the bubble jets varying with D and Φ.

Highly dependent on boundary conditions, the behaviors of underwater explosion (UNDEX) bubbles would be quite unusual near boundaries that are discontinuous with abrupt changes in shape, e.g. ship structures that have already been deformed by previous attacks. The oscillation features of the UNDEX bubble near the bilge with a circular opening representing previous deformation are studied experimentally with electric-spark-generated bubbles and high-speed photographing. The bubble behaviors are found highly dependent on two non-dimensional variables, D and Φ, representing the opening-bubble distance and the opening diameter, respectively. Seven distinctive oscillation scenarios are summarized from 180 experiments, namely the ‘rim-constrained oscillation’, the ‘inward jet’, the ‘outward jet’, the ‘bump and dimple’, the ‘quasi-spherical oscillation’, the ‘spherical oscillation with jet’ and the ‘spherical oscillation without jet’. The occurrence domains of the scenarios are identified as functions of D and Φ. Significantly affected by the opening, the bubble behaviors are quite different from that near a non-opening bilge; the bubble jet might not be formed, or even develop from inside the bilge, which indicates that the bubble load on the bilge should be re-evaluated. Finally the speeds, initiation time and displacements of the jets in different scenarios are measured and noticeable variation trends are found.