Subsonic streamers in water: Initiation, propagation and morphology

In this paper, we focus on the development and morphology of underwater subsonic streamers in a pin-plane electrode configuration. The processes of initiation, propagation and branching of underwater subsonic streamers are studied in detail, and the effects of applied voltage amplitude and polarity are discussed. The results show a similar initiation process for both polarities including the formation of a low-density shadowed region, an initial bubble and streamers. Joule heating is thought to be the dominant intrinsic dynamic in the initiation process. There is a distinct difference in the propagation mode and the morphology between positive and negative subsonic streamers. Positive subsonic streamers develop in the form of a bubble cluster with a constant low speed of 20-80 m s^-1, but the development mode transition from bubble cluster to secondary streamer occurs at the final stage of streamer propagation. On the other hand, negative subsonic streamers develop in the form of tree-like structures with a thinner root and propagate faster at a speed of 100-370 m s^-1, and the propagation velocity increases significantly near the final breakdowns. A specific phenomenon has been observed in which the breakdown voltage of positive polarity can be higher than that of negative polarity in micro-second underwater discharge. The above results indicate that the polarity effect of subsonic streamers is quite different from that of the supersonic streamers.