Experimental study on the dynamics of a spark bubble near the top of a cylinder and associated annular secondary cavitation bubbles

This paper presents an experimental study focused on the annular secondary cavitation caused by the interaction between a high-voltage spark bubble and a cylinder. A 2000 V high-voltage underwater electric discharge was used to create the bubble, which reached a maximum diameter of approximately 31 mm. High-speed photography was employed to capture the behavior of the spark bubble and the induced secondary cavitation bubbles. The formation and evolution of the annular secondary cavitation bubbles near the cylinder's edge were captured and analyzed. The study classified the morphology of the annular secondary cavitation bubbles into three different types based on various bubble distances and cylinder diameters. Notably, unique bubble morphologies, such as the terraced bubble shape, were identified, resulting from the coupling between the spark bubble, the annular secondary cavitation bubble, and the cylinder. The interaction between the annular secondary cavitation bubble and the spark bubble, including coalescence, and the resulting effects on bubble shape and period were investigated. The changes of secondary cavitation bubbles are compared when the cylinder has different chamfer angles. Moreover, the study examined the dynamic characteristics of the bubbles, including their shape evolution, jetting behavior, period length, and pressure pulses upon collapse, at various distances from the top-surface of cylinders with different diameters. The experiment also introduced flow visualization with dye into spark bubble experiments for the first time to trace the vortex around the annular secondary bubble. Additionally, the transparency of bubbles in the images was improved through multi-source lighting techniques.