Cavitation bubble dynamics in a funnel-shaped tube

Control of cavitation in confining geometries is vital to the safety of hydraulic conduits and structures. Here, we investigate cavitation bubble dynamics in one typical type of open-ended tube with varying cross sections, i.e., a funnel-shaped tube, defined as a funnel-shaped section connected to a cylindrical section. In our experiments, single cavitation bubbles are generated by spark along the symmetric axis of the funnel-shaped tube at different positions, including in the funnel-shaped section (regime 1), near the throat (regime 2), and in the cylindrical section (regime 3). With high-speed photography, we observe that at the end of collapse, bubbles produce weak jets in regime 1, jets in a shape of an inverted cone in regime 2, and jets in a cylindrical shape with a rounded end in regime 3. With the help of OpenFOAM simulation, we better understand the surrounding flow fields during the bubble evolution. By capturing the main features of the flow fields, we derive dynamics equations for bubbles in each regime. Scaling laws are proposed for the bubble jet velocity, which depends on the position of bubble generation, the inner diameter of the cylindrical section of the tube, and the maximum distance where the bubble's upper surface can reach before the collapse. Our findings may be inspiring for understanding cavitation bubble dynamics in tubular conduits with complex geometries, e.g., cavitation in pipelines and draft tubes in hydrodynamic systems, and ultrasonic diagnosis in blood vessels.