  The transient process accompanied by extreme acceleration in the conical
sections of hydraulic systems (e.g., draft tube, diffuser) can induce large
cavitation bubbles both at the closed ends and in the bulk liquid. The
collapses of the large cavitation bubbles can cause severe damage to the solid
walls. We conduct experiments in the tubes with different conical-frustum
shaped closed ends with the `tube-arrest' method and observe bubbles generated
at these two locations. For the bubbles generated at the close end of the tube,
we propose the onset criteria, consisting of two universal non-dimensional
parameters $Ca_1$ and $Ca_2$, of large cavitation bubbles separating the water
column. We investigate their dynamics including the collapse time and speed.
The results indicate that the larger the conical angle, the faster the bubbles
collapse. For the bubbles generated in the bulk liquid, we numerically study
the collapse time, the jet characteristics and the pressure pulse at bubble
collapse. We observe a much stronger jet and pressure pulse of bubbles in
tubes, comparing with a bubble near an infinite plate. Our results can provide
guidance in the design and safe operation of hydraulic machinery with complex
geometries, considering the cavitation during the transient process.
