We investigate cavitation of a vortex ring in an impulsively starting jet induced by an in-tube expanding bubble, combining experimental, numerical, and theoretical approaches. As the jet cavitation number decreases, the cavitation evolves from discrete cavitation bubbles in the vortex core to a cavitating vortex ring in which the vapor phase fully envelops the vortex core azimuthally. A simplified one-dimensional model that approximates the in-tube bubble as a piston captures the time-dependent velocity of the starting jet, which features a remarkably high initial acceleration. This velocity history is then used as the inlet condition for nozzle-flow simulations, enabling us to address the following two scientific questions: (i) why vortex cavitation occurs at an exceptionally high cavitation number and (ii) why the incipient cavitation number increases with nozzle radius, known as the size-scale effect. To clarify the relationship between flow dynamics and cavitation, we analyze the vorticity flux and identify a previously underexplored contribution—the over-pressure component. This component, which strongly depends on the nozzle radius, dominates vortex ring circulation when the initial nozzle flow experiences an intense and brief acceleration. Notably, it may outweigh the shearing component predicted by conventional slug models. The impulsive nozzle flow provides a plausible mechanism for the unusually high incipient cavitation number and the observed size-scale effect. These results shed new light on the mechanisms governing cavitation onset in submerged jets and offer guidance for promoting or suppressing cavitation in practical applications.

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• Bubble dynamics
• Cavitation