Cavitation inception during the interaction of a pair of counter-rotating vortices

Pairs of unequal strength, counter-rotating vortices were produced to examine the inception and dynamics of vortex cavitation as the vortices undergo a long-wavelength instability. The instability causes the weaker, secondary vortex to be turned and stretched by the stronger primary vortex. Folding and stretching of the secondary vortices result in sharp reductions of the core pressure. Here, these sharp and transient reductions in the secondary vortex core pressure produced incipient cavitation at static pressures that were as much as 20 times higher than that required for inception in the core of the unstretched secondary vortex. In addition, the majority of nuclei measured was of the order of 1 μm in size, which requires tension on the order of 100 kPa for cavitation inception to occur. The flow parameters that lead to the instability and cavitation inception in the secondary vortex are examined, and the measured event rates are correlated to freestream nuclei populations and static pressure. These measurements, combined with observations of the elongated bubbles themselves, suggest that stretching produced large tensions in the core of the secondary vortex due to both a reduction in the secondary vortex core size and the creation of a jetting flow in the vortex core.