title: The influence of nuclei content on tip vortex cavitation inception
Creators: name: Khoo, MTB
Publisher: name: University of Tasmania
Year: 2021

description: Tip vortex cavitation (TVC) is detrimental to the acoustic stealth performance of naval ship and submarine propellers and control surfaces. Nuclei, or microbubble, content can vary in environmental waters, having implications for TVC inception and acoustic signature. While it is commonly known that TVC inception occurs earlier in flows abundant with nuclei, further research is required to broaden understanding of nucleation effects on TVC behaviour and its associated physics. This includes research in the interrelated areas of natural nuclei population dynamics in test facilities, the statistical nature of TVC inception in nuclei deplete flows, and the influence of different nuclei populations on the physical characteristics of TVC inception and development. To address the lack of published information in these areas, a series of experiments were conducted in variable pressure water tunnels, also known as cavitation tunnels, which are typically used to study TVC and nucleation effects under controlled conditions. The natural nuclei population behaviour in different test facilities was measured using a cavitation susceptibility meter (CSM). This informed the design of subsequent TVC experiments concerning nucleation effects on TVC about lifting surfaces. The statistical nature of TVC inception and desinence about an elliptical hydrofoil in flows deplete of nuclei was quantifed using the optical detection of a large number of cavitation events. A nuclei abundant ow was then introduced to assess the effect of vastly different nuclei populations on TVC behaviour about an elliptical hydrofoil. High-resolution photographs and acoustic measurements were acquired at fixed Reynolds and cavitation numbers. The deplete and abundant nuclei populations were quantified using the CSM and Mie scattering imaging, respectively. The study was then extended to consider the effects of different artificially-generated nuclei populations on TVC inception location and cavity kinematics using synchronised video and acoustic measurements. The natural nuclei population in the cavitation tunnel at the Australian Maritime College Cavitation Research Laboratory is found to follow a power law, and remains invariant at a baseline level over short timescales. The baseline level of the natural nuclei population in the Japanese Acquisition, Technology &amp; Logistics Agency's Flow Noise Simulator is similar to that of the Australian tunnel. The test section pressure of the Japanese tunnel is a better indicator of whether the natural nuclei population has increased above the baseline, which is in contrast to the dissolved oxygen saturation condition in the plenum for the Australian facility. It is discovered that for undersaturated conditions in the plenum of the Australian tunnel, the natural nuclei population remains constant around the tunnel circuit. Therefore, the population in the test section can measured by sampling water into the CSM from the lower-segment resorber, provided the water is undersaturated in the plenum. This is a convenient way to monitor the natural nuclei population during TVC experiments for which the presence of the CSM sampling probe in the test section would be prohibitive. In one such experiment, TVC inception is shown to be a stochastic process that requires a large number of repeated measurements for characterisation. At a given test condition, the probability of inception increases with waiting time. This is due to a larger volume of water, and thus higher number of susceptible nuclei, passing through the streamtube of opportunity. Meanwhile, TVC desinence exhibits much less statistical variation, suggesting it is less dependent on the natural nuclei population. Comparisons of TVC inception behaviour in nuclei deplete and abundant flows confirm that cavitation inception occurs at a higher incidence in a nuclei deplete ow due to a lack of weaker nuclei. In both flows tested, the sound pressure level reaches a local peak at or after inception, although the physics driving this behaviour are different between the seeding cases. In a nuclei abundant ow, intermittent activations occur before the formation of a continuous tip vortex cavity at higher tensions, which results in a higher overall sound level. It is also found that both the nuclei population and cavitation number have a significant impact on the inception location distribution along the trailing vortex of an elliptical hydrofoil, and in particular, on inception event rates. The cavitation number changes the ow volume subjected to tension, and hence the shape of the inception location distribution as well. It is found that once ingested nuclei are activated, cavity kinematic and acoustic properties are influenced by the local pressure (i.e. inception location and cavitation number) rather than the initial nucleus size, at least in the ~50-100 μm diameter range activated in this study. A greater understanding of the effects of nuclei content on TVC has been achieved. Natural nuclei population dynamics in test facilities should be factored into experimental design when testing in flows with low pressures. This was taken into account in the study of TVC in nuclei deplete flows, which quantitatively shows inception to be stochastic in nature and strongly dependent on the nuclei population, in contrast to desinence. The extent to which nuclei content affects TVC behaviour increases as the difference between the populations becomes greater. While the appearance and noise emissions of TVC differ markedly between nuclei deplete and abundant flows, the differences in these characteristics are less pronounced between flows with injected mono- and polydisperse nuclei populations which are more similar with respect to bubble size and concentration distributions. Nevertheless, variations in nuclei concentration between populations are clearly reected by changes in inception event rate. To further understanding about nucleation effects on TVC, additional work is required on nuclei measurement and bubble generation techniques, TVC behaviour across a wider range of nuclei populations and validation of theoretical cavity dynamic and acoustic models using experimental data.