title: Microbubble disperse flows about a lifting surface
Creators: name: Russell, PS
Publisher: name: University of Tasmania
Year: 2020

description: The formation, size and concentration of microbubbles generated in the wake of a cavitating hydro-foil were investigated experimentally in a variable pressure water tunnel for several Reynolds and cavitation numbers, with and without freestream nuclei. In the absence of freestream nuclei, interactions between the cavity, the overlying boundary layer and associated interfacial effects were invesigated qualitatively and quantitatively using a combination of still and high speed photography. The influence of these features on the physics of cavity breakup and condensation, and subsequent microbubble formation, were examined. Coherent spatial and temporal features of the sheet cavitation were found to be functions of both Reynolds and cavitation numbers. Long range microscopic shadowgraphy was used to measure the dense bubble populations present in the wake, and additionally implemented as a reference technique in the development of the Mie- Scattering Imaging (MSI) technique described below. For the range of microbubble sizes measured, concentrations are shown to increase with Reynolds number and reduce with decreasing cavitation number. The presence of freestream nuclei markedly alters cavity topology, and their effect on flow features and associated microbubble production was also evaluated. Wake microbubble concentrations were found to in-crease when low concentrations of nuclei were introduced but to then decrease with further increase in nuclei seeding. Regardless of seeding concentration, microbubble populations in the wake in-creased as the cavitation number was reduced. For high cavitation numbers the increase in concentration is primarily in bubbles of smaller size, whereas the increase in wake concentration at lower cavitation numbers occurs over a greater size range. These experiments demonstrate the im-portance of cavitation nuclei measurement in hydrodynamic test facilities. Application of an interferometric technique known as Mie- Scattering Imaging (MSI) for the measurement of sparse nuclei seeding populations in such facilities has been developed. A separate pressure chamber, with similar optical path properties to the tunnel test section, was used to develop the technique. Monodisperse bubbles (with diameters between 30 and 150 ➭m) were generated by a microfluidic ‘T’ junction, and individual bubbles were simultaneously imaged with shadowgraphy and MSI. In develop-ment of the MSI technique, approximations from Lorenz-Mie theory were experimentally validated, and the influence of fringe uniformity and intensity for each polarisation (perpendicular or parallel) on measurement precision was investigated. Parallel polarisation was preferred for its more uniform fringe spacing despite a lower intensity. The inverse relation between fringe wavelength and bubble diameter was demonstrated at a measurement angle of 90°. The wavelength of the scattered fringe pattern is predicted using Lorentz-Mie theory and the calibration constant for fringe spacing was obtained. A practical method for the calibration of a second constant related to the imaging optics has also been developed. Using this approach the measured bubble diameters from the shadowgraphy and MSI compared to within 1 μm. The precise bubble location within the beam was measured with shadowgraphy and with this information a method for determining the size dependent measurement volume for both axisymmetric and arbitrary beam profiles was developed. Once re-fined, the technique was used to characterise the concentrations and range of microbubble sizes produced by a nuclei seeding system for various tunnel conditions. Nuclei concentrations between 0-24 bubbles per mL were measured and the distribution of bubble sizes was found to follow a power law for high nuclei concentrations.