The microbubble content is considered one of the most important fluid characteristics influencing cavitation inception and dynamics. However, the effects of microbubbles on cavitation are not yet precisely understood. The current work experimentally assesses the influence of microbubbles on cavitation around the runner of a hydraulic turbine model in a hydraulic test facility. Microbubbles are measured upstream of the modelusingIPI, while cavitation is observed visually. IPI measurements are performed under different DO concentrations, thereby changing the microbubble content in the facility, at different runner speeds. The results show an increase in microbubble sizes and concentrations for higher DO
concentrations. The sizes and concentrations of microbubbles increase accordingly with decreasing σ. This increase is not only the result of a pressure reduction at the measurement position but also the appearance of a cavitating vortex at the outlet of the runner, causing additional bubbles measured at the measurement position. These effects are highlighted once again after the onset of gap cavitation, showing the influence of cavitation on the measured microbubble content as no resorber is located in the test facility. A decrease in the microbubble concentration is observed after the onset of gap cavitation for increasing DO concentrations, as oxygen diffuses into cavitation bubble. Moreover, at low values of σ, a vacuum is applied to the system, resulting in a faster decrease in the DO concentration
at a higher initial DO concentration, resulting in a decrease of the microbubble content. A delay in the onset of gap cavitation is observed at lower DO concentrations due to the availability of fewer and smaller microbubbles in the facility. After the onset of cavitation, the development of gap cavitation is significantly altered by a change in the DO concentration. It is shown that gap cavitation covers a larger area of the runner at high DO concentrations, most likely caused by an increase in the vapour volume fraction at the outlet of the labyrinth seal, thereby increasing the number of vapour bubbles around the gap of the runner

Tip vortex cavitation around the marine propeller is the primary source of the underwater noises emitted by ships. Investigating the tip vortex inception is important to understand the aforementioned side effect. The water quality, highly related to the bubbles population, surrounding the tip vortex is crucial to the tip vortex cavitation inception. This research explores how bubble moves around the tip vortex and investigates its background mechanisms. A one-way coupled Lagrangian bubble tracking model is applied to the Lamb-Oseen vortex and the hydrofoil tip vortex flow field, where the tip vortex is simulated with an open-usage CFD software, ReFRESCO. A series of capture time models have been developed by exploiting the bubble force (acceleration) balance in the Lamb-Oseen vortex to identify the crucial parameters. The other aspect of the acceleration analysis reflects the role of the lift force as the system's stabilizer, which has long been ignored. In the CFD tip vortex flow simulation, the bubble population evolution is investigated from upstream to downstream. Several kinds of bubble behaviors around the tip vortex flow have been identified via the numerical simulation, and simplified models based on the bubble force analysis have been proposed in order to explain them. Mainly due to the stagnation bubble events, clustering of large bubbles has been discovered a short distance downstream of the foil tip, implying that this may be a possible hotspot for tip vortex cavitation inception. The stagnation bubble phenomenon still needs to be further investigated, particularly in terms of quantifying its dependence on various flow parameters. The results also need to be rigorously validated which has not been possible as of yet due to insufficient measurement data. Still, this research manages to explore the bubble motion around the tip vortex and their fundamental mechanism.