In tribonucleation, a liquid-to-gas phase transition induced by a local pressure drop (cavitation) is highly undesirable, as it causes surface erosion and noise. A paradigmatic flow characteristic of tribonucleation problems is the flow between two coaxial disks. The flow is produced by the rapid upward movement of the top disk, which is initially at rest and in contact with the bottom disk. An analytical model, the so-called negative squeeze film, is typically used to predict the flow in the gap between the disks in this class of problems. Such a model considers an azimuthally uniform inflow in the gap between the disks. In this study, we experimentally show that if a negligibly small misalignment between the axes of the two disks is introduced, the inflow is not azimuthally uniform as expected from the negative squeeze film, but an entry jet appears in the flow between the disks. This entry jet is associated with the formation of two counter-rotating vortices. From reconstructing the pressure field from PIV velocity data in the vortex regions, we find that the local pressure is lower than the vapor pressure. This indicates that the gaseous phase in the cores of the vortices, which is observed from shadowgraphy visualizations in our study, should be attributed to cavitation. The negative-squeeze-film model, however, largely fails to predict the minimum pressure. Therefore, the onset of cavitation is not correctly captured by the analytical model.

Cavitation occurs when the local pressure, induced by high local velocities, drops below the vapor pressure, leading to the formation of vapor bubbles. The subsequent collapse of these bubbles can cause noise, erosion, and vibrations. Recent studies show that cavitation is sensitive to water quality, i.e., the nuclei populations, the chemical composition of water, and the presence of particles. Motivated by investigating the effects of water quality on cavitation, experiments are performed in a dedicated experimental facility. This consists in two co-axial disks that are initially at rest and mutually in contact, in a tank filled with water. The fast diverging movement of the top disk with respect to the bottom one produces a jet flow inside the gap between the disks, which leads to the formation of two counter-rotating vortices. The local pressure drop induced by high flow velocities leads to a phase change. To characterize the phenomenon, two optical techniques are applied, i.e., shadowgraphy and particle image velocimetry (PIV). In performing PIV reconstruction, the sum of correlation enhances the spatial resolution of the velocity vector fields. The pressure field in the region where the vortices occur is obtained from velocity data. The water quality effects on cavitation are investigated by adding salt and using water with an abundance of nuclei inside the tank.