Numerical investigation of inner structure and its formation mechanism of cloud cavitating flow

The inner structure and the formation mechanism of cloud cavitation around a twisted hydrofoil was numerically investigated using a multi-scale method. A homogeneous mixture model was used to capture the macroscopic vapor structures, while the microbubble behavior was tracked by Lagrangian model. The spatial and temporal evolution of the multi-scale cloud cavitation is analyzed with special emphasis on the microscopic bubble dynamics. A microbubble generation model was proposed to explain the source of the numerous bubbles inside the cloud cavity. The shear layer at the liquid-vapor interface was found to be an important cause of the bubble formation. Furthermore, a statistical analysis of the large number of bubbles inside the cloud cavity showed two distinct bubble size spectra, N(D), which is similar to the theory for breaking ocean waves. Two mechanisms were established to explain this phenomenon. The first is when the attached cavity pinches off and rolls up which creates small bubbles conforming to Ns(D) ∼ D-1.26 with the bubble generation controlled by shearing effects. The second is when the vortex structures absorb bubbles into their core, where the low local pressure allows them to grow resulting in larger bubble size satisfying Nb(D) ∼ D-2.8. When the U-shaped cloud collapses, it breaks into a mass of tiny bubbles with a bubble size spectrum conforming to Ns(D) ∼ D-1.26 as well as a significantly greater number of larger bubbles that conform to Nb(D) ∼ D-2.2, which is thought to be due to both vortex and turbulent fragmentation.