Characteristics and dynamics of compressible multiscale cavitating flows inside a nozzle, with special emphasis on compressible effects

Unsteady cavitating flow usually contains various vapor structures with different length scales and significant fluid compressibility, which poses great challenges to numerical simulation. In this work, we propose a compressible Eulerian–Lagrangian method to investigate cloud cavitation inside a nozzle, with particular emphasis on the effect of compressibility on multiscale structures. The macroscopic cavity evolution is dealt with using large eddy simulation and the volume of fluid method in a compressible Eulerian framework. Microscopic bubble dynamics are described using the equation of bubble motion and the compressible Rayleigh–Plesset equation. A two-way coupling algorithm is established to bridge the two frameworks. The calculated results are validated by comparison with available experimental data. The numerical results show that the compressibility exacerbates the instability of the cavitating flow field, generates more discrete bubbles, and promotes liquid–vapor mass exchange, which further increases the overall cavitation volume. Furthermore, two distinct power laws for bubble size distribution, with exponent −4/3 for small bubbles and −10/3 for large bubbles, are obtained. Owing to the influence of the high flow velocity and high ambient pressure in the nozzle, and the wall effects on bubble dynamics, the number of large-size bubbles calculated by the numerical simulation is greater than that obtained by theoretical analysis. The formation of these bubbles is affected by shear on the liquid–vapor interface, the presence of vortex structures, and turbulence, thus differing significantly from the case of cavitation flow around a hydrofoil.