Numerical Investigation of Cavitation-Regimes in a Converging-Diverging Nozzle

Abstract

In the present study the authors analyze cavitation dynamics, instabilities and detachment mechanisms in an axisymmetric converging-diverging nozzle. Numerical simulation is used to investigate a recent experiment. Following the experimental setup, three operating points are simulated to identify two different cloud detachment mechanisms: re-entrant jet and condensation shock. A homogeneous mixture model is applied to model cavitating two-phase flows. Pure liquid and liquid-vapor mixtures are both treated as fully compressible substances to enable the computation of propagating pressure waves into the liquid bulk. Because the study focuses on inertia-dominated mechanisms, the flow is modeled as inviscid. Our numerical simulation predicts a slightly different cavitation behavior than the one published for the reference experiment. We observe a high-frequency shedding that generates vapor pockets inside the channel. These vapor pockets partially collapse but frequently they integrate into a large coherent structure. Once a coherent vapor cloud reaches a sufficient length, it detaches and is advected downstream. As a result, the total vapor volume fraction shows a low frequency oscillation which is about one order slower than the shedding frequency. Two detachment mechanisms are present during one cycle: the re-entrant jet for shorter cavities and the condensation shock for longer cavities. The maximum cavity length is not constant but features significant cycle-to-cycle variation. Even a slight difference in the cavitation number leads to a difference in the detachment mechanisms: the lower cavitation number leads to a bubbly shock as the dominant detachment mechanism, while for a higher cavitation number shock-driven detachment occurs only once per cycle.