Vortex dynamic characteristics of unsteady tip clearance cavitation in a waterjet pump determined with different vortex identification methods

Abstract

The cavitating flow field in waterjet pumps is complex, especially tip leakage cavitation (TLC), which has been a puzzle to researchers for decades. In this study, high-speed video (HSV) of the instantaneous inner structures of tip cavitation flow is used to show the evolution of TLC. Numerical simulation is conducted with the scale adaptive simulation (SAS) turbulence model and Zwart-Gerber-Belamri (ZGB) cavitation model to understand the cavitation-vortex interaction in the blade tip region. The predicted cavitation performance curve exhibits reasonable agreement with the experimental results, and the time-dependent vapor iso-surfaces (αv = 0.1) are consistent with HSV at different times in a typical cycle. Numerical simulation results show that cavitation can enhance the turbulent kinetic energy in the unstable vortex cavitation region and downstream tip leakage vortex region along the blade. Different vortex identification methods, including vorticity, Q criterion, λci, λ2 criterion, Ω, and Liutex/Rortex, are investigated. Analysis and comparison of the iso-surfaces of the different vortex identification methods indicate that the Ω and Liutex iso-surfaces can effectively predict the tip leakage vortex core and vortex pair in the unstable vortex cavitation region. Analysis of the fifth clip contours shows that no obvious difference exists between Q and λ2 criteria in terms of predicting the vortex core. All vortex identification methods can accurately predict the tip separation vortex in the blade tip region, but only the Ω and Liutex iso-surfaces can predict weak vortices in the cavitation region. The influence of the small parameter e of the Ω method on tip vortex identification is also discussed.