A Lagrangian analysis of partial cavitation growth and cavitation control mechanism

Abstract

Partial cavitation has a strong unsteadiness, which will cause serious damage to the hydraulic machinery. The spanwise obstacle is nearly the most efficient method for controlling unsteady cavitation. In this study, numerical simulations of partial cavitating flows around NACA (National Advisory Committee for Aeronautics) 66 hydrofoils in two dimensions (2D) were carried out both with and without obstruction. The obstruction is placed at 0.37c, and its height is 0.1c. Utilizing the finite-time Lyapunov exponent, the Lagrangian coherent structures (LCSs) were developed to investigate the dynamic characteristics of the unsteady flow. By showing the dynamic evolution of the Lagrangian behaviors, the time-dependent LCSs over the two different flows demonstrate the effectiveness of LCSs in explaining the evolution of the vortex during the partial cavitation process. With the use of LCSs, the vortex boundary and reentrant jet can be easily located, and the link between the vortexes can be readily seen. In the meantime, the vortex's origin and destination are shown by the stable and unstable manifolds, respectively. LCSs were then utilized to examine how the obstruction had an impact, and the following conclusions were reached. First, the obstruction can stop a portion of reentrant jets from passing through it. Second, the obstruction can curve the pathway of the reentrant jet, which has passed through it. Third, the obstruction prevents the cavity from flowing downstream. Finally, the obstruction continuously obliterates the expanding cavity across it. Simply said, the Lagrangian analysis based on LCSs provides a better understanding of the vortex dynamics than traditional visualization techniques, which is essential to understanding the great performance of the cavitation-induced unsteady flow.