Analysis of low-order modal coherent structures in cavitation flow field based on dynamic mode decomposition and finite-time Lyapunov exponent

The finite-time Lyapunov exponent (FTLE) method is a mature and practical method for analyzing the characteristics of Lagrangian coherent structures. It can be used for studying the severe impacts of cavitation on the coherent structure of flow. The reduced-order modeling (ROM) method has also significant advantages in extract key features of flow structure. This study analyzed the cloud cavitation flow structure of National Advisory Committee for Aeronautics (NACA)0015 hydrofoil. The backward FTLE and ROM were combined, and a comparison was made between the low-order modes of FTLE structure and the FTLE obtained from the low-order modes. The results indicate that the two methods have effectively captured the main coherent structural features of cloud cavitation flow fields. The main characteristic structures captured by the FTLE obtained from the low-order modes of the flow field are much clearer. The first two coherent structures of the FTLE obtained from the low-order modes of the flow field decompose the FTLE of the velocity field into three distinct parts: the leading-edge structure of the hydrofoil, the reflux structure in the middle of the hydrofoil, and the wake region of the hydrofoil. It is proved that the combination of FTLE and ROM can provide a new perspective and means for the analysis of turbulent structures.