Furthering the Understanding of Multi-modal Shedding on a Cavitating NACA0015 Hydrofoil using Cluster-Based Reduced-Order Modeling (CROM) and Dynamic Mode Decomposition (DMD)

Cavitation dynamics on a NACA0015 hydrofoil is known to be multi-modal with abrupt changes in shedding frequency corresponding to a change in cavitation number. By using time-resolved X-ray densitometry based void-fraction field measurements, the underlying physical mechanisms responsible for the observed behavior have been found to be the occurrence of a three-step shedding process which involved either partial or full-collapse of growing cavity due to shed cloud-collapse. In addition, occurrence of propagating bubbly shock waves as a dominant mechanism of shedding also coincided with the observance of multi-modal behavior. It was also observed that the shedding process under certain conditions involved abrupt oscillations from three-step shedding to a single-step shedding. Even though the flow structures responsible for this transition have been identified, the driving force behind the transition between modes is not clear. In this study, we use CROM and DMD on instantaneous void-fraction fields to understand the transition dynamics and relate them to physical processes. In addition, comparison between the two methods for a cavitating flow is also presented.