The essence of generating vortex-within-a-vortex structures behind forked caudal fins

This work focuses on vortex-within-a-vortex (VV) structures behind three different shapes of forked caudal fin with a heaving-pitching motion. The results show that the synchronization trailing-edge vortex separation and shedding leading-edge contributes to generation VV structure. fins chord lengths can be observed almost simultaneously. Strouhal number not only determines vortices in wake but also reflects activation time superimposed effect propulsion performance. study provides an important theoretical reference for design control fork propellers use underwater vehicles.