Hydrodynamic performance of swimming fish in the wake region of a semi-cylinder

The characteristics of environmental vortices significantly influence the behavioral strategies of fish. This study investigated how wake vortices from obstacles affected propulsion and stability disturbances in fish behavior by numerically quantifying hydrodynamic effects related to different scales of semi-cylinders. The strong reverse flow in the recirculation regions of semi-cylinders generated trailing edge vortices near the tail, which reduced thrust and induced instability upon wake vortex impingement. Decreasing the gap between the semi-cylinder and the fish increased the duration of disruption. Additionally, passing wake vortices induced drag on fish tails and destabilized the fish when the gap between the vortices was small. Notably, for fish behind small semi-cylinders, wake vortex impingement did not cause drag and instability; rather, the primary source of drag was the shed wake vortices. These factors extended the disruption region laterally from the edges of the semi-cylinder to twice its diameter outside the wake region. Only the far downstream area, beyond the recirculation flow, could provide a beneficial turbulence environment with low-momentum flow. The findings of this study may enhance the understanding of vortex-fish interactions and offer valuable insights for the design and path planning of bio-inspired underwater vehicles.