Vortical flows around rigid-bodied, multi-propulsor swimmers

Abstract

Boxfishes (Teleostei: Ostraciidae) are rigid-body, multi-propulsor swimmers that exhibit small amplitude recoil movements during rectilinear locomotion. The mechanisms by which boxfishes achieve such smooth swimming trajectories are not fully understood, and thus the goal of our research program is to examine the roles of the rigid carapace and fins in maintaining stability. To study the contributions of the carapace to stability, stereolithographic models of four morphologically distinct boxfishes were positioned at various angles of attack in water or wind tunnels, and flow patterns around each model were investigated using three methods: 1) digital particle image velocimetry (DPIV), 2) pressure distribution measurements, and 3) force balance measurements. Three-dimensional defocusing digital particle image velocimetry (DDPIV) also was used to examine the complex interactions between the body and fins of live fishes. Well-developed vortices formed along the ventro-lateral keels of all boxfish models; circulation within regions of concentrated vorticity increased posteriorly and intensified as angles of attack became more positive or negative. These vortices, which developed above and below ventro-lateral keels at positive and negative angles of attack, respectively, produce overall lift and self-correcting forces for pitching. Pressure distribution results were consistent with these findings, with areas of low pressure correlating well with regions of attached vorticity. Lift coefficients of boxfish models were similar to lift coefficients of delta wings, structures that also generate lift through vortex generation. Based on DDPIV results, fin movements interact with body vortex formation, and like the carapace, play an important role in stabilization.