Finlets are a series of small non-retractable fins common to scombrid fishes (mackerels, bonitos and tunas), which are known for their high swimming speed. It is hypothesized that these small fins could potentially affect propulsive performance. Here, we combine experimental and computational approaches to investigate the hydrodynamics of finlets in yellowfin tuna ( Thunnus albacares ) during steady swimming. High-speed videos were obtained to provide kinematic data on the in vivo motion of finlets. High-fidelity simulations were then carried out to examine the hydrodynamic performance and vortex dynamics of a biologically realistic multiple-finlet model with reconstructed kinematics. It was found that finlets undergo both heaving and pitching motion and are delayed in phase from anterior to posterior along the body. Simulation results show that finlets were drag producing and did not produce thrust. The interactions among finlets helped reduce total finlet drag by 21.5%. Pitching motions of finlets helped reduce the power consumed by finlets during swimming by 20.8% compared with non-pitching finlets. Moreover, the pitching finlets created constructive forces to facilitate posterior body flapping. Wake dynamics analysis revealed a unique vortex tube matrix structure and cross-flow streams redirected by the pitching finlets, which supports their hydrodynamic function in scombrid fishes. Limitations on modelling and the generality of results are also discussed.

Dorsoventral undulation is adopted by aquatic mammals for propulsion. However, it is not too common to find invertebrate aquatic animals that undulate their bodies in the vertical plane, which results from antiphasic contractions of dorsal and ventral muscles. To explore the mechanisms of the soft-bodied propulsion, in this work, an annelid swimmer employing up and down undulatory swimming mode is chosen, and the related kinematics and hydrodynamics are studied using a combined experimental and computational approach. A fully calibrated photogrammetry system with three highspeed cameras from different views is used to record the forward swimming motion of this invertebrate swimmer, namely leech. The vertically undulating kinematics are then reconstructed from those videos. With the detailed reconstruction, the undulating wavelength and amplitude distribution the swimmer exhibits during propulsion are quantified. Kinematics analysis results show that the invertebrate swimmer swims in a vertical anguilliform mode and the wavelength is about 0.7BL (body length) when it swims at a velocity of 1.5BL/s. An in-house immersed-boundary-method based flow solver is used to conduct the numerical simulations, with which the hydrodynamic performance and wake structures are investigated. The thrust generation and power consumption of the undulating body are described quantitatively. Furthermore, along the undulating body, the pressure distributions are studied.

No abstract