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.

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