Vortex dynamics in the wake of a mechanical fish

This study focuses on the three-dimensional flow around a mechanical fish model, which reproduces the typical undulatory body and fin motion of a carangiform swimmer. The mechanical model consists of a flexible skeleton embedded in a soft transparent silicone body, which is connected with two cams to a flapping and bending hinge generating a traveling wave motion with increasing amplitude from anterior to posterior, extending to a combined heaving and pitching motion at the fin. The model is submerged in a water tank and towed at the characteristic swimming speed for the neutral swimming mode at U/V = 1. The method of Scanning Particle Image Velocimetry was used to analyze the three-dimensional time-dependent flow field in the axial and saggital planes. The results confirm the earlier observations that the wake develops into a chain of vortex rings which travel sidewards perpendicular to the swimming direction. However, instead of one single vortex shed at each tail beat half-cycle we observed a pair of two vortex rings being shed. Each pair consists of a larger main vortex ring corresponding to the tail beat start–stop vortex, while the second vortex ring is due to the body bending motion. The existence of the second vortex reflects the role of the body in undulatory swimming. A simplified model of the fish body comparing it to a plate with a hinged flap demonstrates the link between the sequence of kinematics and vortex shedding.