Among all aquatic species, mantas and rays swim by oscillating their pectoral fins; this motion is similar to other fishes in term of efficiency, but it gives better agility in turning with respect to fishes moving their caudal fin. The fin motion is featured by a travelling wave going opposite to the forward motion, producing a force thanks to momentum conservation. Another contribution to the generation of thrust is given by the generation of a vortex in correspondence of the leading edge of the fin, which pulls the fish forward thanks to the lower pressure in its centre. In literature these contributions have been highlighted, but it remains to understand which one of these two mechanisms is prevailing according to different conditions of swimming, how they affect each other and what is the influence of the two on energetical efficiency. The object of this activity is to investigate how thrust generation is influenced by geometrical characteristics of the fin, such as size, geometry and flexibility and by parameters of motion, such as speed, amplitude and frequency of fin oscillation and velocity of the travelling wave. A CFD model of the fish has been implemented in OpenFOAM, not only confirming that both upstroke and downstroke contribute positively to the forward movement according to the momentum conservation principle, but also highlighting the formation of a leading-edge vortex enhancing thrust generation. The description of how thrust generation is linked to motion parameters is simulated also coupling the CFD with a multibody to simulate the whole motion in its complexity.
One field in which nature outperforms current technology is fish swimming, because its efficiency, manoeuvrability and noise are far better than those of typical ship propellers. These advantages are not only due to the streamlined shape and the low-drag skin, but also and above all to the propulsion mechanism, which makes thrust generation possible with small energy dissipation in vortices. Nowadays the interest in autonomous underwater vehicles is in constant increase following the emerging needs of underwater mining and fish farming. Batoid fishes produce thrust with their pectoral fins, they essentially produce a wave travelling in the direction opposite to their motion, pushing water backwards and gaining thrust as a consequence of momentum conservation. The motion of the fin has been studied and reproduced with a series of articulated mechanisms. In this work the optimization of the mechanism’s geometry is described and the experimental results on the reconstructed fin are presented. Moreover, a bioinspired robot mimicking cownose ray locomotion has been designed and built. In this paper the functioning of this robot is shown.
Roots are extraordinary diggers because they penetrate the soil adding new material on their tip without moving the already grown part, preventing friction from dissipating too much energy and minimising inertial forces during motion. A robot exploiting this principle can assist operations of search and rescue digging in mud or snow to find people in danger. In this work a soft pneumatic robot inspired to roots’ growth is presented. The body of the robot consists of a cylindric plastic membrane folded inside out; one extremity is kept fixed to the base, whereas the other one is folded inside itself. When air is blown from the base, the body of the robot is inflated, and its tip is everted increasing its length. Inside the tip a head is mounted, where the mechanism controlling the direction of growth is placed. On the external surface of the membrane some hooks are mounted, and tensioned wires connects them longitudinally while they are folded before being everted. These wires are cut when they pass next to the head allowing the robot to unfold; since series of hooks are distributed radially on the body of the robot, the direction of growth is controlled by selecting which wires are to be cut. On the head of the robot can be mounted an infrared sensor or a video-camera needed for the specific application.