Investigation of fluid-dynamic forces on an artificial cownose ray fin

The study of fish swimming is of great interest for engineers and researchers because the investigation of novel propulsion mechanisms is functional to the development of autonomous underwater vehicles. Among the different locomotion strategies adopted by fish species, the one characterizing the cownose ray is known to be very efficient. These fishes move their large pectoral fins and create a wave travelling in a direction opposite to the swimming direction. Thus, thanks to momentum conservation, the fish receives a propulsive thrust. This mechanism has been reproduced in a bioinspired robot mimicking a cownose ray, which has been designed and built. Before testing the robot underwater, some preliminary experiments in a small wind tunnel have been performed on a single fin. These tests are aimed at assessing the fluid dynamics of the fin shape and at verifying the effectiveness of this propulsive strategy. The fluid dynamic forces acting on the profile are measured with the fin still; the velocity of the incoming air is such that the Reynolds number is the same as for the fish swimming in water. Then, the propulsive strategy is tested with the fin in motion; the velocity of the incoming air is the same as the relative velocity of water for the fish during forward swimming, and several experiments were carried out with different frequencies and wavelengths of fin motion. Finally, having the fluids different densities, the measured forces and moments are scaled. These tests have confirmed that the cownose ray swimming strategy is effective and a relation between the measured forces with frequency and wavelength of fin motion has been found.