  It is well known that tuna fish in the ocean can dynamically morph their
median fins to achieve optimal hydrodynamic performance, e.g. linear
acceleration and maneuverability. In this study, based on the previous studies
about the median fin's hydrodynamic effects focusing on tethered conditions, we
continue to explore the hydrodynamic function of tuna morphing dorsal fin in
free swimming conditions for better approaching real-life situations.Here, we
developed a tuna-inspired robotic fish platform that can swim independently in
three dimensions, equipped with a biomimetic morphing dorsal fin magnetically
attached to the robotic fish. Based on the free-swimming robotic fish platform,
we investigated how the erected dorsal fin affects the speed, cost of transport
(COT), and robotic fish's yaw angle at different frequencies and amplitudes.
The erected dorsal fin plays a positive role in improving the yaw stability of
robotic fish. However, it shows little influence on the speed and COT in our
test. This remains to be further investigated in the future.
