Bluegill sunfish have an agility that human engineers can only marvel at. Their repertoire includes hovering, reversing and spinning around, and they achieve all this with deft moves of their fins. James Tangorra from Drexel University, USA, explains that bluegill sunfish pectoral fins are particularly remarkable because they generate forward thrust even when they are swept forward against the flow. Intrigued by the fin's remarkable ability to generate thrust when other fins produce drag, Tangorra and his colleagues, George Lauder, Ian Hunter, Rajat Mittal, Peter Madden and Meliha Bozkurttas, decided to build a robotic fin to see if they could replicate the fish's remarkable fluid dynamics and thrust characteristics (p. 4043).

First the team analysed the fin's complex manoeuvres. Tangorra explains that the fin is composed of 14 rays spanned by a membrane, so Lauder and Madden digitised 300 points on the surface of swimming fish fins to define the fin's movements. Then, Mittal and Bozkurttas analysed the kinematics and calculated the fluid flows over the surface to find out how the fin generated thrust. Having identified key aspects of the fin's motion, the team realised that they could simplify its structure down to five key rays to reproduce the fin's complex motion. Next, Tangorra and Hunter built a series of seven robot fins where they varied the taper and flexibility of the rays in an attempt to reproduce the fin's flowing movements. Sewing flexible polyester/elastane weaves between the rays to reproduce the membrane, Tangorra programmed each of the rays to replicate the sunfish's complex movements as the fin flapped. Then the team measured the forces on the rays and visualised the spinning vortices generated by the fin pushing against the water as they ‘swam’ the fin in still and flowing water.

Analysing movies of the robofin movements, the team could see that the most flexible rays produced the most realistic swimming action, while the fins with stiffer rays move more rigidly. And when the team compared the forces generated by the fins, they found that fins with the most flexible rays produced thrust even when the fin was being swept forward, while stiffer fins produced drag as they were swept forward. Tangorra explains that the fin's flexibility, coupled with the cup shape that it forms as it sweeps forward, produces thrust when other fins generate drag. Finally, when the team visualised the fluid flowing over the robofins, they found that the robots produced realistic fluid flows, replicating the spinning vortices that Lauder had seen when looking at the swimming fish.

Thinking about the ways that fish adjust thrust production as they move, Tangorra says, ‘It was really nice to see how this fin structure got tuned... you can stiffen it up and the forces change drastically’. He adds, ‘Sunfish have a way of modulating the mechanical properties of the fin so that when they want to swim forward and have a continuous thrust, they are able to do so, and we are now able to do the same with this robotic system.’ Considering how the fish control thrust production from an engineering perspective, Tangorra points out that fish produce this remarkably fine control with remarkably low power controls – central pattern generators – and concludes, ‘I think that this is a model that engineers are fascinated by and want to learn from.’

J. L.
G. V.
I. W.
P. G. A.
The effect of fin ray flexural rigidity on the propulsive forces generated by a biorobotic fish pectoral fin
J. Exp. Biol.