Performance of synchronized fins in biomimetic propulsion

By using a two-dimensional model of ray fins, we numerically investigate the thrust generation by closely-coupled fins with an immersed boundary approach. The concentration is on the performance enhancement through fin-fin interactions and the underlying vortex control mechanisms in three representative systems, a two-fin tandem configuration, a two-fin parallel configuration, and a three-fin triangular configuration. In all these systems the thrust generation can be significantly increased in comparison with single fins. Unlike previous studies of tandem fins, in which the gap and phase lag between the two fins were considered separately, our study shows that the dynamics of the system is determined by a parameter that combines these two (the global phase difference). The optimal performance occurs as this parameter is around π (destructive mode), and the worst performance occurs when it is around 0 (constructive mode). Interestingly, contrary to the vorticity cancellation scenario implied by its name, our simulations show that in the destructive mode there is in fact a wake re-organization mechanism, during which vortices with the same rotational direction shed from the two fins are attracted towards each other and merge. Subsequently, the wake downstream becomes a strong and well-organized reverse Kármán vortex street, which explains the increased thrust. In the parallel system, the best performance occurs in cases when the two fins are in opposites phases. Both the thrust and efficiency increase as the gap between the fins decreases, until a symmetry-breaking instability occurs in the wake and the efficiency starts plunging due to the increase in lateral force generation. In the triangular formation, the highest thrust generation also occurs in the destructive mode. However, no further increase in performance is observed compared with the tandem system.