Numerical study on the effects of a semi-free and non-uniform flexible filament in different vortex streets

The variable flexibility of a fish body is believed to play a significant role in improving swimming performance. To explore the effect of non-uniform flexibility on the motion performance of fish under biologically relevant conditions, we set up three different flexible distribution modes for a semi-free filament and compared the motion performance of different flexible distribution modes through numerical simulations. The filament is located in the wake of the front flapping foil; it can swing adaptively in the lateral direction according to the flow situation of the surrounding fluid and finally reach a stable position. The results show that the motion state of the filament will alter with a change in the flexibility of the filament, from moving in the vortex street to moving on the side of the vortex street. In the Benard-von Karman (BvK) vortex streets, the drag coefficient of the filament increases as the flexibility of the filament increases, and the value of the drag coefficient is at a minimum when the flexibility of the filament increases linearly along the length of the filament. Further investigation indicates that at 85%–90% of the filament length (starting from the leading edge), the flexibility of the filament begins to increase significantly, and the filament can obtain its best propulsion performance. The results of this work provide new insights into the role of non-uniform flexibility during the process of fish movement and provide a valuable reference for the design of bionic underwater vehicles.