Study on the propulsion of the rigid-flexible composite plate driven on two points

The fish can be driven by the sarcomere at different locations of the body. To study the hydrodynamic performance of the fish in this process, the self-propulsion of a composite plate driving on two points was investigated. It is supposed that the forces were exerted on the junction point to maintain a given pitch motion of the rigid portion, and an additional external moment was exerted at a point on the flexible plate. The ratio between the lengths of the rigid portion and the flexible portion was fixed as 1/9, which was reasonable for a real fish. The acting point and phase angle of the additional external moment was varying to study the cooperation of the forces on different points. It was found that the additional external moment was not always promoting the propulsion velocity and efficiency of the composite plate. The additional external moment near the joint point was most suitable for improve the propulsive velocity and efficiency of the composite plate. The anti-phase additional external moment was prejudicing the propulsion of the plate. The additional external moment could change the vibration shape of the flexible plate and the flow structure around it. As ϕ = π, the amplitude and the gradient of the deformation was largely reduced, and the wake became long and narrow. The hydrodynamic forces at the head and tail portion of the flexible plate were hindering the propulsion of the plate, while the thrust was mainly produced on the intermediate portion. It was expected that our investigation can be helpful for the design of the soft robotic fish.